Add an alignment property to OverflowBox

This patch changes the default alignment of OverflowBox to center rather than
having a bias towards the upper-left corner.

Fixes #1236

packages/flutter/lib/src/rendering/overflow.dart

@@ -7,146 +7,6 @@ import 'object.dart';
 
 export 'package:flutter/src/painting/box_painter.dart';
 
-/// A render object that imposes different constraints on its child than it gets
-/// from its parent, possibly allowing the child to overflow the parent.
-///
-/// A render overflow box proxies most functions in the render box protocol to
-/// its child, except that when laying out its child, it passes constraints
-/// based on the minWidth, maxWidth, minHeight, and maxHeight fields instead of
-/// just passing the parent's constraints in. Specifically, it overrides any of
-/// the equivalent fields on the constraints given by the parent with the
-/// constraints given by these fields for each such field that is not null. It
-/// then sizes itself based on the parent's constraints' maxWidth and maxHeight,
-/// ignoring the child's dimensions.
-///
-/// For example, if you wanted a box to always render 50 pixels high, regardless
-/// of where it was rendered, you would wrap it in a RenderOverflow with
-/// minHeight and maxHeight set to 50.0. Generally speaking, to avoid confusing
-/// behaviour around hit testing, a RenderOverflowBox should usually be wrapped
-/// in a RenderClipRect.
-///
-/// The child is positioned at the top left of the box. To position a smaller
-/// child inside a larger parent, use [RenderPositionedBox] and
-/// [RenderConstrainedBox] rather than RenderOverflowBox.
-class RenderOverflowBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
-  RenderOverflowBox({
-    RenderBox child,
-    double minWidth,
-    double maxWidth,
-    double minHeight,
-    double maxHeight
-  }) : _minWidth = minWidth, _maxWidth = maxWidth, _minHeight = minHeight, _maxHeight = maxHeight {
-    this.child = child;
-  }
-
-  /// The minimum width constraint to give the child. Set this to null (the
-  /// default) to use the constraint from the parent instead.
-  double get minWidth => _minWidth;
-  double _minWidth;
-  void set minWidth (double value) {
-    if (_minWidth == value)
-      return;
-    _minWidth = value;
-    markNeedsLayout();
-  }
-
-  /// The maximum width constraint to give the child. Set this to null (the
-  /// default) to use the constraint from the parent instead.
-  double get maxWidth => _maxWidth;
-  double _maxWidth;
-  void set maxWidth (double value) {
-    if (_maxWidth == value)
-      return;
-    _maxWidth = value;
-    markNeedsLayout();
-  }
-
-  /// The minimum height constraint to give the child. Set this to null (the
-  /// default) to use the constraint from the parent instead.
-  double get minHeight => _minHeight;
-  double _minHeight;
-  void set minHeight (double value) {
-    if (_minHeight == value)
-      return;
-    _minHeight = value;
-    markNeedsLayout();
-  }
-
-  /// The maximum height constraint to give the child. Set this to null (the
-  /// default) to use the constraint from the parent instead.
-  double get maxHeight => _maxHeight;
-  double _maxHeight;
-  void set maxHeight (double value) {
-    if (_maxHeight == value)
-      return;
-    _maxHeight = value;
-    markNeedsLayout();
-  }
-
-  BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
-    return new BoxConstraints(
-      minWidth: _minWidth ?? constraints.minWidth,
-      maxWidth: _maxWidth ?? constraints.maxWidth,
-      minHeight: _minHeight ?? constraints.minHeight,
-      maxHeight: _maxHeight ?? constraints.maxHeight
-    );
-  }
-
-  double getMinIntrinsicWidth(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainWidth();
-  }
-
-  double getMaxIntrinsicWidth(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainWidth();
-  }
-
-  double getMinIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainHeight();
-  }
-
-  double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    assert(constraints.isNormalized);
-    return constraints.constrainHeight();
-  }
-
-  double computeDistanceToActualBaseline(TextBaseline baseline) {
-    if (child != null)
-      return child.getDistanceToActualBaseline(baseline);
-    return super.computeDistanceToActualBaseline(baseline);
-  }
-
-  bool get sizedByParent => true;
-
-  void performResize() {
-    size = constraints.biggest;
-  }
-
-  void performLayout() {
-    if (child != null)
-      child.layout(_getInnerConstraints(constraints));
-  }
-
-  bool hitTestChildren(HitTestResult result, { Point position }) {
-    return child?.hitTest(result, position: position) ?? false;
-  }
-
-  void paint(PaintingContext context, Offset offset) {
-    if (child != null)
-      context.paintChild(child, offset);
-  }
-
-  void debugDescribeSettings(List<String> settings) {
-    super.debugDescribeSettings(settings);
-    settings.add('minWidth: ${minWidth ?? "use parent minWidth constraint"}');
-    settings.add('maxWidth: ${maxWidth ?? "use parent maxWidth constraint"}');
-    settings.add('minHeight: ${minHeight ?? "use parent minHeight constraint"}');
-    settings.add('maxHeight: ${maxHeight ?? "use parent maxHeight constraint"}');
-  }
-}
-
 /// A render box that's a specific size but passes its original constraints through to its child, which will probably overflow
 class RenderSizedOverflowBox extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
   RenderSizedOverflowBox({

packages/flutter/lib/src/rendering/shifted_box.dart

@@ -356,6 +356,158 @@ class RenderPositionedBox extends RenderShiftedBox {
   }
 }
 
+/// A render object that imposes different constraints on its child than it gets
+/// from its parent, possibly allowing the child to overflow the parent.
+///
+/// A render overflow box proxies most functions in the render box protocol to
+/// its child, except that when laying out its child, it passes constraints
+/// based on the minWidth, maxWidth, minHeight, and maxHeight fields instead of
+/// just passing the parent's constraints in. Specifically, it overrides any of
+/// the equivalent fields on the constraints given by the parent with the
+/// constraints given by these fields for each such field that is not null. It
+/// then sizes itself based on the parent's constraints' maxWidth and maxHeight,
+/// ignoring the child's dimensions.
+///
+/// For example, if you wanted a box to always render 50 pixels high, regardless
+/// of where it was rendered, you would wrap it in a RenderOverflow with
+/// minHeight and maxHeight set to 50.0. Generally speaking, to avoid confusing
+/// behaviour around hit testing, a RenderOverflowBox should usually be wrapped
+/// in a RenderClipRect.
+///
+/// The child is positioned at the top left of the box. To position a smaller
+/// child inside a larger parent, use [RenderPositionedBox] and
+/// [RenderConstrainedBox] rather than RenderOverflowBox.
+class RenderOverflowBox extends RenderShiftedBox {
+  RenderOverflowBox({
+    RenderBox child,
+    double minWidth,
+    double maxWidth,
+    double minHeight,
+    double maxHeight,
+    FractionalOffset alignment: const FractionalOffset(0.5, 0.5)
+  }) : _minWidth = minWidth,
+       _maxWidth = maxWidth,
+       _minHeight = minHeight,
+       _maxHeight = maxHeight,
+       _alignment = alignment,
+       super(child);
+
+  /// The minimum width constraint to give the child. Set this to null (the
+  /// default) to use the constraint from the parent instead.
+  double get minWidth => _minWidth;
+  double _minWidth;
+  void set minWidth (double value) {
+    if (_minWidth == value)
+      return;
+    _minWidth = value;
+    markNeedsLayout();
+  }
+
+  /// The maximum width constraint to give the child. Set this to null (the
+  /// default) to use the constraint from the parent instead.
+  double get maxWidth => _maxWidth;
+  double _maxWidth;
+  void set maxWidth (double value) {
+    if (_maxWidth == value)
+      return;
+    _maxWidth = value;
+    markNeedsLayout();
+  }
+
+  /// The minimum height constraint to give the child. Set this to null (the
+  /// default) to use the constraint from the parent instead.
+  double get minHeight => _minHeight;
+  double _minHeight;
+  void set minHeight (double value) {
+    if (_minHeight == value)
+      return;
+    _minHeight = value;
+    markNeedsLayout();
+  }
+
+  /// The maximum height constraint to give the child. Set this to null (the
+  /// default) to use the constraint from the parent instead.
+  double get maxHeight => _maxHeight;
+  double _maxHeight;
+  void set maxHeight (double value) {
+    if (_maxHeight == value)
+      return;
+    _maxHeight = value;
+    markNeedsLayout();
+  }
+
+  /// 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.
+  FractionalOffset get alignment => _alignment;
+  FractionalOffset _alignment;
+  void set alignment (FractionalOffset newAlignment) {
+    assert(newAlignment != null && newAlignment.dx != null && newAlignment.dy != null);
+    if (_alignment == newAlignment)
+      return;
+    _alignment = newAlignment;
+    markNeedsLayout();
+  }
+
+  BoxConstraints _getInnerConstraints(BoxConstraints constraints) {
+    return new BoxConstraints(
+      minWidth: _minWidth ?? constraints.minWidth,
+      maxWidth: _maxWidth ?? constraints.maxWidth,
+      minHeight: _minHeight ?? constraints.minHeight,
+      maxHeight: _maxHeight ?? constraints.maxHeight
+    );
+  }
+
+  double getMinIntrinsicWidth(BoxConstraints constraints) {
+    assert(constraints.isNormalized);
+    return constraints.minWidth;
+  }
+
+  double getMaxIntrinsicWidth(BoxConstraints constraints) {
+    assert(constraints.isNormalized);
+    return constraints.minWidth;
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    assert(constraints.isNormalized);
+    return constraints.minHeight;
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    assert(constraints.isNormalized);
+    return constraints.minHeight;
+  }
+
+  bool get sizedByParent => true;
+
+  void performResize() {
+    size = constraints.biggest;
+  }
+
+  void performLayout() {
+    if (child != null) {
+      child.layout(_getInnerConstraints(constraints), parentUsesSize: true);
+      final BoxParentData childParentData = child.parentData;
+      childParentData.offset = _alignment.alongOffset(size - child.size);
+    }
+  }
+
+  void debugDescribeSettings(List<String> settings) {
+    super.debugDescribeSettings(settings);
+    settings.add('minWidth: ${minWidth ?? "use parent minWidth constraint"}');
+    settings.add('maxWidth: ${maxWidth ?? "use parent maxWidth constraint"}');
+    settings.add('minHeight: ${minHeight ?? "use parent minHeight constraint"}');
+    settings.add('maxHeight: ${maxHeight ?? "use parent maxHeight constraint"}');
+    settings.add('alignment: $alignment');
+  }
+}
+
 /// A delegate for computing the layout of a render object with a single child.
 class OneChildLayoutDelegate {
   /// Returns the size of this object given the incoming constraints.

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

@@ -597,8 +597,15 @@ class FractionallySizedBox extends OneChildRenderObjectWidget {
 ///
 /// See [RenderOverflowBox] for details.
 class OverflowBox extends OneChildRenderObjectWidget {
-  OverflowBox({ Key key, this.minWidth, this.maxWidth, this.minHeight, this.maxHeight, Widget child })
-    : super(key: key, child: child);
+  OverflowBox({
+    Key key,
+    this.minWidth,
+    this.maxWidth,
+    this.minHeight,
+    this.maxHeight,
+    this.alignment: const FractionalOffset(0.5, 0.5),
+    Widget child
+  }) : super(key: key, child: child);
 
   /// The minimum width constraint to give the child. Set this to null (the
   /// default) to use the constraint from the parent instead.
@@ -616,18 +623,32 @@ class OverflowBox extends OneChildRenderObjectWidget {
   /// default) to use the constraint from the parent instead.
   final double maxHeight;
 
+  /// 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;
+
   RenderOverflowBox createRenderObject() => new RenderOverflowBox(
     minWidth: minWidth,
     maxWidth: maxWidth,
     minHeight: minHeight,
-    maxHeight: maxHeight
+    maxHeight: maxHeight,
+    alignment: alignment
   );
 
   void updateRenderObject(RenderOverflowBox renderObject, OverflowBox oldWidget) {
-    renderObject.minWidth = minWidth;
-    renderObject.maxWidth = maxWidth;
-    renderObject.minHeight = minHeight;
-    renderObject.maxHeight = maxHeight;
+    renderObject
+      ..minWidth = minWidth
+      ..maxWidth = maxWidth
+      ..minHeight = minHeight
+      ..maxHeight = maxHeight
+      ..alignment = alignment;
   }
 }
 
@@ -2047,7 +2068,7 @@ class Semantics extends OneChildRenderObjectWidget {
   /// 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.
@@ -2080,7 +2101,7 @@ class Semantics extends OneChildRenderObjectWidget {
     super.debugFillDescription(description);
     description.add('container: $container');
     if (checked != null);
-      description.add('checked: $checked');       
+      description.add('checked: $checked');
     if (label != null);
       description.add('label: "$label"');
   }

packages/flutter/test/rendering/overflow_test.dart

@@ -30,7 +30,8 @@ void main() {
       child: new RenderCustomPaint(
         child: child = new RenderOverflowBox(
           child: text = new RenderParagraph(new PlainTextSpan('Hello World')),
-          maxHeight: height1 / 2.0
+          maxHeight: height1 / 2.0,
+          alignment: const FractionalOffset(0.0, 0.0)
         ),
         painter: new TestCallbackPainter(
           onPaint: () {

packages/flutter/test/widget/fractionally_sized_box_test.dart

@@ -17,6 +17,7 @@ void main() {
         maxWidth: 100.0,
         minHeight: 0.0,
         maxHeight: 100.0,
+        alignment: const FractionalOffset(0.0, 0.0),
         child: new Center(
           child: new FractionallySizedBox(
             width: 0.5,

packages/flutter/test/widget/overflow_box_test.dart

@@ -0,0 +1,35 @@
+// 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 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('OverflowBox control test', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey inner = new GlobalKey();
+      tester.pumpWidget(new Align(
+        alignment: const FractionalOffset(1.0, 1.0),
+        child: new SizedBox(
+          width: 10.0,
+          height: 20.0,
+          child: new OverflowBox(
+            minWidth: 0.0,
+            maxWidth: 100.0,
+            minHeight: 0.0,
+            maxHeight: 50.0,
+            child: new Container(
+              key: inner
+            )
+          )
+        )
+      ));
+      RenderBox box = inner.currentContext.findRenderObject();
+      expect(box.localToGlobal(Point.origin), equals(const Point(745.0, 565.0)));
+      expect(box.size, equals(const Size(100.0, 50.0)));
+    });
+  });
+}