flutter_flutter/packages/flutter/lib/src/rendering/box.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:math' as math;
import 'dart:ui' as ui;
import 'dart:ui' show hashValues;

import 'package:flutter/animation.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/painting.dart';
import 'package:vector_math/vector_math_64.dart';

import 'debug.dart';
import 'object.dart';

export 'package:flutter/painting.dart' show EdgeDims, FractionalOffset, TextBaseline;

// This class should only be used in debug builds
class _DebugSize extends Size {
  _DebugSize(Size source, this._owner, this._canBeUsedByParent): super.copy(source);
  final RenderBox _owner;
  final bool _canBeUsedByParent;
}

/// The two cardinal directions in two dimensions.
enum Axis {
  /// Left and right
  horizontal,

  /// Up and down
  vertical,
}

/// Immutable layout constraints for box layout.
///
/// A size respects a BoxConstraints if, and only if, all of the following
/// relations hold:
///
/// * `minWidth <= size.width <= maxWidth`
/// * `minHeight <= size.height <= maxHeight`
///
/// The constraints themselves must satisfy these relations:
///
/// * `0.0 <= minWidth <= maxWidth <= double.INFINITY`
/// * `0.0 <= minHeight <= maxHeight <= double.INFINITY`
///
/// Note: `double.INFINITY` is a legal value for each constraint.
class BoxConstraints extends Constraints {
  /// Constructs box constraints with the given constraints
  const BoxConstraints({
    this.minWidth: 0.0,
    this.maxWidth: double.INFINITY,
    this.minHeight: 0.0,
    this.maxHeight: double.INFINITY
  });

  final double minWidth;
  final double maxWidth;
  final double minHeight;
  final double maxHeight;

  /// Constructs box constraints that is respected only by the given size.
  BoxConstraints.tight(Size size)
    : minWidth = size.width,
      maxWidth = size.width,
      minHeight = size.height,
      maxHeight = size.height;

  /// Constructs box constraints that require the given width or height.
  const BoxConstraints.tightFor({
    double width,
    double height
  }): minWidth = width != null ? width : 0.0,
      maxWidth = width != null ? width : double.INFINITY,
      minHeight = height != null ? height : 0.0,
      maxHeight = height != null ? height : double.INFINITY;

  /// Constructs box constraints that forbid sizes larger than the given size.
  BoxConstraints.loose(Size size)
    : minWidth = 0.0,
      maxWidth = size.width,
      minHeight = 0.0,
      maxHeight = size.height;

  /// Constructs box constraints that expand to fill another box contraints.
  ///
  /// If width or height is given, the constraints will require exactly the
  /// given value in the given dimension.
  const BoxConstraints.expand({
    double width,
    double height
  }): minWidth = width != null ? width : double.INFINITY,
      maxWidth = width != null ? width : double.INFINITY,
      minHeight = height != null ? height : double.INFINITY,
      maxHeight = height != null ? height : double.INFINITY;

  /// Returns new box constraints that are smaller by the given edge dimensions.
  BoxConstraints deflate(EdgeDims edges) {
    assert(edges != null);
    assert(isNormalized);
    double horizontal = edges.left + edges.right;
    double vertical = edges.top + edges.bottom;
    double deflatedMinWidth = math.max(0.0, minWidth - horizontal);
    double deflatedMinHeight = math.max(0.0, minHeight - vertical);
    return new BoxConstraints(
      minWidth: deflatedMinWidth,
      maxWidth: math.max(deflatedMinWidth, maxWidth - horizontal),
      minHeight: deflatedMinHeight,
      maxHeight: math.max(deflatedMinHeight, maxHeight - vertical)
    );
  }

  /// Returns new box constraints that remove the minimum width and height requirements.
  BoxConstraints loosen() {
    assert(isNormalized);
    return new BoxConstraints(
      minWidth: 0.0,
      maxWidth: maxWidth,
      minHeight: 0.0,
      maxHeight: maxHeight
    );
  }

  /// Returns new box constraints that respect the given constraints while being
  /// as close as possible to the original constraints.
  BoxConstraints enforce(BoxConstraints constraints) {
    return new BoxConstraints(
      minWidth: minWidth.clamp(constraints.minWidth, constraints.maxWidth),
      maxWidth: maxWidth.clamp(constraints.minWidth, constraints.maxWidth),
      minHeight: minHeight.clamp(constraints.minHeight, constraints.maxHeight),
      maxHeight: maxHeight.clamp(constraints.minHeight, constraints.maxHeight)
    );
  }

  /// Returns new box constraints with a tight width and/or height as close to
  /// the given width and height as possible while still respecting the original
  /// box constraints.
  BoxConstraints tighten({ double width, double height }) {
    return new BoxConstraints(minWidth: width == null ? minWidth : width.clamp(minWidth, maxWidth),
                              maxWidth: width == null ? maxWidth : width.clamp(minWidth, maxWidth),
                              minHeight: height == null ? minHeight : height.clamp(minHeight, maxHeight),
                              maxHeight: height == null ? maxHeight : height.clamp(minHeight, maxHeight));
  }

  /// Returns box constraints with the same width constraints but with
  /// unconstrainted height.
  BoxConstraints widthConstraints() => new BoxConstraints(minWidth: minWidth, maxWidth: maxWidth);

  /// Returns box constraints with the same height constraints but with
  /// unconstrainted width
  BoxConstraints heightConstraints() => new BoxConstraints(minHeight: minHeight, maxHeight: maxHeight);

  /// Returns the width that both satisfies the constraints and is as close as
  /// possible to the given width.
  double constrainWidth([double width = double.INFINITY]) {
    assert(isNormalized);
    return width.clamp(minWidth, maxWidth);
  }

  /// Returns the height that both satisfies the constraints and is as close as
  /// possible to the given height.
  double constrainHeight([double height = double.INFINITY]) {
    assert(isNormalized);
    return height.clamp(minHeight, maxHeight);
  }

  /// Returns the size that both satisfies the constraints and is as close as
  /// possible to the given size.
  Size constrain(Size size) {
    Size result = new Size(constrainWidth(size.width), constrainHeight(size.height));
    assert(() {
      if (size is _DebugSize)
        result = new _DebugSize(result, size._owner, size._canBeUsedByParent);
      return true;
    });
    return result;
  }

  /// The biggest size that satisifes the constraints.
  Size get biggest => new Size(constrainWidth(), constrainHeight());

  /// The smallest size that satisfies the constraints.
  Size get smallest => new Size(constrainWidth(0.0), constrainHeight(0.0));

  /// Whether there is exactly one width value that satisfies the constraints.
  bool get hasTightWidth => minWidth >= maxWidth;

  /// Whether there is exactly one height value that satisfies the constraints.
  bool get hasTightHeight => minHeight >= maxHeight;

  /// Whether there is exactly one size that satifies the constraints.
  bool get isTight => hasTightWidth && hasTightHeight;

  /// Whether the given size satisfies the constraints.
  bool isSatisfiedBy(Size size) {
    assert(isNormalized);
    return (minWidth <= size.width) && (size.width <= maxWidth) &&
           (minHeight <= size.height) && (size.height <= maxHeight);
  }

  BoxConstraints operator*(double other) {
    return new BoxConstraints(
      minWidth: minWidth * other,
      maxWidth: maxWidth * other,
      minHeight: minHeight * other,
      maxHeight: maxHeight * other
    );
  }

  BoxConstraints operator/(double other) {
    return new BoxConstraints(
      minWidth: minWidth / other,
      maxWidth: maxWidth / other,
      minHeight: minHeight / other,
      maxHeight: maxHeight / other
    );
  }

  BoxConstraints operator~/(double other) {
    return new BoxConstraints(
      minWidth: (minWidth ~/ other).toDouble(),
      maxWidth: (maxWidth ~/ other).toDouble(),
      minHeight: (minHeight ~/ other).toDouble(),
      maxHeight: (maxHeight ~/ other).toDouble()
    );
  }

  BoxConstraints operator%(double other) {
    return new BoxConstraints(
      minWidth: minWidth % other,
      maxWidth: maxWidth % other,
      minHeight: minHeight % other,
      maxHeight: maxHeight % other
    );
  }

  /// Linearly interpolate between two BoxConstraints.
  ///
  /// If either is null, this function interpolates from [BoxConstraints.zero].
  static BoxConstraints lerp(BoxConstraints a, BoxConstraints b, double t) {
    if (a == null && b == null)
      return null;
    if (a == null)
      return b * t;
    if (b == null)
      return a * (1.0 - t);
    assert(a.isNormalized);
    assert(b.isNormalized);
    return new BoxConstraints(
      minWidth: ui.lerpDouble(a.minWidth, b.minWidth, t),
      maxWidth: ui.lerpDouble(a.maxWidth, b.maxWidth, t),
      minHeight: ui.lerpDouble(a.minHeight, b.minHeight, t),
      maxHeight: ui.lerpDouble(a.maxHeight, b.maxHeight, t)
    );
  }

  bool get isNormalized => minWidth <= maxWidth && minHeight <= maxHeight;

  BoxConstraints normalize() {
    return new BoxConstraints(
      minWidth: minWidth,
      maxWidth: minWidth > maxWidth ? minWidth : maxWidth,
      minHeight: minHeight,
      maxHeight: minHeight > maxHeight ? minHeight : maxHeight
    );
  }

  bool operator ==(dynamic other) {
    assert(isNormalized);
    if (identical(this, other))
      return true;
    if (other is! BoxConstraints)
      return false;
    final BoxConstraints typedOther = other;
    assert(typedOther.isNormalized);
    return minWidth == typedOther.minWidth &&
           maxWidth == typedOther.maxWidth &&
           minHeight == typedOther.minHeight &&
           maxHeight == typedOther.maxHeight;
  }

  int get hashCode {
    assert(isNormalized);
    return hashValues(minWidth, maxWidth, minHeight, maxHeight);
  }

  String toString() {
    String annotation = isNormalized ? '' : '; NOT NORMALIZED';
    if (minWidth == double.INFINITY && minHeight == double.INFINITY)
      return 'BoxConstraints(biggest$annotation)';
    if (minWidth == 0 && maxWidth == double.INFINITY &&
        minHeight == 0 && maxHeight == double.INFINITY)
      return 'BoxConstraints(unconstrained$annotation)';
    String describe(double min, double max, String dim) {
      if (min == max)
        return '$dim=${min.toStringAsFixed(1)}';
      return '${min.toStringAsFixed(1)}<=$dim<=${max.toStringAsFixed(1)}';
    }
    final String width = describe(minWidth, maxWidth, 'w');
    final String height = describe(minHeight, maxHeight, 'h');
    return 'BoxConstraints($width, $height$annotation)';
  }
}

/// A hit test entry used by [RenderBox].
class BoxHitTestEntry extends HitTestEntry {
  const BoxHitTestEntry(RenderBox target, this.localPosition) : super(target);

  RenderBox get target => super.target;

  /// The position of the hit test in the local coordinates of [target].
  final Point localPosition;

  String toString() => '${target.runtimeType}@$localPosition';
}

/// Parent data used by [RenderBox] and its subclasses.
class BoxParentData extends ParentData {
  /// The offset at which to paint the child in the parent's coordinate system
  Offset get offset => _offset;
  Offset _offset = Offset.zero;
  void set offset(Offset value) {
    assert(RenderObject.debugDoingLayout);
    _offset = value;
  }

  String toString() => 'offset=$offset';
}

/// Abstract ParentData subclass for RenderBox subclasses that want the
/// ContainerRenderObjectMixin.
abstract class ContainerBoxParentDataMixin<ChildType extends RenderObject> extends BoxParentData with ContainerParentDataMixin<ChildType> { }

/// A render object in a 2D cartesian coordinate system.
///
/// The size of each box is expressed as a width and a height. Each box has its
/// own coordinate system in which its upper left corner is placed at (0, 0).
/// The lower right corner of the box is therefore at (width, height). The box
/// contains all the points including the upper left corner and extending to,
/// but not including, the lower right corner.
///
/// Box layout is performed by passing a [BoxConstraints] object down the tree.
/// The box constraints establish a min and max value for the child's width
/// and height. In determining its size, the child must respect the constraints
/// given to it by its parent.
///
/// This protocol is sufficient for expressing a number of common box layout
/// data flows.  For example, to implement a width-in-height-out data flow, call
/// your child's [layout] function with a set of box constraints with a tight
/// width value (and pass true for parentUsesSize). After the child determines
/// its height, use the child's height to determine your size.
abstract class RenderBox extends RenderObject {

  void setupParentData(RenderObject child) {
    if (child.parentData is! BoxParentData)
      child.parentData = new BoxParentData();
  }

  /// Returns the minimum width that this box could be without failing to paint
  /// its contents within itself.
  ///
  /// Override in subclasses that implement [performLayout].
  double getMinIntrinsicWidth(BoxConstraints constraints) {
    assert(constraints.isNormalized);
    return constraints.constrainWidth(0.0);
  }

  /// Returns the smallest width beyond which increasing the width never
  /// decreases the height.
  ///
  /// Override in subclasses that implement [performLayout].
  double getMaxIntrinsicWidth(BoxConstraints constraints) {
    assert(constraints.isNormalized);
    return constraints.constrainWidth(0.0);
  }

  /// Return the minimum height that this box could be without failing to paint
  /// its contents within itself.
  ///
  /// Override in subclasses that implement [performLayout].
  double getMinIntrinsicHeight(BoxConstraints constraints) {
    assert(constraints.isNormalized);
    return constraints.constrainHeight(0.0);
  }

  /// Returns the smallest height beyond which increasing the height never
  /// decreases the width.
  ///
  /// If the layout algorithm used is width-in-height-out, i.e. the height
  /// depends on the width and not vice versa, then this will return the same
  /// as getMinIntrinsicHeight().
  ///
  /// Override in subclasses that implement [performLayout].
  double getMaxIntrinsicHeight(BoxConstraints constraints) {
    assert(constraints.isNormalized);
    return constraints.constrainHeight(0.0);
  }

  /// The size of this render box computed during layout.
  ///
  /// This value is stale whenever this object is marked as needing layout.
  /// During [performLayout], do not read the size of a child unless you pass
  /// true for parentUsesSize when calling the child's [layout] function.
  ///
  /// The size of a box should be set only during the box's [performLayout] or
  /// [performResize] functions. If you wish to change the size of a box outside
  /// of those functins, call [markNeedsLayout] instead to schedule a layout of
  /// the box.
  Size get size {
    assert(hasSize);
    assert(() {
      if (_size is _DebugSize) {
        final _DebugSize _size = this._size;
        assert(_size._owner == this);
        if (RenderObject.debugActiveLayout != null) {
          // We are always allowed to access our own size (for print debugging
          // and asserts if nothing else). Other than us, the only object that's
          // allowed to read our size is our parent, if they've said they will.
          // If you hit this assert trying to access a child's size, pass
          // "parentUsesSize: true" to that child's layout().
          assert(debugDoingThisResize || debugDoingThisLayout ||
                 (RenderObject.debugActiveLayout == parent && _size._canBeUsedByParent));
        }
        assert(_size == this._size);
      }
      return true;
    });
    return _size;
  }
  bool get hasSize => _size != null;
  Size _size;
  void set size(Size value) {
    assert((sizedByParent && debugDoingThisResize) ||
           (!sizedByParent && debugDoingThisLayout));
    assert(() {
      if (value is _DebugSize) {
        if (value._owner != this) {
          assert(value._owner.parent == this);
          assert(value._canBeUsedByParent);
        }
      }
      return true;
    });
    _size = value;
    assert(() {
      _size = new _DebugSize(_size, this, debugCanParentUseSize);
      return true;
    });
    assert(debugDoesMeetConstraints());
  }

  void debugResetSize() {
    // updates the value of size._canBeUsedByParent if necessary
    size = size;
  }

  Map<TextBaseline, double> _cachedBaselines;
  bool _ancestorUsesBaseline = false;
  static bool _debugDoingBaseline = false;
  static bool _debugSetDoingBaseline(bool value) {
    _debugDoingBaseline = value;
    return true;
  }

  /// Returns the distance from the y-coordinate of the position of the box to
  /// the y-coordinate of the first given baseline in the box's contents.
  ///
  /// Used by certain layout models to align adjacent boxes on a common
  /// baseline, regardless of padding, font size differences, etc. If there is
  /// no baseline, this function returns the distance from the y-coordinate of
  /// the position of the box to the y-coordinate of the bottom of the box
  /// (i.e., the height of the box) unless the the caller passes true
  /// for `onlyReal`, in which case the function returns null.
  ///
  /// Only call this function calling [layout] on this box. You are only
  /// allowed to call this from the parent of this box during that parent's
  /// [performLayout] or [paint] functions.
  double getDistanceToBaseline(TextBaseline baseline, { bool onlyReal: false }) {
    assert(!needsLayout);
    assert(!_debugDoingBaseline);
    final RenderObject parent = this.parent;
    assert(() {
      if (RenderObject.debugDoingLayout)
        return (RenderObject.debugActiveLayout == parent) && parent.debugDoingThisLayout;
      if (RenderObject.debugDoingPaint)
        return ((RenderObject.debugActivePaint == parent) && parent.debugDoingThisPaint) ||
               ((RenderObject.debugActivePaint == this) && debugDoingThisPaint);
      return false;
    });
    assert(_debugSetDoingBaseline(true));
    double result = getDistanceToActualBaseline(baseline);
    assert(_debugSetDoingBaseline(false));
    assert(parent == this.parent);
    if (result == null && !onlyReal)
      return size.height;
    return result;
  }

  /// Calls [computeDistanceToActualBaseline] and caches the result.
  ///
  /// This function must only be called from [getDistanceToBaseline] and
  /// [computeDistanceToActualBaseline]. Do not call this function directly from
  /// outside those two methods.
  double getDistanceToActualBaseline(TextBaseline baseline) {
    assert(_debugDoingBaseline);
    _ancestorUsesBaseline = true;
    if (_cachedBaselines == null)
      _cachedBaselines = new Map<TextBaseline, double>();
    _cachedBaselines.putIfAbsent(baseline, () => computeDistanceToActualBaseline(baseline));
    return _cachedBaselines[baseline];
  }

  /// Returns the distance from the y-coordinate of the position of the box to
  /// the y-coordinate of the first given baseline in the box's contents, if
  /// any, or null otherwise.
  ///
  /// Do not call this function directly. Instead, call [getDistanceToBaseline]
  /// if you need to know the baseline of a child from an invocation of
  /// [performLayout] or [paint] and call [getDistanceToActualBaseline] if you
  /// are implementing [computeDistanceToActualBaseline] and need to defer to a
  /// child.
  ///
  /// Subclasses should override this function to supply the distances to their
  /// baselines.
  double computeDistanceToActualBaseline(TextBaseline baseline) {
    assert(_debugDoingBaseline);
    return null;
  }

  /// The box constraints most recently received from the parent.
  BoxConstraints get constraints => super.constraints;
  bool debugDoesMeetConstraints() {
    assert(constraints != null);
    assert(_size != null);
    assert(() {
      'See https://flutter.io/layout/#unbounded-constraints';
      return !_size.isInfinite;
    });
    bool result = constraints.isSatisfiedBy(_size);
    if (!result)
      debugPrint("${this.runtimeType} does not meet its constraints. Constraints: $constraints, size: $_size");
    return result;
  }

  void markNeedsLayout() {
    if (_cachedBaselines != null && _cachedBaselines.isNotEmpty) {
      // if we have cached data, then someone must have used our data
      assert(_ancestorUsesBaseline);
      final RenderObject parent = this.parent;
      parent.markNeedsLayout();
      assert(parent == this.parent);
      // Now that they're dirty, we can forget that they used the
      // baseline. If they use it again, then we'll set the bit
      // again, and if we get dirty again, we'll notify them again.
      _ancestorUsesBaseline = false;
      _cachedBaselines.clear();
    } else {
      // if we've never cached any data, then nobody can have used it
      assert(!_ancestorUsesBaseline);
    }
    super.markNeedsLayout();
  }
  void performResize() {
    // default behaviour for subclasses that have sizedByParent = true
    size = constraints.constrain(Size.zero);
    assert(!size.isInfinite);
  }
  void performLayout() {
    assert(() {
      if (!sizedByParent) {
        debugPrint('$runtimeType needs to either override performLayout() to\n'
          'set size and lay out children, or, set sizedByParent to true\n'
          'so that performResize() sizes the render object.');
        assert(sizedByParent);
      }
      return true;
    });
  }

  /// Determines the set of render objects located at the given position.
  ///
  /// Returns true if the given point is contained in this render object or one
  /// of its descendants. Adds any render objects that contain the point to the
  /// given hit test result.
  ///
  /// The caller is responsible for transforming [position] into the local
  /// coordinate space of the callee.  The callee is responsible for checking
  /// whether the given position is within its bounds.
  bool hitTest(HitTestResult result, { Point position }) {
    assert(!needsLayout);
    assert(_size != null && 'Missing size. Did you set a size during layout?' != null);
    if (position.x >= 0.0 && position.x < _size.width &&
        position.y >= 0.0 && position.y < _size.height) {
      if (hitTestChildren(result, position: position) || hitTestSelf(position)) {
        result.add(new BoxHitTestEntry(this, position));
        return true;
      }
    }
    return false;
  }

  /// Override this function if this render object can be hit even if its
  /// children were not hit.
  bool hitTestSelf(Point position) => false;

  /// Override this function to check whether any children are located at the
  /// given position.
  ///
  /// Typically children should be hit tested in reverse paint order so that
  /// hit tests at locations where children overlap hit the child that is
  /// visually "on top" (i.e., paints later).
  bool hitTestChildren(HitTestResult result, { Point position }) => false;

  static Point _transformPoint(Matrix4 transform, Point point) {
    Vector3 position3 = new Vector3(point.x, point.y, 0.0);
    Vector3 transformed3 = transform.transform3(position3);
    return new Point(transformed3.x, transformed3.y);
  }

  /// Multiply the transform from the parent's coordinate system to this box's
  /// coordinate system into the given transform.
  ///
  /// This function is used to convert coordinate systems between boxes.
  /// Subclasses that apply transforms during painting should override this
  /// function to factor those transforms into the calculation.
  ///
  /// The RenderBox implementation takes care of adjusting the matrix for the
  /// position of the given child.
  void applyPaintTransform(RenderObject child, Matrix4 transform) {
    assert(child.parent == this);
    BoxParentData childParentData = child.parentData;
    Offset offset = childParentData.offset;
    transform.translate(offset.dx, offset.dy);
  }

  /// Convert the given point from the global coodinate system to the local
  /// coordinate system for this box.
  ///
  /// If the transform from global coordinates to local coordinates is
  /// degenerate, this function returns Point.origin.
  Point globalToLocal(Point point) {
    assert(attached);
    Matrix4 transform = new Matrix4.identity();
    RenderObject renderer = this;
    while (renderer.parent is RenderObject) {
      RenderObject rendererParent = renderer.parent;
      rendererParent.applyPaintTransform(renderer, transform);
      renderer = rendererParent;
    }
    double det = transform.invert();
    if (det == 0.0)
      return Point.origin;
    return _transformPoint(transform, point);
  }

  /// Convert the given point from the local coordinate system for this box to
  /// the global coordinate system.
  Point localToGlobal(Point point) {
    List<RenderObject> renderers = <RenderObject>[];
    for (RenderObject renderer = this; renderer != null; renderer = renderer.parent)
      renderers.add(renderer);
    Matrix4 transform = new Matrix4.identity();
    for (int index = renderers.length - 1; index > 0; index -= 1)
      renderers[index].applyPaintTransform(renderers[index - 1], transform);
    return _transformPoint(transform, point);
  }

  /// Returns a rectangle that contains all the pixels painted by this box.
  ///
  /// The paint bounds can be larger or smaller than [size], which is the amount
  /// of space this box takes up during layout. For example, if this box casts a
  /// shadow, that shadow might extend beyond the space allocated to this box
  /// during layout.
  ///
  /// The paint bounds are used to size the buffers into which this box paints.
  /// If the box attempts to paints outside its paint bounds, there might not be
  /// enough memory allocated to represent the box's visual appearance, which
  /// can lead to undefined behavior.
  ///
  /// The returned paint bounds are in the local coordinate system of this box.
  Rect get paintBounds => Point.origin & size;

  int _debugActivePointers = 0;
  void handleEvent(PointerEvent event, HitTestEntry entry) {
    super.handleEvent(event, entry);
    assert(() {
      if (debugPaintPointersEnabled) {
        if (event is PointerDownEvent) {
          _debugActivePointers += 1;
        } else if (event is PointerUpEvent || event is PointerCancelEvent) {
          _debugActivePointers -= 1;
        }
        markNeedsPaint();
      }
      return true;
    });
  }

  void debugPaint(PaintingContext context, Offset offset) {
    assert(() {
      if (debugPaintSizeEnabled)
        debugPaintSize(context, offset);
      if (debugPaintBaselinesEnabled)
        debugPaintBaselines(context, offset);
      if (debugPaintPointersEnabled)
        debugPaintPointers(context, offset);
      return true;
    });
  }
  void debugPaintSize(PaintingContext context, Offset offset) {
    assert(() {
      Paint paint = new Paint()
       ..style = ui.PaintingStyle.stroke
       ..strokeWidth = 1.0
       ..color = debugPaintSizeColor;
      context.canvas.drawRect((offset & size).deflate(0.5), paint);
      return true;
    });
  }
  void debugPaintBaselines(PaintingContext context, Offset offset) {
    assert(() {
      Paint paint = new Paint()
       ..style = ui.PaintingStyle.stroke
       ..strokeWidth = 0.25;
      Path path;
      // ideographic baseline
      double baselineI = getDistanceToBaseline(TextBaseline.ideographic, onlyReal: true);
      if (baselineI != null) {
        paint.color = debugPaintIdeographicBaselineColor;
        path = new Path();
        path.moveTo(offset.dx, offset.dy + baselineI);
        path.lineTo(offset.dx + size.width, offset.dy + baselineI);
        context.canvas.drawPath(path, paint);
      }
      // alphabetic baseline
      double baselineA = getDistanceToBaseline(TextBaseline.alphabetic, onlyReal: true);
      if (baselineA != null) {
        paint.color = debugPaintAlphabeticBaselineColor;
        path = new Path();
        path.moveTo(offset.dx, offset.dy + baselineA);
        path.lineTo(offset.dx + size.width, offset.dy + baselineA);
        context.canvas.drawPath(path, paint);
      }
      return true;
    });
  }
  void debugPaintPointers(PaintingContext context, Offset offset) {
    assert(() {
      if (_debugActivePointers > 0) {
        Paint paint = new Paint()
         ..color = new Color(debugPaintPointersColorValue | ((0x04000000 * depth) & 0xFF000000));
        context.canvas.drawRect(offset & size, paint);
      }
      return true;
    });
  }

  void debugDescribeSettings(List<String> settings) {
    super.debugDescribeSettings(settings);
    settings.add('size: ${ hasSize ? size : "MISSING" }');
  }
}

/// A mixin that provides useful default behaviors for boxes with children
/// managed by the [ContainerRenderObjectMixin] mixin.
///
/// By convention, this class doesn't override any members of the superclass.
/// Instead, it provides helpful functions that subclasses can call as
/// appropriate.
abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerBoxParentDataMixin<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> {

  /// Returns the baseline of the first child with a baseline.
  ///
  /// Useful when the children are displayed vertically in the same order they
  /// appear in the child list.
  double defaultComputeDistanceToFirstActualBaseline(TextBaseline baseline) {
    assert(!needsLayout);
    RenderBox child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      double result = child.getDistanceToActualBaseline(baseline);
      if (result != null)
        return result + childParentData.offset.dy;
      child = childParentData.nextSibling;
    }
    return null;
  }

  /// Returns the minimum baseline value among every child.
  ///
  /// Useful when the vertical position of the children isn't determined by the
  /// order in the child list.
  double defaultComputeDistanceToHighestActualBaseline(TextBaseline baseline) {
    assert(!needsLayout);
    double result;
    RenderBox child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      double candidate = child.getDistanceToActualBaseline(baseline);
      if (candidate != null) {
        candidate += childParentData.offset.dy;
        if (result != null)
          result = math.min(result, candidate);
        else
          result = candidate;
      }
      child = childParentData.nextSibling;
    }
    return result;
  }

  /// Performs a hit test on each child by walking the child list backwards.
  ///
  /// Stops walking once after the first child reports that it contains the
  /// given point. Returns whether any children contain the given point.
  bool defaultHitTestChildren(HitTestResult result, { Point position }) {
    // the x, y parameters have the top left of the node's box as the origin
    ChildType child = lastChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      Point transformed = new Point(position.x - childParentData.offset.dx,
                                    position.y - childParentData.offset.dy);
      if (child.hitTest(result, position: transformed))
        return true;
      child = childParentData.previousSibling;
    }
    return false;
  }

  /// Paints each child by walking the child list forwards.
  void defaultPaint(PaintingContext context, Offset offset) {
    RenderBox child = firstChild;
    while (child != null) {
      final ParentDataType childParentData = child.parentData;
      context.paintChild(child, childParentData.offset + offset);
      child = childParentData.nextSibling;
    }
  }
}

class AnimatedFractionalOffsetValue extends AnimatedValue<FractionalOffset> {
  AnimatedFractionalOffsetValue(FractionalOffset begin, { FractionalOffset end, Curve curve, Curve reverseCurve })
    : super(begin, end: end, curve: curve, reverseCurve: reverseCurve);

  FractionalOffset lerp(double t) => FractionalOffset.lerp(begin, end, t);
}