flutter_flutter/packages/flutter/lib/src/widgets/basic.dart

// Copyright 2015 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

import 'dart:ui' as ui show Image, ImageFilter;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter/services.dart';

import 'debug.dart';
import 'framework.dart';

export 'package:flutter/animation.dart';
export 'package:flutter/painting.dart';
export 'package:flutter/rendering.dart' show
  Axis,
  BoxConstraints,
  CrossAxisAlignment,
  CustomClipper,
  CustomPainter,
  FixedColumnCountGridDelegate,
  FlexFit,
  FlowDelegate,
  FlowPaintingContext,
  FractionalOffsetTween,
  GridChildPlacement,
  GridDelegate,
  GridDelegateWithInOrderChildPlacement,
  GridSpecification,
  HitTestBehavior,
  MainAxisAlignment,
  MainAxisSize,
  MaxTileWidthGridDelegate,
  MultiChildLayoutDelegate,
  Overflow,
  PaintingContext,
  PointerCancelEvent,
  PointerCancelEventListener,
  PointerDownEvent,
  PointerDownEventListener,
  PointerEvent,
  PointerMoveEvent,
  PointerMoveEventListener,
  PointerUpEvent,
  PointerUpEventListener,
  RelativeRect,
  ShaderCallback,
  SingleChildLayoutDelegate,
  TextOverflow,
  ValueChanged,
  ValueGetter,
  ViewportAnchor,
  ViewportDimensions,
  ViewportDimensionsChangeCallback;

// PAINTING NODES

/// A widget that makes its child partially transparent.
///
/// This class paints its child into an intermediate buffer and then blends the
/// child back into the scene partially transparent.
///
/// For values of opacity other than 0.0 and 1.0, this class is relatively
/// expensive because it requires painting the child into an intermediate
/// buffer. For the value 0.0, the child is simply not painted at all. For the
/// value 1.0, the child is painted immediately without an intermediate buffer.
class Opacity extends SingleChildRenderObjectWidget {
  /// Creates a widget that makes its child partially transparent.
  ///
  /// The [opacity] argument must not be null and must be between 0.0 and 1.0
  /// (inclusive).
  Opacity({
    Key key,
    @required this.opacity,
    Widget child
  }) : super(key: key, child: child) {
    assert(opacity != null && opacity >= 0.0 && opacity <= 1.0);
  }

  /// The fraction to scale the child's alpha value.
  ///
  /// An opacity of 1.0 is fully opaque. An opacity of 0.0 is fully transparent
  /// (i.e., invisible).
  ///
  /// The opacity must not be null.
  ///
  /// Values 1.0 and 0.0 are painted with a fast path. Other values
  /// require painting the child into an intermediate buffer, which is
  /// expensive.
  final double opacity;

  @override
  RenderOpacity createRenderObject(BuildContext context) => new RenderOpacity(opacity: opacity);

  @override
  void updateRenderObject(BuildContext context, RenderOpacity renderObject) {
    renderObject.opacity = opacity;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('opacity: $opacity');
  }
}

/// A widget that applies a mask generated by a [Shader] to its child.
///
/// For example, [ShaderMask] can be used to gradually fade out the edge
/// of a child by using a [ui.Gradient.linear] mask.
class ShaderMask extends SingleChildRenderObjectWidget {
  /// Creates a widget that applies a mask generated by a [Shader] to its child.
  ///
  /// The [shaderCallback] and [transferMode] arguments must not be null.
  ShaderMask({
    Key key,
    @required this.shaderCallback,
    this.transferMode: TransferMode.modulate,
    Widget child
  }) : super(key: key, child: child) {
    assert(shaderCallback != null);
    assert(transferMode != null);
  }

  /// Called to creates the [Shader] that generates the mask.
  ///
  /// The shader callback is called with the current size of the child so that
  /// it can customize the shader to the size and location of the child.
  final ShaderCallback shaderCallback;

  /// The [TransferMode] to use when applying the shader to the child.
  ///
  /// The default, [TransferMode.modulate], is useful for applying an alpha blend
  /// to the child. Other transfer modes can be used to create other effects.
  final TransferMode transferMode;

  @override
  RenderShaderMask createRenderObject(BuildContext context) {
    return new RenderShaderMask(
      shaderCallback: shaderCallback,
      transferMode: transferMode
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderShaderMask renderObject) {
    renderObject
      ..shaderCallback = shaderCallback
      ..transferMode = transferMode;
  }
}

/// A widget that applies a filter to the existing painted content and then paints [child].
///
/// This effect is relatively expensive, especially if the filter is non-local,
/// such as a blur.
class BackdropFilter extends SingleChildRenderObjectWidget {
  /// Creates a backdrop filter.
  ///
  /// The [filter] argument must not be null.
  BackdropFilter({
    Key key,
    @required this.filter,
    Widget child
  }) : super(key: key, child: child) {
    assert(filter != null);
  }

  /// The image filter to apply to the existing painted content before painting the child.
  ///
  /// For example, consider using [ui.ImageFilter.blur] to create a backdrop
  /// blur effect
  final ui.ImageFilter filter;

  @override
  RenderBackdropFilter createRenderObject(BuildContext context) {
    return new RenderBackdropFilter(filter: filter);
  }

  @override
  void updateRenderObject(BuildContext context, RenderBackdropFilter renderObject) {
    renderObject.filter = filter;
  }
}

/// A widget that provides a canvas on which to draw during the paint phase.
///
/// When asked to paint, [CustomPaint] first asks its [painter] to paint on the
/// current canvas, then it paints its child, and then, after painting its
/// child, it asks its [foregroundPainter] to paint. The coodinate system of the
/// canvas matches the coordinate system of the [CustomPaint] object. The
/// painters are expected to paint within a rectangle starting at the origin and
/// encompassing a region of the given size. (If the painters paint outside
/// those bounds, there might be insufficient memory allocated to rasterize the
/// painting commands and the resulting behavior is undefined.)
///
/// Painters are implemented by subclassing [CustomPainter].
///
/// Because custom paint calls its painters during paint, you cannot call
/// `setState` or `markNeedsLayout` during the callback (the layout for this
/// frame has already happened).
///
/// Custom painters normally size themselves to their child. If they do not have
/// a child, they attempt to size themselves to the [size], which defaults to
/// [Size.zero].
///
/// See also:
///
///  * [CustomPainter].
///  * [Canvas].
class CustomPaint extends SingleChildRenderObjectWidget {
  /// Creates a widget that delegates its painting.
  CustomPaint({ Key key, this.painter, this.foregroundPainter, this.size: Size.zero, Widget child })
    : super(key: key, child: child) {
    assert(size != null);
  }

  /// The painter that paints before the children.
  final CustomPainter painter;

  /// The painter that paints after the children.
  final CustomPainter foregroundPainter;

  /// The size that this [CustomPaint] should aim for, given the layout
  /// constraints, if there is no child.
  ///
  /// Defaults to [Size.zero].
  ///
  /// If there's a child, this is ignored, and the size of the child is used
  /// instead.
  final Size size;

  @override
  RenderCustomPaint createRenderObject(BuildContext context) => new RenderCustomPaint(
    painter: painter,
    foregroundPainter: foregroundPainter,
    preferredSize: size,
  );

  @override
  void updateRenderObject(BuildContext context, RenderCustomPaint renderObject) {
    renderObject
      ..painter = painter
      ..foregroundPainter = foregroundPainter
      ..preferredSize = size;
  }

  @override
  void didUnmountRenderObject(RenderCustomPaint renderObject) {
    renderObject
      ..painter = null
      ..foregroundPainter = null;
  }
}

/// A widget that clips its child using a rectangle.
///
/// By default, [ClipRect] prevents its child from painting outside its
/// bounds, but the size and location of the clip rect can be customized using a
/// custom [clipper].
class ClipRect extends SingleChildRenderObjectWidget {
  /// Creates a rectangular clip.
  ///
  /// If [clipper] is null, the clip will match the layout size and position of
  /// the child.
  ClipRect({ Key key, this.clipper, Widget child }) : super(key: key, child: child);

  /// If non-null, determines which clip to use.
  final CustomClipper<Rect> clipper;

  @override
  RenderClipRect createRenderObject(BuildContext context) => new RenderClipRect(clipper: clipper);

  @override
  void updateRenderObject(BuildContext context, RenderClipRect renderObject) {
    renderObject.clipper = clipper;
  }

  @override
  void didUnmountRenderObject(RenderClipRect renderObject) {
    renderObject.clipper = null;
  }
}

/// A widget that clips its child using a rounded rectangle.
///
/// By default, [ClipRRect] uses its own bounds as the base rectangle for the
/// clip, but the size and location of the clip can be customized using a custom
/// [clipper].
class ClipRRect extends SingleChildRenderObjectWidget {
  /// Creates a rounded-rectangular clip.
  ///
  /// The [borderRadius] defaults to [BorderRadius.zero], i.e. a rectangle with
  /// right-angled corners.
  ///
  /// If [clipper] is non-null, then [borderRadius] is ignored.
  ClipRRect({
    Key key,
    this.borderRadius,
    this.clipper,
    Widget child,
  }) : super(key: key, child: child) {
    assert(borderRadius != null || clipper != null);
  }

  /// The border radius of the rounded corners.
  ///
  /// Values are clamped so that horizontal and vertical radii sums do not
  /// exceed width/height.
  ///
  /// This value is ignored if [clipper] is non-null.
  final BorderRadius borderRadius;

  /// If non-null, determines which clip to use.
  final CustomClipper<RRect> clipper;

  @override
  RenderClipRRect createRenderObject(BuildContext context) => new RenderClipRRect(borderRadius: borderRadius, clipper: clipper);

  @override
  void updateRenderObject(BuildContext context, RenderClipRRect renderObject) {
    renderObject
      ..borderRadius = borderRadius
      ..clipper = clipper;
  }
}

/// A widget that clips its child using an oval.
///
/// By default, inscribes an axis-aligned oval into its layout dimensions and
/// prevents its child from painting outside that oval, but the size and
/// location of the clip oval can be customized using a custom [clipper].
class ClipOval extends SingleChildRenderObjectWidget {
  /// Creates an oval-shaped clip.
  ///
  /// If [clipper] is null, the oval will be inscribed into the layout size and
  /// position of the child.
  ClipOval({ Key key, this.clipper, Widget child }) : super(key: key, child: child);

  /// If non-null, determines which clip to use.
  ///
  /// The delegate returns a rectangle that describes the axis-aligned
  /// bounding box of the oval. The oval's axes will themselves also
  /// be axis-aligned.
  ///
  /// If the [clipper] delegate is null, then the oval uses the
  /// widget's bounding box (the layout dimensions of the render
  /// object) instead.
  final CustomClipper<Rect> clipper;

  @override
  RenderClipOval createRenderObject(BuildContext context) => new RenderClipOval(clipper: clipper);

  @override
  void updateRenderObject(BuildContext context, RenderClipOval renderObject) {
    renderObject.clipper = clipper;
  }

  @override
  void didUnmountRenderObject(RenderClipOval renderObject) {
    renderObject.clipper = null;
  }
}

/// A widget that clips its child using a path.
///
/// Calls a callback on a delegate whenever the widget is to be
/// painted. The callback returns a path and the widget prevents the
/// child from painting outside the path.
///
/// Clipping to a path is expensive. Certain shapes have more
/// optimized widgets:
///
///  * To clip to a rectangle, consider [ClipRect].
///  * To clip to an oval or circle, consider [ClipOval].
///  * To clip to a rounded rectangle, consider [ClipRRect].
class ClipPath extends SingleChildRenderObjectWidget {
  /// Creates a path clip.
  ///
  /// If [clipper] is null, the clip will be a rectangle that matches the layout
  /// size and location of the child. However, rather than use this default,
  /// consider using a [ClipRect], which can achieve the same effect more
  /// efficiently.
  ClipPath({ Key key, this.clipper, Widget child }) : super(key: key, child: child);

  /// If non-null, determines which clip to use.
  ///
  /// The default clip, which is used if this property is null, is the
  /// bounding box rectangle of the widget. [ClipRect] is a more
  /// efficient way of obtaining that effect.
  final CustomClipper<Path> clipper;

  @override
  RenderClipPath createRenderObject(BuildContext context) => new RenderClipPath(clipper: clipper);

  @override
  void updateRenderObject(BuildContext context, RenderClipPath renderObject) {
    renderObject.clipper = clipper;
  }

  @override
  void didUnmountRenderObject(RenderClipPath renderObject) {
    renderObject.clipper = null;
  }
}


// POSITIONING AND SIZING NODES

/// A widget that applies a transformation before painting its child.
class Transform extends SingleChildRenderObjectWidget {
  /// Creates a widget that transforms its child.
  ///
  /// The [transform] argument must not be null.
  Transform({
    Key key,
    @required this.transform,
    this.origin,
    this.alignment,
    this.transformHitTests: true,
    Widget child
  }) : super(key: key, child: child) {
    assert(transform != null);
  }

  /// The matrix to transform the child by during painting.
  final Matrix4 transform;

  /// The origin of the coordinate system (relative to the upper left corder of
  /// this render object) in which to apply the matrix.
  ///
  /// Setting an origin is equivalent to conjugating the transform matrix by a
  /// translation. This property is provided just for convenience.
  final Offset origin;

  /// The alignment of the origin, relative to the size of the box.
  ///
  /// This is equivalent to setting an origin based on the size of the box.
  /// If it is specified at the same time as an offset, both are applied.
  final FractionalOffset alignment;

  /// Whether to apply the translation when performing hit tests.
  final bool transformHitTests;

  @override
  RenderTransform createRenderObject(BuildContext context) => new RenderTransform(
    transform: transform,
    origin: origin,
    alignment: alignment,
    transformHitTests: transformHitTests
  );

  @override
  void updateRenderObject(BuildContext context, RenderTransform renderObject) {
    renderObject
      ..transform = transform
      ..origin = origin
      ..alignment = alignment
      ..transformHitTests = transformHitTests;
  }
}

/// Scales and positions its child within itself according to [fit].
class FittedBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that scales and positions its child within itself according to [fit].
  ///
  /// The [fit] and [alignment] arguments must not be null.
  FittedBox({
    Key key,
    this.fit: ImageFit.contain,
    this.alignment: FractionalOffset.center,
    Widget child
  }) : super(key: key, child: child) {
    assert(fit != null);
    assert(alignment != null && alignment.dx != null && alignment.dy != null);
  }

  /// How to inscribe the child into the space allocated during layout.
  final ImageFit fit;

  /// How to align the child within its parent's bounds.
  ///
  /// An alignment of (0.0, 0.0) aligns the child to the top-left corner of its
  /// parent's bounds.  An alignment of (1.0, 0.5) aligns the child to the middle
  /// of the right edge of its parent's bounds.
  final FractionalOffset alignment;

  @override
  RenderFittedBox createRenderObject(BuildContext context) => new RenderFittedBox(fit: fit, alignment: alignment);

  @override
  void updateRenderObject(BuildContext context, RenderFittedBox renderObject) {
    renderObject
      ..fit = fit
      ..alignment = alignment;
  }
}

/// A widget that applies a translation expressed as a fraction of the box's
/// size before painting its child.
class FractionalTranslation extends SingleChildRenderObjectWidget {
  /// Creates a widget that translates its child's painting.
  ///
  /// The [translation] argument must not be null.
  FractionalTranslation({
    Key key,
    @required this.translation,
    this.transformHitTests: true,
    Widget child
  }) : super(key: key, child: child) {
    assert(translation != null);
  }

  /// The offset by which to translate the child, as a multiple of its size.
  final FractionalOffset translation;

  /// Whether to apply the translation when performing hit tests.
  final bool transformHitTests;

  @override
  RenderFractionalTranslation createRenderObject(BuildContext context) => new RenderFractionalTranslation(translation: translation, transformHitTests: transformHitTests);

  @override
  void updateRenderObject(BuildContext context, RenderFractionalTranslation renderObject) {
    renderObject
      ..translation = translation
      ..transformHitTests = transformHitTests;
  }
}

/// A widget that rotates its child by a integral number of quarter turns.
///
/// Unlike [Transform], which applies a transform just prior to painting,
/// this object applies its rotation prior to layout, which means the entire
/// rotated box consumes only as much space as required by the rotated child.
class RotatedBox extends SingleChildRenderObjectWidget {
  /// A widget that rotates its child.
  ///
  /// The [quarterTurns] argument must not be null.
  RotatedBox({
    Key key,
    @required this.quarterTurns,
    Widget child
  }) : super(key: key, child: child) {
    assert(quarterTurns != null);
  }

  /// The number of clockwise quarter turns the child should be rotated.
  final int quarterTurns;

  @override
  RenderRotatedBox createRenderObject(BuildContext context) => new RenderRotatedBox(quarterTurns: quarterTurns);

  @override
  void updateRenderObject(BuildContext context, RenderRotatedBox renderObject) {
    renderObject.quarterTurns = quarterTurns;
  }
}

/// A widget that insets its child by the given padding.
///
/// When passing layout constraints to its child, padding shrinks the
/// constraints by the given padding, causing the child to layout at a smaller
/// size. Padding then sizes itself to its child's size, inflated by the
/// padding, effectively creating empty space around the child.
class Padding extends SingleChildRenderObjectWidget {
  /// Creates a widget that insets its child.
  ///
  /// The [padding] argument must not be null.
  Padding({
    Key key,
    @required this.padding,
    Widget child
  }) : super(key: key, child: child) {
    assert(padding != null);
  }

  /// The amount of space by which to inset the child.
  final EdgeInsets padding;

  @override
  RenderPadding createRenderObject(BuildContext context) => new RenderPadding(padding: padding);

  @override
  void updateRenderObject(BuildContext context, RenderPadding renderObject) {
    renderObject.padding = padding;
  }
}

/// A widget that aligns its child within itself and optionally sizes itself
/// based on the child's size.
///
/// For example, to align a box at the bottom right, you would pass this box a
/// tight constraint that is bigger than the child's natural size,
/// with an alignment of [FractionalOffset.bottomRight].
///
/// This widget will be as big as possible if its dimensions are constrained and
/// [widthFactor] and [heightFactor] are null. If a dimension is unconstrained
/// and the corresponding size factor is null then the widget will match its
/// child's size in that dimension. If a size factor is non-null then the
/// corresponding dimension of this widget will be the product of the child's
/// dimension and the size factor. For example if widthFactor is 2.0 then
/// the width of this widget will always be twice its child's width.
///
/// See also:
///
///  * [CustomSingleChildLayout], which uses a delegate to control the layout of
///    a single child.
///  * [Center], which is the same as [Align] but with the [alignment] always
///    set to [FractionalOffset.center].
///  * [FractionallySizedBox], which sizes its child based on a fraction of its own
///    size and positions the child according to a [FractionalOffset] value.
class Align extends SingleChildRenderObjectWidget {
  /// Creates an alignment widget.
  ///
  /// The alignment defaults to [FractionalOffset.center].
  Align({
    Key key,
    this.alignment: FractionalOffset.center,
    this.widthFactor,
    this.heightFactor,
    Widget child
  }) : super(key: key, child: child) {
    assert(alignment != null && alignment.dx != null && alignment.dy != null);
    assert(widthFactor == null || widthFactor >= 0.0);
    assert(heightFactor == null || heightFactor >= 0.0);
  }

  /// How to align the child.
  ///
  /// The x and y values of the alignment control the horizontal and vertical
  /// alignment, respectively.  An x value of 0.0 means that the left edge of
  /// the child is aligned with the left edge of the parent whereas an x value
  /// of 1.0 means that the right edge of the child is aligned with the right
  /// edge of the parent. Other values interpolate (and extrapolate) linearly.
  /// For example, a value of 0.5 means that the center of the child is aligned
  /// with the center of the parent.
  final FractionalOffset alignment;

  /// If non-null, sets its width to the child's width multipled by this factor.
  ///
  /// Can be both greater and less than 1.0 but must be positive.
  final double widthFactor;

  /// If non-null, sets its height to the child's height multipled by this factor.
  ///
  /// Can be both greater and less than 1.0 but must be positive.
  final double heightFactor;

  @override
  RenderPositionedBox createRenderObject(BuildContext context) => new RenderPositionedBox(alignment: alignment, widthFactor: widthFactor, heightFactor: heightFactor);

  @override
  void updateRenderObject(BuildContext context, RenderPositionedBox renderObject) {
    renderObject
      ..alignment = alignment
      ..widthFactor = widthFactor
      ..heightFactor = heightFactor;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('alignment: $alignment');
    if (widthFactor != null)
      description.add('widthFactor: $widthFactor');
    if (heightFactor != null)
      description.add('heightFactor: $heightFactor');
  }
}

/// A widget that centers its child within itself.
///
/// See also:
///
///  * [Align], which lets you arbitrarily position a child within itself,
///    rather than just centering it.
class Center extends Align {
  /// Creates a widget that centers its child.
  Center({ Key key, double widthFactor, double heightFactor, Widget child })
    : super(key: key, widthFactor: widthFactor, heightFactor: heightFactor, child: child);
}

/// A widget that defers the layout of its single child to a delegate.
///
/// The delegate can determine the layout constraints for the child and can
/// decide where to position the child. The delegate can also determine the size
/// of the parent, but the size of the parent cannot depend on the size of the
/// child.
///
/// See also:
///
///  * [SingleChildLayoutDelegate], which controls the layout of the child.
///  * [Align], which sizes itself based on its child's size and positions
///    the child according to a [FractionalOffset] value.
///  * [FractionallySizedBox], which sizes its child based on a fraction of its own
///    size and positions the child according to a [FractionalOffset] value.
///  * [CustomMultiChildLayout], which uses a delegate to position multiple
///    children.
class CustomSingleChildLayout extends SingleChildRenderObjectWidget {
  /// Creates a custom single child layout.
  ///
  /// The [delegate] argument must not be null.
  CustomSingleChildLayout({
    Key key,
    @required this.delegate,
    Widget child
  }) : super(key: key, child: child) {
    assert(delegate != null);
  }

  /// The delegate that controls the layout of the child.
  final SingleChildLayoutDelegate delegate;

  @override
  RenderCustomSingleChildLayoutBox createRenderObject(BuildContext context) => new RenderCustomSingleChildLayoutBox(delegate: delegate);

  @override
  void updateRenderObject(BuildContext context, RenderCustomSingleChildLayoutBox renderObject) {
    renderObject.delegate = delegate;
  }
}

/// Meta data for identifying children in a [CustomMultiChildLayout].
///
/// The [MultiChildLayoutDelegate] hasChild, layoutChild, and positionChild
/// methods use these identifiers.
class LayoutId extends ParentDataWidget<CustomMultiChildLayout> {
  /// Marks a child with a layout identifier.
  ///
  /// Both the child and the id arguments must not be null.
  LayoutId({
    Key key,
    @required Object id,
    @required Widget child
  }) : id = id, super(key: key ?? new ValueKey<Object>(id), child: child) {
    assert(child != null);
    assert(id != null);
  }

  /// An object representing the identity of this child.
  final Object id;

  @override
  void applyParentData(RenderObject renderObject) {
    assert(renderObject.parentData is MultiChildLayoutParentData);
    final MultiChildLayoutParentData parentData = renderObject.parentData;
    if (parentData.id != id) {
      parentData.id = id;
      AbstractNode targetParent = renderObject.parent;
      if (targetParent is RenderObject)
        targetParent.markNeedsLayout();
    }
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('id: $id');
  }
}

/// A widget that uses a delegate to size and position multiple children.
///
/// The delegate can determine the layout constraints for each child and can
/// decide where to position each child. The delegate can also determine the
/// size of the parent, but the size of the parent cannot depend on the sizes of
/// the children.
///
/// [CustomMultiChildLayout] is appropriate when there are complex relationships
/// between the size and positioning of a multiple widgets. To control the
/// layout of a single child, [CustomSingleChildLayout] is more appropriate. For
/// simple cases, such as aligning a widget to one or another edge, the [Stack]
/// widget is more appropriate.
///
/// Each child must be wrapped in a [LayoutId] widget to identify the widget for
/// the delegate.
///
/// See also:
///
/// * [MultiChildLayoutDelegate], for details about how to control the layout of
///   the children.
/// * [CustomSingleChildLayout], which uses a delegate to control the layout of
///   a single child.
/// * [Stack], which arranges children relative to the edges of the container.
/// * [Flow], which provides paint-time control of its children using transform
///   matrices.
class CustomMultiChildLayout extends MultiChildRenderObjectWidget {
  /// Creates a custom multi-child layout.
  ///
  /// The [delegate] argument must not be null.
  CustomMultiChildLayout({
    Key key,
    @required this.delegate,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(delegate != null);
  }

  /// The delegate that controls the layout of the children.
  final MultiChildLayoutDelegate delegate;

  @override
  RenderCustomMultiChildLayoutBox createRenderObject(BuildContext context) {
    return new RenderCustomMultiChildLayoutBox(delegate: delegate);
  }

  @override
  void updateRenderObject(BuildContext context, RenderCustomMultiChildLayoutBox renderObject) {
    renderObject.delegate = delegate;
  }
}

/// A box with a specified size.
///
/// If given a child, this widget forces its child to have a specific width
/// and/or height (assuming values are permitted by this widget's parent). If
/// either the width or height is null, this widget will size itself to match
/// the child's size in that dimension.
///
/// If not given a child, this widget will size itself to the given width and
/// height, treating nulls as zero.
///
/// The [new SizedBox.expand] constructor can be used to make a [SizedBox] that
/// sizes itself to fit the parent. It is equivalent to setting [width] and
/// [height] to [double.INFINITY].
class SizedBox extends SingleChildRenderObjectWidget {
  /// Creates a box of a specific size.
  const SizedBox({ Key key, this.width, this.height, Widget child })
    : super(key: key, child: child);

  const SizedBox.expand({ Key key, Widget child })
    : width = double.INFINITY,
      height = double.INFINITY,
      super(key: key, child: child);

  /// If non-null, requires the child to have exactly this width.
  final double width;

  /// If non-null, requires the child to have exactly this height.
  final double height;

  @override
  RenderConstrainedBox createRenderObject(BuildContext context) => new RenderConstrainedBox(
    additionalConstraints: _additionalConstraints
  );

  BoxConstraints get _additionalConstraints {
    return new BoxConstraints.tightFor(width: width, height: height);
  }

  @override
  void updateRenderObject(BuildContext context, RenderConstrainedBox renderObject) {
    renderObject.additionalConstraints = _additionalConstraints;
  }

  @override
  String toStringShort() {
    String type = (width == double.INFINITY && height == double.INFINITY) ?
                  '$runtimeType.expand' : '$runtimeType';
    return key == null ? '$type' : '$type-$key';
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    if (width != double.INFINITY || height != double.INFINITY) {
      if (width != null)
        description.add('width: $width');
      if (height != null)
        description.add('height: $height');
    }
  }
}

/// A widget that imposes additional constraints on its child.
///
/// For example, if you wanted [child] to have a minimum height of 50.0 logical
/// pixels, you could use `const BoxConstraints(minHeight: 50.0)`` as the
/// [additionalConstraints].
class ConstrainedBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that imposes additional constraints on its child.
  ///
  /// The [constraints] argument must not be null.
  ConstrainedBox({
    Key key,
    @required this.constraints,
    Widget child
  }) : super(key: key, child: child) {
    assert(constraints != null);
    assert(constraints.debugAssertIsValid());
  }

  /// The additional constraints to impose on the child.
  final BoxConstraints constraints;

  @override
  RenderConstrainedBox createRenderObject(BuildContext context) => new RenderConstrainedBox(additionalConstraints: constraints);

  @override
  void updateRenderObject(BuildContext context, RenderConstrainedBox renderObject) {
    renderObject.additionalConstraints = constraints;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('$constraints');
  }
}

/// A widget that sizes its child to a fraction of the total available space.
/// For more details about the layout algorithm, see [RenderFractionallySizedOverflowBox].
///
/// See also:
///
/// * [Align] (which sizes itself based on its child's size and positions
///   the child according to a [FractionalOffset] value)
/// * [OverflowBox]
class FractionallySizedBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that sizes its child to a fraction of the total available space.
  ///
  /// If non-null, the [widthFactor] and [heightFactor] arguments must be
  /// non-negative.
  FractionallySizedBox({
    Key key,
    this.alignment: FractionalOffset.center,
    this.widthFactor,
    this.heightFactor,
    Widget child
  }) : super(key: key, child: child) {
    assert(alignment != null && alignment.dx != null && alignment.dy != null);
    assert(widthFactor == null || widthFactor >= 0.0);
    assert(heightFactor == null || heightFactor >= 0.0);
  }

  /// If non-null, the fraction of the incoming width given to the child.
  ///
  /// If non-null, the child is given a tight width constraint that is the max
  /// incoming width constraint multipled by this factor.
  ///
  /// If null, the incoming width constraints are passed to the child
  /// unmodified.
  final double widthFactor;

  /// If non-null, the fraction of the incoming height given to the child.
  ///
  /// If non-null, the child is given a tight height constraint that is the max
  /// incoming height constraint multipled by this factor.
  ///
  /// If null, the incoming height constraints are passed to the child
  /// unmodified.
  final double heightFactor;

  /// How to align the child.
  ///
  /// The x and y values of the alignment control the horizontal and vertical
  /// alignment, respectively.  An x value of 0.0 means that the left edge of
  /// the child is aligned with the left edge of the parent whereas an x value
  /// of 1.0 means that the right edge of the child is aligned with the right
  /// edge of the parent. Other values interpolate (and extrapolate) linearly.
  /// For example, a value of 0.5 means that the center of the child is aligned
  /// with the center of the parent.
  final FractionalOffset alignment;

  @override
  RenderFractionallySizedOverflowBox createRenderObject(BuildContext context) => new RenderFractionallySizedOverflowBox(
    alignment: alignment,
    widthFactor: widthFactor,
    heightFactor: heightFactor
  );

  @override
  void updateRenderObject(BuildContext context, RenderFractionallySizedOverflowBox renderObject) {
    renderObject
      ..alignment = alignment
      ..widthFactor = widthFactor
      ..heightFactor = heightFactor;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('alignment: $alignment');
    if (widthFactor != null)
      description.add('widthFactor: $widthFactor');
    if (heightFactor != null)
      description.add('heightFactor: $heightFactor');
  }
}

/// A box that limits its size only when it's unconstrained.
///
/// If this widget's maximum width is unconstrained then its child's width is
/// limited to maxWidth. Similarly, if this widget's maximum height is unconstrained
/// then its child's height is limited to to maxHeight.
class LimitedBox extends SingleChildRenderObjectWidget {
  /// Creates a box that limits its size only when it's unconstrained.
  ///
  /// The [maxWidth] and [maxHeight] arguments must not be null and must not be
  /// negative.
  LimitedBox({
    Key key,
    this.maxWidth: double.INFINITY,
    this.maxHeight: double.INFINITY,
    Widget child
  }) : super(key: key, child: child) {
    assert(maxWidth != null && maxWidth >= 0.0);
    assert(maxHeight != null && maxHeight >= 0.0);
  }

  /// The maximum width limit to apply in the absence of a maxWidth constraint.
  final double maxWidth;

  /// The maximum height limit to apply in the absence of a maxHeight constraint.
  final double maxHeight;

  @override
  RenderLimitedBox createRenderObject(BuildContext context) => new RenderLimitedBox(
    maxWidth: maxWidth,
    maxHeight: maxHeight
  );

  @override
  void updateRenderObject(BuildContext context, RenderLimitedBox renderObject) {
    renderObject
      ..maxWidth = maxWidth
      ..maxHeight = maxHeight;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    if (maxWidth != double.INFINITY)
      description.add('maxWidth: $maxWidth');
    if (maxHeight != double.INFINITY)
      description.add('maxHeight: $maxHeight');
  }
}

/// A widget that imposes different constraints on its child than it gets
/// from its parent, possibly allowing the child to overflow the parent.
///
/// See [RenderOverflowBox] for details.
class OverflowBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that lets its child overflow itself.
  OverflowBox({
    Key key,
    this.alignment: FractionalOffset.center,
    this.minWidth,
    this.maxWidth,
    this.minHeight,
    this.maxHeight,
    Widget child
  }) : super(key: key, child: child);

  /// How to align the child.
  ///
  /// The x and y values of the alignment control the horizontal and vertical
  /// alignment, respectively.  An x value of 0.0 means that the left edge of
  /// the child is aligned with the left edge of the parent whereas an x value
  /// of 1.0 means that the right edge of the child is aligned with the right
  /// edge of the parent. Other values interpolate (and extrapolate) linearly.
  /// For example, a value of 0.5 means that the center of the child is aligned
  /// with the center of the parent.
  final FractionalOffset alignment;

  /// The minimum width constraint to give the child. Set this to null (the
  /// default) to use the constraint from the parent instead.
  final double minWidth;

  /// The maximum width constraint to give the child. Set this to null (the
  /// default) to use the constraint from the parent instead.
  final double maxWidth;

  /// The minimum height constraint to give the child. Set this to null (the
  /// default) to use the constraint from the parent instead.
  final double minHeight;

  /// The maximum height constraint to give the child. Set this to null (the
  /// default) to use the constraint from the parent instead.
  final double maxHeight;

  @override
  RenderConstrainedOverflowBox createRenderObject(BuildContext context) => new RenderConstrainedOverflowBox(
    alignment: alignment,
    minWidth: minWidth,
    maxWidth: maxWidth,
    minHeight: minHeight,
    maxHeight: maxHeight
  );

  @override
  void updateRenderObject(BuildContext context, RenderConstrainedOverflowBox renderObject) {
    renderObject
      ..alignment = alignment
      ..minWidth = minWidth
      ..maxWidth = maxWidth
      ..minHeight = minHeight
      ..maxHeight = maxHeight;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('alignment: $alignment');
    if (minWidth != null)
      description.add('minWidth: $minWidth');
    if (maxWidth != null)
      description.add('maxWidth: $maxWidth');
    if (minHeight != null)
      description.add('minHeight: $minHeight');
    if (maxHeight != null)
      description.add('maxHeight: $maxHeight');
  }
}

/// A widget that is a specific size but passes its original constraints
/// through to its child, which will probably overflow.
class SizedOverflowBox extends SingleChildRenderObjectWidget {
  /// Creates a widget of a given size that lets its child overflow.
  ///
  /// The [size] argument must not be null.
  SizedOverflowBox({
    Key key,
    @required this.size,
    this.alignment: FractionalOffset.center,
    Widget child
  }) : super(key: key, child: child) {
    assert(size != null);
    assert(alignment != null && alignment.dx != null && alignment.dy != null);
  }

  /// How to align the child.
  ///
  /// The x and y values of the alignment control the horizontal and vertical
  /// alignment, respectively.  An x value of 0.0 means that the left edge of
  /// the child is aligned with the left edge of the parent whereas an x value
  /// of 1.0 means that the right edge of the child is aligned with the right
  /// edge of the parent. Other values interpolate (and extrapolate) linearly.
  /// For example, a value of 0.5 means that the center of the child is aligned
  /// with the center of the parent.
  final FractionalOffset alignment;

  /// The size this widget should attempt to be.
  final Size size;

  @override
  RenderSizedOverflowBox createRenderObject(BuildContext context) {
    return new RenderSizedOverflowBox(
      alignment: alignment,
      requestedSize: size
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderSizedOverflowBox renderObject) {
    renderObject
      ..alignment = alignment
      ..requestedSize = size;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('alignment: $alignment');
    if (size != null)
      description.add('size: $size');
  }
}

/// A widget that lays the child out as if it was in the tree, but without painting anything,
/// without making the child available for hit testing, and without taking any
/// room in the parent.
class Offstage extends SingleChildRenderObjectWidget {
  /// Creates a widget that visually hides its child.
  Offstage({ Key key, this.offstage: true, Widget child })
    : super(key: key, child: child) {
    assert(offstage != null);
  }

  /// Whether the child is hidden from the rest of the tree.
  ///
  /// If true, the child is laid out as if it was in the tree, but without
  /// painting anything, without making the child available for hit testing, and
  /// without taking any room in the parent.
  ///
  /// If false, the child is included in the tree as normal.
  final bool offstage;

  @override
  RenderOffstage createRenderObject(BuildContext context) => new RenderOffstage(offstage: offstage);

  @override
  void updateRenderObject(BuildContext context, RenderOffstage renderObject) {
    renderObject.offstage = offstage;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('offstage: $offstage');
  }

  @override
  _OffstageElement createElement() => new _OffstageElement(this);
}

class _OffstageElement extends SingleChildRenderObjectElement {
  _OffstageElement(Offstage widget) : super(widget);

  @override
  Offstage get widget => super.widget;

  @override
  void visitChildrenForSemantics(ElementVisitor visitor) {
    if (!widget.offstage)
      super.visitChildrenForSemantics(visitor);
  }
}

/// A widget that attempts to size the child to a specific aspect ratio.
///
/// The widget first tries the largest width permited by the layout
/// constraints. The height of the widget is determined by applying the
/// given aspect ratio to the width, expressed as a ratio of width to height.
///
/// For example, a 16:9 width:height aspect ratio would have a value of
/// 16.0/9.0. If the maximum width is infinite, the initial width is determined
/// by applying the aspect ratio to the maximum height.
///
/// Now consider a second example, this time with an aspect ratio of 2.0 and
/// layout constraints that require the width to be between 0.0 and 100.0 and
/// the height to be between 0.0 and 100.0. We'll select a width of 100.0 (the
/// biggest allowed) and a height of 50.0 (to match the aspect ratio).
///
/// In that same situation, if the aspect ratio is 0.5, we'll also select a
/// width of 100.0 (still the biggest allowed) and we'll attempt to use a height
/// of 200.0. Unfortunately, that violates the constraints because the child can
/// be at most 100.0 pixels tall. The widget will then take that value
/// and apply the aspect ratio again to obtain a width of 50.0. That width is
/// permitted by the constraints and the child receives a width of 50.0 and a
/// height of 100.0. If the width were not permitted, the widget would
/// continue iterating through the constraints. If the widget does not
/// find a feasible size after consulting each constraint, the widget
/// will eventually select a size for the child that meets the layout
/// constraints but fails to meet the aspect ratio constraints.
class AspectRatio extends SingleChildRenderObjectWidget {
  /// Creates a widget with a specific aspect ratio.
  ///
  /// The [aspectRatio] argument must not be null.
  AspectRatio({
    Key key,
    @required this.aspectRatio,
    Widget child
  }) : super(key: key, child: child) {
    assert(aspectRatio != null);
  }

  /// The aspect ratio to attempt to use.
  ///
  /// The aspect ratio is expressed as a ratio of width to height. For example,
  /// a 16:9 width:height aspect ratio would have a value of 16.0/9.0.
  final double aspectRatio;

  @override
  RenderAspectRatio createRenderObject(BuildContext context) => new RenderAspectRatio(aspectRatio: aspectRatio);

  @override
  void updateRenderObject(BuildContext context, RenderAspectRatio renderObject) {
    renderObject.aspectRatio = aspectRatio;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('aspectRatio: $aspectRatio');
  }
}

/// A widget that sizes its child to the child's intrinsic width.
///
/// Sizes its child's width to the child's maximum intrinsic width. If
/// [stepWidth] is non-null, the child's width will be snapped to a multiple of
/// the [stepWidth]. Similarly, if [stepHeight] is non-null, the child's height
/// will be snapped to a multiple of the [stepHeight].
///
/// This class is useful, for example, when unlimited width is available and
/// you would like a child that would otherwise attempt to expand infinitely to
/// instead size itself to a more reasonable width.
///
/// This class is relatively expensive. Avoid using it where possible.
class IntrinsicWidth extends SingleChildRenderObjectWidget {
  /// Creates a widget that sizes its child to the child's intrinsic width.
  ///
  /// This class is relatively expensive. Avoid using it where possible.
  IntrinsicWidth({ Key key, this.stepWidth, this.stepHeight, Widget child })
    : super(key: key, child: child);

  /// If non-null, force the child's width to be a multiple of this value.
  final double stepWidth;

  /// If non-null, force the child's height to be a multiple of this value.
  final double stepHeight;

  @override
  RenderIntrinsicWidth createRenderObject(BuildContext context) => new RenderIntrinsicWidth(stepWidth: stepWidth, stepHeight: stepHeight);

  @override
  void updateRenderObject(BuildContext context, RenderIntrinsicWidth renderObject) {
    renderObject
      ..stepWidth = stepWidth
      ..stepHeight = stepHeight;
  }
}

/// A widget that sizes its child to the child's intrinsic height.
///
/// This class is useful, for example, when unlimited height is available and
/// you would like a child that would otherwise attempt to expand infinitely to
/// instead size itself to a more reasonable height.
///
/// This class is relatively expensive. Avoid using it where possible.
class IntrinsicHeight extends SingleChildRenderObjectWidget {
  /// Creates a widget that sizes its child to the child's intrinsic height.
  ///
  /// This class is relatively expensive. Avoid using it where possible.
  IntrinsicHeight({ Key key, Widget child }) : super(key: key, child: child);

  @override
  RenderIntrinsicHeight createRenderObject(BuildContext context) => new RenderIntrinsicHeight();
}

/// A widget that positions its child according to the child's baseline.
///
/// This widget shifts the child down such that the child's baseline (or the
/// bottom of the child, if the child has no baseline) is [baseline]
/// logical pixels below the top of this box, then sizes this box to
/// contain the child. If [baseline] is less than the distance from
/// the top of the child to the baseline of the child, then the child
/// is top-aligned instead.
class Baseline extends SingleChildRenderObjectWidget {
  /// Creates a widget that positions its child according to the child's baseline.
  ///
  /// The [baseline] and [baselineType] arguments must not be null.
  Baseline({
    Key key,
    @required this.baseline,
    @required this.baselineType,
    Widget child
  }) : super(key: key, child: child) {
    assert(baseline != null);
    assert(baselineType != null);
  }

  /// The number of logical pixels from the top of this box at which to position
  /// the child's baseline.
  final double baseline;

  /// The type of baseline to use for positioning the child.
  final TextBaseline baselineType;

  @override
  RenderBaseline createRenderObject(BuildContext context) => new RenderBaseline(baseline: baseline, baselineType: baselineType);

  @override
  void updateRenderObject(BuildContext context, RenderBaseline renderObject) {
    renderObject
      ..baseline = baseline
      ..baselineType = baselineType;
  }
}

/// A widget that's bigger on the inside.
///
/// The child of a viewport can layout to a larger size along the viewport's
/// [mainAxis] than the viewport itself. If that happens, only a portion of the
/// child will be visible through the viewport. The portion of the child that is
/// visible is controlled by the scroll offset.
///
/// Viewport is the core scrolling primitive in the system, but it can be used
/// in other situations.
class Viewport extends SingleChildRenderObjectWidget {
  /// Creates a widget that's bigger on the inside.
  ///
  /// The [mainAxis] and [paintOffset] arguments must not be null.
  Viewport({
    Key key,
    this.paintOffset: Offset.zero,
    this.mainAxis: Axis.vertical,
    this.anchor: ViewportAnchor.start,
    this.onPaintOffsetUpdateNeeded,
    Widget child
  }) : super(key: key, child: child) {
    assert(mainAxis != null);
    assert(paintOffset != null);
  }

  /// The offset at which to paint the child.
  ///
  /// The offset can be non-zero only in the [mainAxis].
  final Offset paintOffset;

  /// The direction in which the child is permitted to be larger than the viewport.
  ///
  /// The child is given layout constraints that are fully unconstrained along
  /// the main axis (e.g., the child can be as tall as it wants if the main axis
  /// is vertical).
  final Axis mainAxis;

  /// The end of the viewport from which the paint offset is computed.
  ///
  /// See [ViewportAnchor] for more detail.
  final ViewportAnchor anchor;

  /// Called when the interior or exterior dimensions of the viewport change.
  final ViewportDimensionsChangeCallback onPaintOffsetUpdateNeeded;

  @override
  RenderViewport createRenderObject(BuildContext context) {
    return new RenderViewport(
      paintOffset: paintOffset,
      mainAxis: mainAxis,
      anchor: anchor,
      onPaintOffsetUpdateNeeded: onPaintOffsetUpdateNeeded
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderViewport renderObject) {
    // Order dependency: RenderViewport validates scrollOffset based on mainAxis.
    renderObject
      ..mainAxis = mainAxis
      ..anchor = anchor
      ..paintOffset = paintOffset
      ..onPaintOffsetUpdateNeeded = onPaintOffsetUpdateNeeded;
  }
}


// LAYOUT NODES

/// A widget that uses the block layout algorithm for its children.
///
/// This widget is rarely used directly. Instead, consider using [Block], which
/// combines the block layout algorithm with scrolling behavior.
///
/// For details about the block layout algorithm, see [RenderBlockBase].
///
/// See also:
///
///  * [Block], which combines block layout with scrolling.
class BlockBody extends MultiChildRenderObjectWidget {
  /// Creates a block layout widget.
  ///
  /// By default, the [mainAxis] is [Axis.vertical].
  BlockBody({
    Key key,
    this.mainAxis: Axis.vertical,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(mainAxis != null);
  }

  /// The direction to use as the main axis.
  final Axis mainAxis;

  @override
  RenderBlock createRenderObject(BuildContext context) => new RenderBlock(mainAxis: mainAxis);

  @override
  void updateRenderObject(BuildContext context, RenderBlock renderObject) {
    renderObject.mainAxis = mainAxis;
  }
}

/// A widget that uses the stack layout algorithm for its children.
///
/// This class is useful if you want to overlap several children in a simple
/// way, for example having some text and an image, overlaid with a gradient and
/// a button attached to the bottom.
///
/// Each child of a [Stack] widget is either _positioned_ or _non-positioned_.
/// Positioned children are those wrapped in a [Positioned] widget that has at
/// least one non-null property. The stack sizes itself to contain all the
/// non-positioned children, which are positioned according to [alignment]
/// (which defaults to the top-left corner). The positioned children are then
/// placed relative to the stack according to their top, right, bottom, and left
/// properties.
///
/// For more details about the stack layout algorithm, see [RenderStack].
///
/// If you want to lay a number of children out in a particular pattern, or if
/// you want to make a custom layout manager, you probably want to use
/// [CustomMultiChildLayout] instead. In particular, when using a [Stack] you
/// can't position children relative to their size or the stack's own size.
///
/// See also:
///
///  * [Align], which sizes itself based on its child's size and positions
///    the child according to a [FractionalOffset] value.
///  * [CustomSingleChildLayout], which uses a delegate to control the layout of
///    a single child.
///  * [CustomMultiChildLayout], which uses a delegate to position multiple
///    children.
///  * [Flow], which provides paint-time control of its children using transform
///    matrices.
class Stack extends MultiChildRenderObjectWidget {
  /// Creates a stack layout widget.
  ///
  /// By default, the non-positioned children of the stack are aligned by their
  /// top left corners.
  Stack({
    Key key,
    this.alignment: FractionalOffset.topLeft,
    this.overflow: Overflow.clip,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children);

  /// How to align the non-positioned children in the stack.
  ///
  /// The non-positioned children are placed relative to each other such that
  /// the points determined by [alignment] are co-located. For example, if the
  /// [alignment] is [FractionalOffset.topLeft], then the top left corner of
  /// each non-positioned child will be located at the same global coordinate.
  final FractionalOffset alignment;

  /// Whether overflowing children should be clipped. See [Overflow].
  ///
  /// Some children in a stack might overflow its box. When this flag is set to
  /// [Overflow.clipped], children cannot paint outside of the stack's box.
  final Overflow overflow;

  @override
  RenderStack createRenderObject(BuildContext context) {
    return new RenderStack(
      alignment: alignment,
      overflow: overflow
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderStack renderObject) {
    renderObject
      ..alignment = alignment
      ..overflow = overflow;
  }
}

/// A [Stack] that shows a single child from a list of children.
///
/// The displayed child is the one with the given [index]. The stack is
/// always as big as the largest child.
///
/// If value is null, then nothing is displayed.
///
/// For more details, see [Stack].
class IndexedStack extends Stack {
  /// Creates a [Stack] widget that paints a single child.
  ///
  /// The [index] argument must not be null.
  IndexedStack({
    Key key,
    FractionalOffset alignment: FractionalOffset.topLeft,
    this.index: 0,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, alignment: alignment, children: children);

  /// The index of the child to show.
  final int index;

  @override
  RenderIndexedStack createRenderObject(BuildContext context) => new RenderIndexedStack(index: index, alignment: alignment);

  @override
  void updateRenderObject(BuildContext context, RenderIndexedStack renderObject) {
    renderObject
      ..index = index
      ..alignment = alignment;
  }
}

/// A widget that controls where a child of a [Stack] is positioned.
///
/// A [Positioned] widget must be a descendant of a [Stack], and the path from
/// the [Positioned] widget to its enclosing [Stack] must contain only
/// [StatelessWidget]s or [StatefulWidget]s (not other kinds of widgets, like
/// [RenderObjectWidget]s).
///
/// If a widget is wrapped in a [Positioned], then it is a _positioned_ widget
/// in its [Stack]. If the [top] property is non-null, the top edge of this child
/// will be positioned [top] layout units from the top of the stack widget. The
/// [right], [bottom], and [left] properties work analogously.
///
/// If both the [top] and [bottom] properties are non-null, then the child will
/// be forced to have exactly the height required to satisfy both constraints.
/// Similarly, setting the [right] and [left] properties to non-null values will
/// force the child to have a particular width. Alternatively the [width] and
/// [height] properties can be used to give the dimensions, with one
/// corresponding position property (e.g. [top] and [height]).
class Positioned extends ParentDataWidget<Stack> {
  /// Creates a widget that controls where a child of a [Stack] is positioned.
  ///
  /// Only two out of the three horizontal values ([left], [right],
  /// [width]), and only two out of the three vertical values ([top],
  /// [bottom], [height]), can be set. In each case, at least one of
  /// the three must be null.
  Positioned({
    Key key,
    this.left,
    this.top,
    this.right,
    this.bottom,
    this.width,
    this.height,
    @required Widget child
  }) : super(key: key, child: child) {
    assert(left == null || right == null || width == null);
    assert(top == null || bottom == null || height == null);
  }

  /// Creates a Positioned object with the values from the given [Rect].
  ///
  /// This sets the [left], [top], [width], and [height] properties
  /// from the given [Rect]. The [right] and [bottom] properties are
  /// set to null.
  Positioned.fromRect({
    Key key,
    Widget child,
    Rect rect
  }) : left = rect.left,
       top = rect.top,
       width = rect.width,
       height = rect.height,
       right = null,
       bottom = null,
       super(key: key, child: child);

  /// Creates a Positioned object with [left], [top], [right], and [bottom] set
  /// to 0.0 unless a value for them is passed.
  Positioned.fill({
    Key key,
    Widget child,
    this.left: 0.0,
    this.top: 0.0,
    this.right: 0.0,
    this.bottom: 0.0
  }) : width = null,
       height = null,
       super(key: key, child: child);

  /// The distance that the child's left edge is inset from the left of the stack.
  ///
  /// Only two out of the three horizontal values ([left], [right], [width]) can be
  /// set. The third must be null.
  final double left;

  /// The distance that the child's top edge is inset from the top of the stack.
  ///
  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
  /// set. The third must be null.
  final double top;

  /// The distance that the child's right edge is inset from the right of the stack.
  ///
  /// Only two out of the three horizontal values ([left], [right], [width]) can be
  /// set. The third must be null.
  final double right;

  /// The distance that the child's bottom edge is inset from the bottom of the stack.
  ///
  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
  /// set. The third must be null.
  final double bottom;

  /// The child's width.
  ///
  /// Only two out of the three horizontal values ([left], [right], [width]) can be
  /// set. The third must be null.
  final double width;

  /// The child's height.
  ///
  /// Only two out of the three vertical values ([top], [bottom], [height]) can be
  /// set. The third must be null.
  final double height;

  @override
  void applyParentData(RenderObject renderObject) {
    assert(renderObject.parentData is StackParentData);
    final StackParentData parentData = renderObject.parentData;
    bool needsLayout = false;

    if (parentData.left != left) {
      parentData.left = left;
      needsLayout = true;
    }

    if (parentData.top != top) {
      parentData.top = top;
      needsLayout = true;
    }

    if (parentData.right != right) {
      parentData.right = right;
      needsLayout = true;
    }

    if (parentData.bottom != bottom) {
      parentData.bottom = bottom;
      needsLayout = true;
    }

    if (parentData.width != width) {
      parentData.width = width;
      needsLayout = true;
    }

    if (parentData.height != height) {
      parentData.height = height;
      needsLayout = true;
    }

    if (needsLayout) {
      AbstractNode targetParent = renderObject.parent;
      if (targetParent is RenderObject)
        targetParent.markNeedsLayout();
    }
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    if (left != null)
      description.add('left: $left');
    if (top != null)
      description.add('top: $top');
    if (right != null)
      description.add('right: $right');
    if (bottom != null)
      description.add('bottom: $bottom');
    if (width != null)
      description.add('width: $width');
    if (height != null)
      description.add('height: $height');
  }
}

/// A [MultiChildRenderObjectWidget] for use with [RenderGrid].
///
/// Typically not used directly. Instead, consider using one of its subclasses,
/// such as:
///
///  * [FixedColumnCountGrid] (which creates a grid with a fixed number of columns)
///  * [MaxTileWidthGrid] (which creates a grid whose tiles have a maximum width)
///  * [CustomGrid] (which lets you supply your own [GridDelegate])
abstract class GridRenderObjectWidget extends MultiChildRenderObjectWidget {
  /// Initializes fields for subclasses.
  GridRenderObjectWidget({
    Key key,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    _delegate = createDelegate();
  }

  GridDelegate _delegate;

  /// The delegate that controls the layout of the children.
  GridDelegate createDelegate();

  @override
  RenderGrid createRenderObject(BuildContext context) => new RenderGrid(delegate: _delegate);

  @override
  void updateRenderObject(BuildContext context, RenderGrid renderObject) {
    renderObject.delegate = _delegate;
  }
}

/// A widget that creates a grid using a custom [delegate].
///
/// For details about the grid layout algorithm, see [RenderGrid].
///
/// To pass data to your delegate's [GridDelegate.getChildPlacement] function,
/// consider using [GridPlacementData].
class CustomGrid extends GridRenderObjectWidget {
  /// Creates a grid with a custom [delegate].
  ///
  /// The [delegate] argument must not be null.
  CustomGrid({
    Key key,
    @required this.delegate,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(delegate != null);
  }

  /// The delegate that controls the layout of the children.
  ///
  /// For example, a [FixedColumnCountGridDelegate] for grids that have a fixed
  /// number of columns or a [MaxTileWidthGridDelegate] for grids that have a
  /// maximum tile width.
  final GridDelegate delegate;

  @override
  GridDelegate createDelegate() => delegate;
}

/// A widget that uses a grid layout with a fixed column count.
///
/// For details about the grid layout algorithm, see [FixedColumnCountGridDelegate].
class FixedColumnCountGrid extends GridRenderObjectWidget {
  /// Creates a grid with a fixed number of columns.
  ///
  /// The [columnCount] argument must not be negative.
  FixedColumnCountGrid({
    Key key,
    @required this.columnCount,
    this.columnSpacing: 0.0,
    this.rowSpacing: 0.0,
    this.tileAspectRatio: 1.0,
    this.padding: EdgeInsets.zero,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(columnCount != null && columnCount >= 0);
    assert(tileAspectRatio != null && tileAspectRatio > 0.0);
  }

  /// The number of columns in the grid.
  final int columnCount;

  /// The horizontal distance between columns.
  final double columnSpacing;

  /// The vertical distance between rows.
  final double rowSpacing;

  /// The ratio of the width to the height of each tile in the grid.
  final double tileAspectRatio;

  /// The amount of padding to apply to each child.
  final EdgeInsets padding;

  @override
  FixedColumnCountGridDelegate createDelegate() {
    return new FixedColumnCountGridDelegate(
      columnCount: columnCount,
      columnSpacing: columnSpacing,
      rowSpacing: rowSpacing,
      tileAspectRatio: tileAspectRatio,
      padding: padding
    );
  }
}

/// A widget that uses a grid layout with a maximum tile width.
///
/// For details about the grid layout algorithm, see [MaxTileWidthGridDelegate].
class MaxTileWidthGrid extends GridRenderObjectWidget {
  /// Creates a grid with a maximum tile width.
  ///
  /// The [columnCount] argument must not be negative.
  MaxTileWidthGrid({
    Key key,
    @required this.maxTileWidth,
    this.columnSpacing: 0.0,
    this.rowSpacing: 0.0,
    this.tileAspectRatio: 1.0,
    this.padding: EdgeInsets.zero,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(maxTileWidth != null && maxTileWidth >= 0.0);
    assert(tileAspectRatio != null && tileAspectRatio > 0.0);
  }

  /// The maximum width of a tile in the grid.
  final double maxTileWidth;

  /// The ratio of the width to the height of each tile in the grid.
  final double tileAspectRatio;

  /// The horizontal distance between columns.
  final double columnSpacing;

  /// The vertical distance between rows.
  final double rowSpacing;

  /// The amount of padding to apply to each child.
  final EdgeInsets padding;

  @override
  MaxTileWidthGridDelegate createDelegate() {
    return new MaxTileWidthGridDelegate(
      maxTileWidth: maxTileWidth,
      tileAspectRatio: tileAspectRatio,
      columnSpacing: columnSpacing,
      rowSpacing: rowSpacing,
      padding: padding
    );
  }
}

/// A widget that controls the placement of a child in a grid.
///
/// The [placementData] this widget associates with the child is interpreted by
/// the grid's [GridDelegate] in its [GridDelegate.getChildPlacement] function.
/// During layout, the grid calls the delegate's
/// [GridDelegate.getChildPlacement] function for each child, passing the
/// [placementData] associated with that child as context. The return value of
/// [GridDelegate.getChildPlacement] is then used to determine the size and
/// position of that child within the grid.
///
/// A [GridPlacementData] widget must be a descendant of a
/// [GridRenderObjectWidget], and the path from the [GridPlacementData] widget
/// to its enclosing [GridRenderObjectWidget] must contain only
/// [StatelessWidget]s or [StatefulWidget]s (not other kinds of widgets, like
/// [RenderObjectWidget]s).
class GridPlacementData<DataType> extends ParentDataWidget<GridRenderObjectWidget> {
  /// Creates a widget that controls placement of a child in a grid.
  ///
  /// The [placementData] and [child] arguments are required.
  GridPlacementData({
    Key key,
    @required this.placementData,
    @required Widget child
  }) : super(key: key, child: child);

  /// Opaque data passed to the getChildPlacement method of the grid's [GridDelegate].
  final DataType placementData;

  @override
  void applyParentData(RenderObject renderObject) {
    assert(renderObject.parentData is GridParentData);
    final GridParentData parentData = renderObject.parentData;
    if (parentData.placementData != placementData) {
      parentData.placementData = placementData;
      AbstractNode targetParent = renderObject.parent;
      if (targetParent is RenderObject)
        targetParent.markNeedsLayout();
    }
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('placementData: $placementData');
  }
}

/// A widget that displays its children in a one-dimensional array.
///
/// The [Flex] widget allows you to control the axis along which the children are
/// placed (horizontal or vertical). This is referred to as the _main axis_. If
/// you know the main axis in advance, then consider using a [Row] (if it's
/// horizontal) or [Column] (if it's vertical) instead, because that will be less
/// verbose.
///
/// To cause a child to expand to fill the available vertical space, wrap the
/// child in an [Expanded] widget.
///
/// The [Flex] widget does not scroll (and in general it is considered an error
/// to have more children in a [Flex] than will fit in the available room). If
/// you have some widgets and want them to be able to scroll if there is
/// insufficient room, consider using a [Block].
///
/// If you only have one child, then rather than using [Flex], [Row], or
/// [Column], consider using [Align] or [Center] to position the child.
///
/// ## Layout algorithm
///
/// Layout for a [Flex] proceeds in six steps:
///
/// 1. Layout each child a null or zero flex factor (e.g., those that are not
///    [Expanded]) with unbounded main axis constraints and the incoming
///    cross axis constraints. If the [crossAxisAlignment] is
///    [CrossAxisAlignment.stretch], instead use tight cross axis constraints
///    that match the incoming max extent in the cross axis.
/// 2. Divide the remaining main axis space among the children with non-zero
///    flex factors (e.g., those that are [Expanded]) according to their flex
///    factor. For example, a child with a flex factor of 2.0 will receive twice
///    the amount of main axis space as a child with a flex factor of 1.0.
/// 3. Layout each of the remaining children with the same cross axis
///    constraints as in step 1, but instead of using unbounded main axis
///    constraints, use max axis constraints based on the amount of space
///    allocated in step 2. Children with [Flexible.fit] properties that are
///    [FlexFit.tight] are given tight constraints (i.e., forced to fill the
///    allocated space), and children with [Flexible.fit] properties that are
///    [FlexFit.loose] are given loose constraints (i.e., not forced to fill the
///    allocated space).
/// 4. The cross axis extent of the [Flex] is the maximum cross axis extent of
///    the children (which will always satisfy the incoming constraints).
/// 5. The main axis extent of the [Flex] is determined by the [mainAxisSize]
///    property. If the [mainAxisSize] property is [MainAxisSize.max], then the
///    main axis extent of the [Flex] is the max extent of the incoming main
///    axis constraints. If the [mainAxisSize] property is [MainAxisSize.min],
///    then the main axis extent of the [Flex] is the sum of the main axis
///    extents of the children (subject to the incoming constraints).
/// 6. Determine the position for each child according to the
///    [mainAxisAlignment] and the [crossAxisAlignment]. For example, if the
///    [mainAxisAlignment] is [MainAxisAlignment.spaceBetween], any main axis
///    space that has not been allocated to children is divided evenly and
///    placed between the children.
class Flex extends MultiChildRenderObjectWidget {
  /// Creates a flex layout.
  ///
  /// The [direction] is required.
  ///
  /// The [direction], [mainAxisAlignment], and [crossAxisAlignment] arguments
  /// must not be null. If [crossAxisAlignment] is
  /// [CrossAxisAlignment.baseline], then [textBaseline] must not be null.
  Flex({
    Key key,
    @required this.direction,
    this.mainAxisAlignment: MainAxisAlignment.start,
    this.mainAxisSize: MainAxisSize.max,
    this.crossAxisAlignment: CrossAxisAlignment.center,
    this.textBaseline,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(direction != null);
    assert(mainAxisAlignment != null);
    assert(mainAxisSize != null);
    assert(crossAxisAlignment != null);
    assert(crossAxisAlignment != CrossAxisAlignment.baseline || textBaseline != null);
  }

  /// The direction to use as the main axis.
  ///
  /// If you know the axis in advance, then consider using a [Row] (if it's
  /// horizontal) or [Column] (if it's vertical) instead of a [Flex], since that
  /// will be less verbose. (For [Row] and [Column] this property is fixed to
  /// the appropriate axis.)
  final Axis direction;

  /// How the children should be placed along the main axis.
  ///
  /// For example, [MainAxisAlignment.start], the default, places the children
  /// at the start (i.e., the left for a [Row] or the top for a [Column]) of the
  /// main axis.
  final MainAxisAlignment mainAxisAlignment;

  /// How much space space should be occupied in the main axis.
  ///
  /// After allocating space to children, there might be some remaining free
  /// space. This value controls whether to maximize or minimize the amount of
  /// free space, subject to the incoming layout constraints.
  ///
  /// If some children have a non-zero flex factors (and none have a fit of
  /// [FlexFit.loose]), they will expand to consume all the available space and
  /// there will be no remaining free space to maximize or minimize, making this
  /// value irrelevant to the final layout.
  final MainAxisSize mainAxisSize;

  /// How the children should be placed along the cross axis.
  ///
  /// For example, [CrossAxisAlignment.center], the default, centers the
  /// children in the cross axis (e.g., horizontally for a [Column]).
  final CrossAxisAlignment crossAxisAlignment;

  /// If aligning items according to their baseline, which baseline to use.
  final TextBaseline textBaseline;

  @override
  RenderFlex createRenderObject(BuildContext context) {
    return new RenderFlex(
      direction: direction,
      mainAxisAlignment: mainAxisAlignment,
      mainAxisSize: mainAxisSize,
      crossAxisAlignment: crossAxisAlignment,
      textBaseline: textBaseline
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderFlex renderObject) {
    renderObject
      ..direction = direction
      ..mainAxisAlignment = mainAxisAlignment
      ..mainAxisSize = mainAxisSize
      ..crossAxisAlignment = crossAxisAlignment
      ..textBaseline = textBaseline;
  }
}

/// A widget that displays its children in a horizontal array.
///
/// To cause a child to expand to fill the available horizontal space, wrap the
/// child in an [Expanded] widget.
///
/// The [Row] widget does not scroll (and in general it is considered an error
/// to have more children in a [Row] than will fit in the available room). If
/// you have a line of widgets and want them to be able to scroll if there is
/// insufficient room, consider using a [Block].
///
/// For a vertical variant, see [Column].
///
/// If you only have one child, then consider using [Align] or [Center] to
/// position the child.
///
/// ## Layout algorithm
///
/// Layout for a [Row] proceeds in six steps:
///
/// 1. Layout each child a null or zero flex factor (e.g., those that are not
///    [Expanded]) with unbounded horizontal constraints and the incoming
///    vertical constraints. If the [crossAxisAlignment] is
///    [CrossAxisAlignment.stretch], instead use tight vertical constraints that
///    match the incoming max height.
/// 2. Divide the remaining horizontal space among the children with non-zero
///    flex factors (e.g., those that are [Expanded]) according to their flex
///    factor. For example, a child with a flex factor of 2.0 will receive twice
///    the amount of horizontal space as a child with a flex factor of 1.0.
/// 3. Layout each of the remaining children with the same vertical constraints
///    as in step 1, but instead of using unbounded horizontal constraints, use
///    horizontal constraints based on the amount of space allocated in step 2.
///    Children with [Flexible.fit] properties that are [FlexFit.tight] are
///    given tight constraints (i.e., forced to fill the allocated space), and
///    children with [Flexible.fit] properties that are [FlexFit.loose] are
///    given loose constraints (i.e., not forced to fill the allocated space).
/// 4. The height of the [Row] is the maximum height of the children (which will
///    always satisfy the incoming vertical constraints).
/// 5. The width of the [Row] is determined by the [mainAxisSize] property. If
///    the [mainAxisSize] property is [MainAxisSize.max], then the width of the
///    [Row] is the max width of the incoming constraints. If the [mainAxisSize]
///    property is [MainAxisSize.min], then the width of the [Row] is the sum
///    of widths of the children (subject to the incoming constraints).
/// 6. Determine the position for each child according to the
///    [mainAxisAlignment] and the [crossAxisAlignment]. For example, if the
///    [mainAxisAlignment] is [MainAxisAlignment.spaceBetween], any horizontal
///    space that has not been allocated to children is divided evenly and
///    placed between the children.
class Row extends Flex {
  /// Creates a horizontal array of children.
  ///
  /// The [direction], [mainAxisAlignment], [mainAxisSize], and
  /// [crossAxisAlignment] arguments must not be null. If [crossAxisAlignment]
  /// is [CrossAxisAlignment.baseline], then [textBaseline] must not be null.
  Row({
    Key key,
    MainAxisAlignment mainAxisAlignment: MainAxisAlignment.start,
    MainAxisSize mainAxisSize: MainAxisSize.max,
    CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center,
    TextBaseline textBaseline,
    List<Widget> children: const <Widget>[],
  }) : super(
    children: children,
    key: key,
    direction: Axis.horizontal,
    mainAxisAlignment: mainAxisAlignment,
    mainAxisSize: mainAxisSize,
    crossAxisAlignment: crossAxisAlignment,
    textBaseline: textBaseline
  );
}

/// A widget that displays its children in a vertical array.
///
/// To cause a child to expand to fill the available vertical space, wrap the
/// child in an [Expanded] widget.
///
/// The [Column] widget does not scroll (and in general it is considered an error
/// to have more children in a [Column] than will fit in the available room). If
/// you have a line of widgets and want them to be able to scroll if there is
/// insufficient room, consider using a [Block].
///
/// For a horizontal variant, see [Row].
///
/// If you only have one child, then consider using [Align] or [Center] to
/// position the child.
///
/// ## Layout algorithm
///
/// Layout for a [Column] proceeds in six steps:
///
/// 1. Layout each child a null or zero flex factor (e.g., those that are not
///    [Expanded]) with unbounded vertical constraints and the incoming
///    horizontal constraints. If the [crossAxisAlignment] is
///    [CrossAxisAlignment.stretch], instead use tight horizontal constraints
///    that match the incoming max width.
/// 2. Divide the remaining vertical space among the children with non-zero
///    flex factors (e.g., those that are [Expanded]) according to their flex
///    factor. For example, a child with a flex factor of 2.0 will receive twice
///    the amount of vertical space as a child with a flex factor of 1.0.
/// 3. Layout each of the remaining children with the same horizontal
///    constraints as in step 1, but instead of using unbounded vertical
///    constraints, use vertical constraints based on the amount of space
///    allocated in step 2. Children with [Flexible.fit] properties that are
///    [FlexFit.tight] are given tight constraints (i.e., forced to fill the
///    allocated space), and children with [Flexible.fit] properties that are
///    [FlexFit.loose] are given loose constraints (i.e., not forced to fill the
///    allocated space).
/// 4. The width of the [Column] is the maximum width of the children (which
///    will always satisfy the incoming horizontal constraints).
/// 5. The height of the [Column] is determined by the [mainAxisSize] property.
///    If the [mainAxisSize] property is [MainAxisSize.max], then the height of
///    the [Column] is the max height of the incoming constraints. If the
///    [mainAxisSize] property is [MainAxisSize.min], then the height of the
///    [Column] is the sum of heights of the children (subject to the incoming
///    constraints).
/// 6. Determine the position for each child according to the
///    [mainAxisAlignment] and the [crossAxisAlignment]. For example, if the
///    [mainAxisAlignment] is [MainAxisAlignment.spaceBetween], any vertical
///    space that has not been allocated to children is divided evenly and
///    placed between the children.
class Column extends Flex {
  /// Creates a vertical array of children.
  ///
  /// The [direction], [mainAxisAlignment], [mainAxisSize], and
  /// [crossAxisAlignment] arguments must not be null. If [crossAxisAlignment]
  /// is [CrossAxisAlignment.baseline], then [textBaseline] must not be null.
  Column({
    Key key,
    MainAxisAlignment mainAxisAlignment: MainAxisAlignment.start,
    MainAxisSize mainAxisSize: MainAxisSize.max,
    CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center,
    TextBaseline textBaseline,
    List<Widget> children: const <Widget>[],
  }) : super(
    children: children,
    key: key,
    direction: Axis.vertical,
    mainAxisAlignment: mainAxisAlignment,
    mainAxisSize: mainAxisSize,
    crossAxisAlignment: crossAxisAlignment,
    textBaseline: textBaseline
  );
}

/// A widget that controls how a child of a [Row], [Column], or [Flex] flexes.
///
/// A [Flexible] widget must be a descendant of a [Row], [Column], or [Flex],
/// and the path from the [Flexible] widget to its enclosing [Row], [Column], or
/// [Flex] must contain only [StatelessWidget]s or [StatefulWidget]s (not other
/// kinds of widgets, like [RenderObjectWidget]s).
class Flexible extends ParentDataWidget<Flex> {
  /// Creates a widget that controls how a child of a [Row], [Column], or [Flex]
  /// flexes.
  Flexible({
    Key key,
    this.flex: 1,
    this.fit: FlexFit.tight,
    @required Widget child,
  }) : super(key: key, child: child);

  /// The flex factor to use for this child
  ///
  /// If null or zero, the child is inflexible and determines its own size. If
  /// non-zero, the amount of space the child's can occupy in the main axis is
  /// determined by dividing the free space (after placing the inflexible
  /// children) according to the flex factors of the flexible children.
  final int flex;

  /// How a flexible child is inscribed into the available space.
  ///
  /// If [flex] is non-zero, the [fit] determines whether the child fills the
  /// space the parent makes available during layout. If the fit is
  /// [FlexFit.tight], the child is required to fill the available space. If the
  /// fit is [FlexFit.loose], the child can be at most as large as the available
  /// space (but is allowed to be smaller).
  final FlexFit fit;

  @override
  void applyParentData(RenderObject renderObject) {
    assert(renderObject.parentData is FlexParentData);
    final FlexParentData parentData = renderObject.parentData;
    bool needsLayout = false;

    if (parentData.flex != flex) {
      parentData.flex = flex;
      needsLayout = true;
    }

    if (parentData.fit != fit) {
      parentData.fit = fit;
      needsLayout = true;
    }

    if (needsLayout) {
      AbstractNode targetParent = renderObject.parent;
      if (targetParent is RenderObject)
        targetParent.markNeedsLayout();
    }
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('flex: $flex');
  }
}

/// A widget that expands a child of a [Row], [Column], or [Flex].
///
/// Using an [Expanded] widget to make a child of a [Row], [Column], or [Flex]
/// expand to fill the available space in the main axis (e.g., horizontally for
/// a [Row] or vertically for a [Column]). If multiple children are expanded,
/// the available space is divided amoung them according to the [flex] factor.
///
/// An [Expanded] widget must be a descendant of a [Row], [Column], or [Flex],
/// and the path from the [Flexible] widget to its enclosing [Row], [Column], or
/// [Flex] must contain only [StatelessWidget]s or [StatefulWidget]s (not other
/// kinds of widgets, like [RenderObjectWidget]s).
class Expanded extends Flexible {
  /// Creates a widget that expands a child of a [Row], [Column], or [Flex]
  /// expand to fill the available space in the main axis.
  Expanded({
    Key key,
    int flex: 1,
    @required Widget child,
  }) : super(key: key, flex: flex, fit: FlexFit.tight, child: child) {
    assert(flex > 0);
  }
}

/// A widget that implements the flow layout algorithm.
///
/// Flow layouts are optimized for repositioning children using transformation
/// matrices.
///
/// The flow container is sized independently from the children by the
/// [FlowDelegate.getSize] function of the delegate. The children are then sized
/// independently given the constraints from the
/// [FlowDelegate.getConstraintsForChild] function.
///
/// Rather than positioning the children during layout, the children are
/// positioned using transformation matrices during the paint phase using the
/// matrices from the [FlowDelegate.paintChildren] function. The children can be
/// repositioned efficiently by simply repainting the flow.
///
/// The most efficient way to trigger a repaint of the flow is to supply a
/// repaint argument to the constructor of the [FlowDelegate]. The flow will
/// listen to this animation and repaint whenever the animation ticks, avoiding
/// both the build and layout phases of the pipeline.
///
/// See also:
///
///  * [FlowDelegate], which controls the visual presentation of the children.
///  * [Stack], which arranges children relative to the edges of the container.
///  * [CustomSingleChildLayout], which uses a delegate to control the layout of
///    a single child.
///  * [CustomMultiChildLayout], which uses a delegate to position multiple
///    children.
class Flow extends MultiChildRenderObjectWidget {
  /// Creates a flow layout.
  ///
  /// Wraps each of the given children in a [RepaintBoundary] to avoid
  /// repainting the children when the flow repaints.
  ///
  /// The [delegate] argument must not be null.
  Flow({
    Key key,
    @required this.delegate,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: RepaintBoundary.wrapAll(children)) {
    assert(delegate != null);
  }

  /// Creates a flow layout.
  ///
  /// Does not wrap the given children in repaint boundaries, unlike the default
  /// constructor. Useful when the child is trivial to paint or already contains
  /// a repaint boundary.
  ///
  /// The [delegate] argument must not be null.
  Flow.unwrapped({
    Key key,
    this.delegate,
    List<Widget> children: const <Widget>[],
  }) : super(key: key, children: children) {
    assert(delegate != null);
  }

  /// The delegate that controls the transformation matrices of the children.
  final FlowDelegate delegate;

  @override
  RenderFlow createRenderObject(BuildContext context) => new RenderFlow(delegate: delegate);

  @override
  void updateRenderObject(BuildContext context, RenderFlow renderObject) {
    renderObject
      ..delegate = delegate;
  }
}

/// A paragraph of rich text.
///
/// The [RichText] widget displays text that uses multiple different styles. The
/// text to display is described using a tree of [TextSpan] objects, each of
/// which has an associated style that is used for that subtree. The text might
/// break across multiple lines or might all be displayed on the same line
/// depending on the layout constraints.
///
/// Text displayed in a [RichText] widget must be explicitly styled. When
/// picking which style to use, consider using [DefaultTextStyle.of] the current
/// [BuildContext] to provide defaults.
///
/// When all the text uses the same style, consider using the [Text] widget,
/// which is less verbose and integrates with [DefaultTextStyle] for default
/// styling.
///
/// Example:
///
/// ```dart
/// new RichText(
///   text: new TextSpan(
///     text: 'Hello ',
///     style: DefaultTextStyle.of(context).style,
///     children: <TextSpan>[
///       new TextSpan(text: 'bold', style: new TextStyle(fontWeight: FontWeight.bold)),
///       new TextSpan(text: ' world!'),
///     ],
///   ),
/// ),
/// ```
///
/// See also:
///
///  * [Text]
///  * [TextSpan]
///  * [DefaultTextStyle]
class RichText extends LeafRenderObjectWidget {
  /// Creates a paragraph of rich text.
  ///
  /// The [text], [softWrap], and [overflow] arguments must not be null.
  RichText({
    Key key,
    @required this.text,
    this.textAlign,
    this.softWrap: true,
    this.overflow: TextOverflow.clip,
    this.textScaleFactor: 1.0
  }) : super(key: key) {
    assert(text != null);
    assert(softWrap != null);
    assert(overflow != null);
    assert(textScaleFactor != null);
  }

  /// The text to display in this widget.
  final TextSpan text;

  /// How the text should be aligned horizontally.
  final TextAlign textAlign;

  /// Whether the text should break at soft line breaks.
  ///
  /// If false, the glyphs in the text will be positioned as if there was unlimited horizontal space.
  final bool softWrap;

  /// How visual overflow should be handled.
  final TextOverflow overflow;

  /// The number of font pixels for each logical pixel.
  ///
  /// For example, if the text scale factor is 1.5, text will be 50% larger than
  /// the specified font size.
  final double textScaleFactor;

  @override
  RenderParagraph createRenderObject(BuildContext context) {
    return new RenderParagraph(text,
      textAlign: textAlign,
      softWrap: softWrap,
      overflow: overflow,
      textScaleFactor: textScaleFactor
    );
  }

  @override
  void updateRenderObject(BuildContext context, RenderParagraph renderObject) {
    renderObject
      ..text = text
      ..textAlign = textAlign
      ..softWrap = softWrap
      ..overflow = overflow
      ..textScaleFactor = textScaleFactor;
  }
}

/// A widget that displays a [ui.Image] directly.
///
/// The image is painted using [paintImage], which describes the meanings of the
/// various fields on this class in more detail.
///
/// This widget is rarely used directly. Instead, consider using [Image].
class RawImage extends LeafRenderObjectWidget {
  /// Creates a widget that displays an image.
  ///
  /// The [scale] and [repeat] arguments must not be null.
  RawImage({
    Key key,
    this.image,
    this.width,
    this.height,
    this.scale: 1.0,
    this.color,
    this.fit,
    this.alignment,
    this.repeat: ImageRepeat.noRepeat,
    this.centerSlice
  }) : super(key: key) {
    assert(scale != null);
    assert(repeat != null);
  }

  /// The image to display.
  final ui.Image image;

  /// If non-null, require the image to have this width.
  ///
  /// If null, the image will pick a size that best preserves its intrinsic
  /// aspect ratio.
  final double width;

  /// If non-null, require the image to have this height.
  ///
  /// If null, the image will pick a size that best preserves its intrinsic
  /// aspect ratio.
  final double height;

  /// Specifies the image's scale.
  ///
  /// Used when determining the best display size for the image.
  final double scale;

  /// If non-null, apply this color filter to the image before painting.
  final Color color;

  /// How to inscribe the image into the space allocated during layout.
  ///
  /// The default varies based on the other fields. See the discussion at
  /// [paintImage].
  final ImageFit fit;

  /// How to align the image within its bounds.
  ///
  /// An alignment of (0.0, 0.0) aligns the image to the top-left corner of its
  /// layout bounds.  An alignment of (1.0, 0.5) aligns the image to the middle
  /// of the right edge of its layout bounds.
  final FractionalOffset alignment;

  /// How to paint any portions of the layout bounds not covered by the image.
  final ImageRepeat repeat;

  /// The center slice for a nine-patch image.
  ///
  /// The region of the image inside the center slice will be stretched both
  /// horizontally and vertically to fit the image into its destination. The
  /// region of the image above and below the center slice will be stretched
  /// only horizontally and the region of the image to the left and right of
  /// the center slice will be stretched only vertically.
  final Rect centerSlice;

  @override
  RenderImage createRenderObject(BuildContext context) => new RenderImage(
    image: image,
    width: width,
    height: height,
    scale: scale,
    color: color,
    fit: fit,
    alignment: alignment,
    repeat: repeat,
    centerSlice: centerSlice
  );

  @override
  void updateRenderObject(BuildContext context, RenderImage renderObject) {
    renderObject
      ..image = image
      ..width = width
      ..height = height
      ..scale = scale
      ..color = color
      ..alignment = alignment
      ..fit = fit
      ..repeat = repeat
      ..centerSlice = centerSlice;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('image: $image');
    if (width != null)
      description.add('width: $width');
    if (height != null)
      description.add('height: $height');
    if (scale != 1.0)
      description.add('scale: $scale');
    if (color != null)
      description.add('color: $color');
    if (fit != null)
      description.add('fit: $fit');
    if (alignment != null)
      description.add('alignment: $alignment');
    if (repeat != ImageRepeat.noRepeat)
      description.add('repeat: $repeat');
    if (centerSlice != null)
      description.add('centerSlice: $centerSlice');
  }
}

/// A widget that determines the default asset bundle for its descendants.
///
/// For example, used by [Image] to determine which bundle to use for
/// [AssetImage]s if no bundle is specified explicitly.
class DefaultAssetBundle extends InheritedWidget {
  /// Creates a widget that determines the default asset bundle for its descendants.
  ///
  /// The [bundle] and [child] arguments must not be null.
  DefaultAssetBundle({
    Key key,
    @required this.bundle,
    @required Widget child
  }) : super(key: key, child: child) {
    assert(bundle != null);
    assert(child != null);
  }

  /// The bundle to use as a default.
  final AssetBundle bundle;

  /// The bundle from the closest instance of this class that encloses
  /// the given context.
  ///
  /// If there is no [DefaultAssetBundle] ancestor widget in the tree
  /// at the given context, then this will return the [rootBundle].
  ///
  /// Typical usage is as follows:
  ///
  /// ```dart
  /// AssetBundle bundle = DefaultAssetBundle.of(context);
  /// ```
  static AssetBundle of(BuildContext context) {
    DefaultAssetBundle result = context.inheritFromWidgetOfExactType(DefaultAssetBundle);
    return result?.bundle ?? rootBundle;
  }

  @override
  bool updateShouldNotify(DefaultAssetBundle old) => bundle != old.bundle;
}

/// An adapter for placing a specific [RenderBox] in the widget tree.
///
/// A given render object can be placed at most once in the widget tree. This
/// widget enforces that restriction by keying itself using a [GlobalObjectKey]
/// for the given render object.
class WidgetToRenderBoxAdapter extends LeafRenderObjectWidget {
  /// Creates an adapter for placing a specific [RenderBox] in the widget tree.
  ///
  /// The [renderBox] argument must not be null.
  WidgetToRenderBoxAdapter({
    @required RenderBox renderBox,
    this.onBuild
  }) : renderBox = renderBox,
       // WidgetToRenderBoxAdapter objects are keyed to their render box. This
       // prevents the widget being used in the widget hierarchy in two different
       // places, which would cause the RenderBox to get inserted in multiple
       // places in the RenderObject tree.
       super(key: new GlobalObjectKey(renderBox)) {
    assert(renderBox != null);
  }

  /// The render box to place in the widget tree.
  final RenderBox renderBox;

  /// Called when it is safe to update the render box and its descendants. If
  /// you update the RenderObject subtree under this widget outside of
  /// invocations of this callback, features like hit-testing will fail as the
  /// tree will be dirty.
  final VoidCallback onBuild;

  @override
  RenderBox createRenderObject(BuildContext context) => renderBox;

  @override
  void updateRenderObject(BuildContext context, RenderBox renderObject) {
    if (onBuild != null)
      onBuild();
  }
}


// EVENT HANDLING

/// A widget that calls callbacks in response to pointer events.
///
/// Rather than listening for raw pointer events, consider listening for
/// higher-level gestures using [GestureDetector].
///
/// If it has a child, this widget defers to the child for sizing behavior. If
/// it does not have a child, it grows to fit the parent instead.
class Listener extends SingleChildRenderObjectWidget {
  /// Creates a widget that forwards point events to callbacks.
  ///
  /// The [behavior] argument defaults to [HitTestBehavior.deferToChild].
  Listener({
    Key key,
    this.onPointerDown,
    this.onPointerMove,
    this.onPointerUp,
    this.onPointerCancel,
    this.behavior: HitTestBehavior.deferToChild,
    Widget child
  }) : super(key: key, child: child) {
    assert(behavior != null);
  }

  /// Called when a pointer comes into contact with the screen at this object.
  final PointerDownEventListener onPointerDown;

  /// Called when a pointer that triggered an [onPointerDown] changes position.
  final PointerMoveEventListener onPointerMove;

  /// Called when a pointer that triggered an [onPointerDown] is no longer in contact with the screen.
  final PointerUpEventListener onPointerUp;

  /// Called when the input from a pointer that triggered an [onPointerDown] is no longer directed towards this receiver.
  final PointerCancelEventListener onPointerCancel;

  /// How to behave during hit testing.
  final HitTestBehavior behavior;

  @override
  RenderPointerListener createRenderObject(BuildContext context) => new RenderPointerListener(
    onPointerDown: onPointerDown,
    onPointerMove: onPointerMove,
    onPointerUp: onPointerUp,
    onPointerCancel: onPointerCancel,
    behavior: behavior
  );

  @override
  void updateRenderObject(BuildContext context, RenderPointerListener renderObject) {
    renderObject
      ..onPointerDown = onPointerDown
      ..onPointerMove = onPointerMove
      ..onPointerUp = onPointerUp
      ..onPointerCancel = onPointerCancel
      ..behavior = behavior;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    List<String> listeners = <String>[];
    if (onPointerDown != null)
      listeners.add('down');
    if (onPointerMove != null)
      listeners.add('move');
    if (onPointerUp != null)
      listeners.add('up');
    if (onPointerCancel != null)
      listeners.add('cancel');
    if (listeners.isEmpty)
      listeners.add('<none>');
    description.add('listeners: ${listeners.join(", ")}');
    switch (behavior) {
      case HitTestBehavior.translucent:
        description.add('behavior: translucent');
        break;
      case HitTestBehavior.opaque:
        description.add('behavior: opaque');
        break;
      case HitTestBehavior.deferToChild:
        description.add('behavior: defer-to-child');
        break;
    }
  }
}

/// A widget that creates a separate display list for its child.
///
/// This widget creates a separate display list for its child, which
/// can improve performance if the subtree repaints at different times than
/// the surrounding parts of the tree. Specifically, when the child does not
/// repaint but its parent does, we can re-use the display list we recorded
/// previously. Similarly, when the child repaints but the surround tree does
/// not, we can re-record its display list without re-recording the display list
/// for the surround tree.
class RepaintBoundary extends SingleChildRenderObjectWidget {
  /// Creates a widget that isolates repaints.
  RepaintBoundary({ Key key, Widget child }) : super(key: key, child: child);

  /// Wraps the given child in a [RepaintBoundary].
  ///
  /// The key for the [RepaintBoundary] is derived either from the child's key
  /// (if the child has a non-null key) or from the given `childIndex`.
  factory RepaintBoundary.wrap(Widget child, int childIndex) {
    Key key = child.key != null ? new ValueKey<Key>(child.key) : new ValueKey<int>(childIndex);
    return new RepaintBoundary(key: key, child: child);
  }

  /// Wraps each of the given children in [RepaintBoundary]s.
  ///
  /// The key for each [RepaintBoundary] is derived either from the wrapped
  /// child's key (if the wrapped child has a non-null key) or from the wrapped
  /// child's index in the list.
  static List<RepaintBoundary> wrapAll(List<Widget> widgets) {
    List<RepaintBoundary> result = new List<RepaintBoundary>(widgets.length);
    for (int i = 0; i < result.length; ++i)
      result[i] = new RepaintBoundary.wrap(widgets[i], i);
    return result;
  }

  @override
  RenderRepaintBoundary createRenderObject(BuildContext context) => new RenderRepaintBoundary();
}

/// A widget that is invisible during hit testing.
///
/// When [ignoring] is `true`, this widget (and its subtree) is invisible
/// to hit testing. It still consumes space during layout and paints its child
/// as usual. It just cannot be the target of located events, because it returns
/// `false` from [hitTest].
///
/// When [ignoringSemantics] is `true`, the subtree will be invisible to
/// the semantics layer (and thus e.g. accessibility tools). If
/// [ignoringSemantics] is null, it uses the value of [ignoring].
///
/// See also:
///
///  * [AbsorbPointer], which also prevents its children from receiving pointer
///    events but is itself visible to hit testing.
class IgnorePointer extends SingleChildRenderObjectWidget {
  /// Creates a widget that is invisible to hit testing.
  ///
  /// The [ignoring] argument must not be null. If [ignoringSemantics], this
  /// render object will be ignored for semantics if [ignoring] is true.
  IgnorePointer({
    Key key,
    this.ignoring: true,
    this.ignoringSemantics,
    Widget child
  }) : super(key: key, child: child) {
    assert(ignoring != null);
  }

  /// Whether this widget is ignored during hit testing.
  ///
  /// Regardless of whether this widget is ignored during hit testing, it will
  /// still consume space during layout and be visible during painting.
  final bool ignoring;

  /// Whether the semantics of this widget is ignored when compiling the semantics tree.
  ///
  /// If null, defaults to value of [ignoring].
  ///
  /// See [SemanticsNode] for additional information about the semantics tree.
  final bool ignoringSemantics;

  @override
  RenderIgnorePointer createRenderObject(BuildContext context) => new RenderIgnorePointer(
    ignoring: ignoring,
    ignoringSemantics: ignoringSemantics
  );

  @override
  void updateRenderObject(BuildContext context, RenderIgnorePointer renderObject) {
    renderObject
      ..ignoring = ignoring
      ..ignoringSemantics = ignoringSemantics;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('ignoring: $ignoring');
    if (ignoringSemantics != null)
      description.add('ignoringSemantics: $ignoringSemantics');
  }
}

/// A widget that absorbs pointers during hit testing.
///
/// When [absorbing] is `true`, this widget prevents its subtree from receiving
/// pointer events by terminating hit testing at itself. It still consumes space
/// during layout and paints its child as usual. It just prevents its children
/// from being the target of located events, because it returns `true` from
/// [hitTest].
///
/// See also:
///
///  * [IgnorePointer], which also prevents its children from receiving pointer
///    events but is itself invisible to hit testing.
class AbsorbPointer extends SingleChildRenderObjectWidget {
  /// Creates a widget that absorbs pointers during hit testing.
  ///
  /// The [absorbing] argument must not be null
  AbsorbPointer({
    Key key,
    this.absorbing: true,
    Widget child
  }) : super(key: key, child: child) {
    assert(absorbing != null);
  }

  /// Whether this widget absorbs pointers during hit testing.
  ///
  /// Regardless of whether this render object absorbs pointers during hit
  /// testing, it will still consume space during layout and be visible during
  /// painting.
  final bool absorbing;

  @override
  RenderAbsorbPointer createRenderObject(BuildContext context) => new RenderAbsorbPointer(absorbing: absorbing);

  @override
  void updateRenderObject(BuildContext context, RenderAbsorbPointer renderObject) {
    renderObject.absorbing = absorbing;
  }
}

/// Holds opaque meta data in the render tree.
///
/// Useful for decorating the render tree with information that will be consumed
/// later. For example, you could store information in the render tree that will
/// be used when the user interacts with the render tree but has no visual
/// impact prior to the interaction.
class MetaData extends SingleChildRenderObjectWidget {
  /// Creates a widget that hold opaque meta data.
  ///
  /// The [behavior] argument defaults to [HitTestBehavior.deferToChild].
  MetaData({
    Key key,
    this.metaData,
    this.behavior: HitTestBehavior.deferToChild,
    Widget child
  }) : super(key: key, child: child);

  /// Opaque meta data ignored by the render tree
  final dynamic metaData;

  /// How to behave during hit testing.
  final HitTestBehavior behavior;

  @override
  RenderMetaData createRenderObject(BuildContext context) => new RenderMetaData(
    metaData: metaData,
    behavior: behavior
  );

  @override
  void updateRenderObject(BuildContext context, RenderMetaData renderObject) {
    renderObject
      ..metaData = metaData
      ..behavior = behavior;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('behavior: $behavior');
    description.add('metaData: $metaData');
  }
}


// UTILITY NODES

/// A widget that annotates the widget tree with a description of the meaning of
/// the widgets.
///
/// Used by accessibility tools, search engines, and other semantic analysis
/// software to determine the meaning of the application.
///
/// See also:
///
/// * [MergeSemantics], which marks a subtree as being a single node for
///   accessibility purposes.
/// * [ExcludeSemantics], which excludes a subtree from the semantics tree
///   (which might be useful if it is, e.g., totally decorative and not
///   important to the user).
/// * [RenderObject.semanticsAnnotator], the rendering library API through which
///   the [Semantics] widget is actually implemented.
/// * [SemanticsNode], the object used by the rendering library to represent
///   semantics in the semantics tree.
/// * [SemanticsDebugger], an overlay to help visualize the semantics tree. Can
///   be enabled using [WidgetsApp.showSemanticsDebugger] or
///   [MaterialApp.showSemanticsDebugger].
class Semantics extends SingleChildRenderObjectWidget {
  /// Creates a semantic annotation.
  ///
  /// The [container] argument must not be null.
  Semantics({
    Key key,
    Widget child,
    this.container: false,
    this.checked,
    this.label
  }) : super(key: key, child: child) {
    assert(container != null);
  }

  /// If 'container' is true, this Widget will introduce a new node in
  /// the semantics tree. Otherwise, the semantics will be merged with
  /// the semantics of any ancestors.
  ///
  /// The 'container' flag is implicitly set to true on the immediate
  /// semantics-providing descendants of a node where multiple
  /// children have semantics or have descendants providing semantics.
  /// In other words, the semantics of siblings are not merged. To
  /// merge the semantics of an entire subtree, including siblings,
  /// you can use a [MergeSemantics] widget.
  final bool container;

  /// If non-null, indicates that this subtree represents a checkbox
  /// or similar widget with a "checked" state, and what its current
  /// state is.
  final bool checked;

  /// Provides a textual description of the widget.
  final String label;

  @override
  RenderSemanticsAnnotations createRenderObject(BuildContext context) => new RenderSemanticsAnnotations(
    container: container,
    checked: checked,
    label: label
  );

  @override
  void updateRenderObject(BuildContext context, RenderSemanticsAnnotations renderObject) {
    renderObject
      ..container = container
      ..checked = checked
      ..label = label;
  }

  @override
  void debugFillDescription(List<String> description) {
    super.debugFillDescription(description);
    description.add('container: $container');
    if (checked != null)
      description.add('checked: $checked');
    if (label != null)
      description.add('label: "$label"');
  }
}

/// A widget that merges the semantics of its descendants.
///
/// Causes all the semantics of the subtree rooted at this node to be
/// merged into one node in the semantics tree. For example, if you
/// have a widget with a Text node next to a checkbox widget, this
/// could be used to merge the label from the Text node with the
/// "checked" semantic state of the checkbox into a single node that
/// had both the label and the checked state. Otherwise, the label
/// would be presented as a separate feature than the checkbox, and
/// the user would not be able to be sure that they were related.
///
/// Be aware that if two nodes in the subtree have conflicting
/// semantics, the result may be nonsensical. For example, a subtree
/// with a checked checkbox and an unchecked checkbox will be
/// presented as checked. All the labels will be merged into a single
/// string (with newlines separating each label from the other). If
/// multiple nodes in the merged subtree can handle semantic gestures,
/// the first one in tree order will be the one to receive the
/// callbacks.
class MergeSemantics extends SingleChildRenderObjectWidget {
  /// Creates a widget that merges the semantics of its descendants.
  MergeSemantics({ Key key, Widget child }) : super(key: key, child: child);

  @override
  RenderMergeSemantics createRenderObject(BuildContext context) => new RenderMergeSemantics();
}

/// A widget that drops all the semantics of its descendants.
///
/// This can be used to hide subwidgets that would otherwise be
/// reported but that would only be confusing. For example, the
/// material library's [Chip] widget hides the avatar since it is
/// redundant with the chip label.
class ExcludeSemantics extends SingleChildRenderObjectWidget {
  /// Creates a widget that drops all the semantics of its descendants.
  ExcludeSemantics({ Key key, Widget child }) : super(key: key, child: child);

  @override
  RenderExcludeSemantics createRenderObject(BuildContext context) => new RenderExcludeSemantics();
}

/// A widget that builds its child.
///
/// Useful for attaching a key to an existing widget.
class KeyedSubtree extends StatelessWidget {
  /// Creates a widget that builds its child.
  KeyedSubtree({
    Key key,
    @required this.child
  }) : super(key: key) {
    assert(child != null);
  }

  /// The widget below this widget in the tree.
  final Widget child;

  /// Creates a KeyedSubtree for child with a key that's based on the child's existing key or childIndex.
  factory KeyedSubtree.wrap(Widget child, int childIndex) {
    Key key = child.key != null ? new ValueKey<Key>(child.key) : new ValueKey<int>(childIndex);
    return new KeyedSubtree(key: key, child: child);
  }

  /// Wrap each item in a KeyedSubtree whose key is based on the item's existing key or
  /// the sum of its list index and `baseIndex`.
  static List<Widget> ensureUniqueKeysForList(Iterable<Widget> items, { int baseIndex: 0 }) {
    if (items == null || items.isEmpty)
      return items;

    List<Widget> itemsWithUniqueKeys = <Widget>[];
    int itemIndex = baseIndex;
    for (Widget item in items) {
      itemsWithUniqueKeys.add(new KeyedSubtree.wrap(item, itemIndex));
      itemIndex += 1;
    }

    assert(!debugItemsHaveDuplicateKeys(itemsWithUniqueKeys));
    return itemsWithUniqueKeys;
  }

  @override
  Widget build(BuildContext context) => child;
}

/// A platonic widget that calls a closure to obtain its child widget.
///
/// See also:
///
///  * [StatefulBuilder], a platonic widget which also has state.
class Builder extends StatelessWidget {
  /// Creates a widget that delegates its build to a callback.
  ///
  /// The [builder] argument must not be null.
  Builder({
    Key key,
    @required this.builder
  }) : super(key: key) {
    assert(builder != null);
  }

  /// Called to obtain the child widget.
  ///
  /// This function is called whenever this widget is included in its parent's
  /// build and the old widget (if any) that it synchronizes with has a distinct
  /// object identity. Typically the parent's build method will construct
  /// a new tree of widgets and so a new Builder child will not be [identical]
  /// to the corresponding old one.
  final WidgetBuilder builder;

  @override
  Widget build(BuildContext context) => builder(context);
}

/// Signature for the builder callback used by [StatefulBuilder].
///
/// Call [setState] to schedule the [StatefulBuilder] to rebuild.
typedef Widget StatefulWidgetBuilder(BuildContext context, StateSetter setState);

/// A platonic widget that both has state and calls a closure to obtain its child widget.
///
/// See also:
///
///  * [Builder], the platonic stateless widget.
class StatefulBuilder extends StatefulWidget {
  /// Creates a widget that both has state and delegates its build to a callback.
  ///
  /// The [builder] argument must not be null.
  StatefulBuilder({
    Key key,
    @required this.builder
  }) : super(key: key) {
    assert(builder != null);
  }

  /// Called to obtain the child widget.
  ///
  /// This function is called whenever this widget is included in its parent's
  /// build and the old widget (if any) that it synchronizes with has a distinct
  /// object identity. Typically the parent's build method will construct
  /// a new tree of widgets and so a new Builder child will not be [identical]
  /// to the corresponding old one.
  final StatefulWidgetBuilder builder;

  @override
  _StatefulBuilderState createState() => new _StatefulBuilderState();
}

class _StatefulBuilderState extends State<StatefulBuilder> {
  @override
  Widget build(BuildContext context) => config.builder(context, setState);
}