Add an AspectRatio widget for doing proportional height sizing

sky/sdk/lib/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:sky' as sky;
 
 import 'package:sky/base/debug.dart';
 import 'package:sky/painting/box_painter.dart';
 import 'package:sky/rendering/object.dart';
@@ skipping 573 matching lines @@
       child.layout(_additionalConstraints.apply(constraints), parentUsesSize: true);
       size = child.size;
     } else {
       size = _additionalConstraints.apply(constraints).constrain(Size.zero);
     }
   }
 
   String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}additionalConstraints: ${additionalConstraints}\n';
 }
 
+class RenderAspectRatio extends RenderProxyBox {
+  RenderAspectRatio({
+    RenderBox child,
+    double aspectRatio
+  }) : super(child), _aspectRatio = aspectRatio {
+    assert(_aspectRatio != null);
+  }
+
+  double _aspectRatio;
+  double get aspectRatio => _aspectRatio;
+  void set aspectRatio (double value) {
+    assert(value != null);
+    if (_aspectRatio == value)
+      return;
+    _aspectRatio = value;
+    markNeedsLayout();
+  }
+
+  double getMinIntrinsicHeight(BoxConstraints constraints) {
+    return _applyAspectRatio(constraints).height;
+  }
+
+  double getMaxIntrinsicHeight(BoxConstraints constraints) {
+    return _applyAspectRatio(constraints).height;
+  }
+
+  Size _applyAspectRatio(BoxConstraints constraints) {
+    double width = constraints.constrainWidth();
+    double height = constraints.constrainHeight(width / _aspectRatio);
+    return new Size(width, height);
+  }
+
+  bool get sizedByParent => true;
+
+  void performResize() {
+    size = _applyAspectRatio(constraints);
+  }
+
+  void performLayout() {
+    if (child != null)
+      child.layout(new BoxConstraints.tight(size));
+  }
+
+  String debugDescribeSettings(String prefix) => '${super.debugDescribeSettings(prefix)}${prefix}aspectRatio: ${aspectRatio}\n';
+}
+
 class RenderShrinkWrapWidth extends RenderProxyBox {
 
   // This class will attempt to size its child to the child's maximum
   // intrinsic width, snapped to a multiple of the stepWidth, if one
   // is provided, and given the provided constraints; and will then
   // adopt the child's resulting dimensions.
 
   // Note: laying out this class is relatively expensive. Avoid using
   // it where possible.
 
@@ skipping 1018 matching lines @@
 
   void defaultPaint(PaintingCanvas canvas, Offset offset) {
     RenderBox child = firstChild;
     while (child != null) {
       assert(child.parentData is ParentDataType);
       canvas.paintChild(child, child.parentData.position + offset);
       child = child.parentData.nextSibling;
     }
   }
 }