Add support for autolayout to widgets

This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.

examples/layers/rendering/autolayout.dart

-// Copyright 2015 The Chromium Authors. All rights reserved.
+// Copyright 2016 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.
 
@@ -8,33 +8,16 @@
 import 'package:cassowary/cassowary.dart' as al;
 import 'package:flutter/rendering.dart';
 
-void main() {
-  RenderDecoratedBox c1 = new RenderDecoratedBox(
-    decoration: new BoxDecoration(backgroundColor: const Color(0xFFFF0000))
-  );
+class _MyAutoLayoutDelegate extends AutoLayoutDelegate {
+  AutoLayoutParams p1 = new AutoLayoutParams();
+  AutoLayoutParams p2 = new AutoLayoutParams();
+  AutoLayoutParams p3 = new AutoLayoutParams();
+  AutoLayoutParams p4 = new AutoLayoutParams();
 
-  RenderDecoratedBox c2 = new RenderDecoratedBox(
-    decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FF00))
-  );
-
-  RenderDecoratedBox c3 = new RenderDecoratedBox(
-    decoration: new BoxDecoration(backgroundColor: const Color(0xFF0000FF))
-  );
-
-  RenderDecoratedBox c4 = new RenderDecoratedBox(
-    decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF))
-  );
-
-  RenderAutoLayout root = new RenderAutoLayout(children: <RenderBox>[c1, c2, c3, c4]);
-
-  AutoLayoutParentData p1 = c1.parentData;
-  AutoLayoutParentData p2 = c2.parentData;
-  AutoLayoutParentData p3 = c3.parentData;
-  AutoLayoutParentData p4 = c4.parentData;
-
-  root.addConstraints(<al.Constraint>[
+  List<al.Constraint> getConstraints(AutoLayoutParams parentParams) {
+    return <al.Constraint>[
       // Sum of widths of each box must be equal to that of the container
-    (p1.width + p2.width + p3.width == root.width) as al.Constraint,
+      (p1.width + p2.width + p3.width == parentParams.width) as al.Constraint,
 
       // The boxes must be stacked left to right
       p1.rightEdge <= p2.leftEdge,
@@ -50,7 +33,7 @@ void main() {
       // The height of the three boxes should be equal to that of the container
       (p1.height == p2.height) as al.Constraint,
       (p2.height == p3.height) as al.Constraint,
-    (p3.height == root.height) as al.Constraint,
+      (p3.height == parentParams.height) as al.Constraint,
 
       // The fourth box should be half as wide as the second and must be attached
       // to the right edge of the same (by its center)
@@ -58,7 +41,45 @@ void main() {
       (p4.height == al.cm(50.0)) as al.Constraint,
       (p4.horizontalCenter == p2.rightEdge) as al.Constraint,
       (p4.verticalCenter == p2.height / al.cm(2.0)) as al.Constraint,
-  ]);
+    ];
+  }
+
+  bool shouldUpdateConstraints(AutoLayoutDelegate oldDelegate) => true;
+}
+
+void main() {
+  RenderDecoratedBox c1 = new RenderDecoratedBox(
+    decoration: new BoxDecoration(backgroundColor: const Color(0xFFFF0000))
+  );
+
+  RenderDecoratedBox c2 = new RenderDecoratedBox(
+    decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FF00))
+  );
+
+  RenderDecoratedBox c3 = new RenderDecoratedBox(
+    decoration: new BoxDecoration(backgroundColor: const Color(0xFF0000FF))
+  );
+
+  RenderDecoratedBox c4 = new RenderDecoratedBox(
+    decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFFFF))
+  );
+
+  _MyAutoLayoutDelegate delegate = new _MyAutoLayoutDelegate();
+
+  RenderAutoLayout root = new RenderAutoLayout(
+    delegate: delegate,
+    children: <RenderBox>[c1, c2, c3, c4]
+  );
+
+  AutoLayoutParentData parentData1 = c1.parentData;
+  AutoLayoutParentData parentData2 = c2.parentData;
+  AutoLayoutParentData parentData3 = c3.parentData;
+  AutoLayoutParentData parentData4 = c4.parentData;
+
+  parentData1.params = delegate.p1;
+  parentData2.params = delegate.p2;
+  parentData3.params = delegate.p3;
+  parentData4.params = delegate.p4;
 
   new RenderingFlutterBinding(root: root);
 }

examples/layers/widgets/autolayout.dart

+// Copyright 2016 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.
+
+// This example shows how to use the Cassowary autolayout system with widgets.
+
+import 'package:cassowary/cassowary.dart' as al;
+import 'package:flutter/widgets.dart';
+
+class _MyAutoLayoutDelegate extends AutoLayoutDelegate {
+  AutoLayoutParams p1 = new AutoLayoutParams();
+  AutoLayoutParams p2 = new AutoLayoutParams();
+  AutoLayoutParams p3 = new AutoLayoutParams();
+  AutoLayoutParams p4 = new AutoLayoutParams();
+
+  List<al.Constraint> getConstraints(AutoLayoutParams parentParams) {
+    return <al.Constraint>[
+      // Sum of widths of each box must be equal to that of the container
+      (p1.width + p2.width + p3.width == parentParams.width) as al.Constraint,
+
+      // The boxes must be stacked left to right
+      p1.rightEdge <= p2.leftEdge,
+      p2.rightEdge <= p3.leftEdge,
+
+      // The widths of the first and the third boxes should be equal
+      (p1.width == p3.width) as al.Constraint,
+
+      // The width of the second box should be twice as much as that of the first
+      // and third
+      (p2.width * al.cm(2.0) == p1.width) as al.Constraint,
+
+      // The height of the three boxes should be equal to that of the container
+      (p1.height == p2.height) as al.Constraint,
+      (p2.height == p3.height) as al.Constraint,
+      (p3.height == parentParams.height) as al.Constraint,
+
+      // The fourth box should be half as wide as the second and must be attached
+      // to the right edge of the same (by its center)
+      (p4.width == p2.width / al.cm(2.0)) as al.Constraint,
+      (p4.height == al.cm(50.0)) as al.Constraint,
+      (p4.horizontalCenter == p2.rightEdge) as al.Constraint,
+      (p4.verticalCenter == p2.height / al.cm(2.0)) as al.Constraint,
+    ];
+  }
+
+  bool shouldUpdateConstraints(AutoLayoutDelegate oldDelegate) => true;
+}
+
+class ColoredBox extends StatelessComponent {
+  ColoredBox({ Key key, this.params, this.color }) : super(key: key);
+
+  final AutoLayoutParams params;
+  final Color color;
+
+  Widget build(BuildContext context) {
+    return new AutoLayoutChild(
+      params: params,
+      child: new DecoratedBox(
+        decoration: new BoxDecoration(backgroundColor: color)
+      )
+    );
+  }
+}
+
+class ColoredBoxes extends StatefulComponent {
+  _ColoredBoxesState createState() => new _ColoredBoxesState();
+}
+
+class _ColoredBoxesState extends State<ColoredBoxes> {
+  final _MyAutoLayoutDelegate delegate = new _MyAutoLayoutDelegate();
+
+  Widget build(BuildContext context) {
+    return new AutoLayout(
+      delegate: delegate,
+      children: <Widget>[
+        new ColoredBox(params: delegate.p1, color: const Color(0xFFFF0000)),
+        new ColoredBox(params: delegate.p2, color: const Color(0xFF00FF00)),
+        new ColoredBox(params: delegate.p3, color: const Color(0xFF0000FF)),
+        new ColoredBox(params: delegate.p4, color: const Color(0xFFFFFFFF)),
+      ]
+    );
+  }
+}
+
+void main() {
+  runApp(new ColoredBoxes());
+}

packages/flutter/lib/src/widgets/auto_layout.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 'package:flutter/rendering.dart';
+
+import 'framework.dart';
+
+export 'package:flutter/rendering.dart' show
+    AutoLayoutParams,
+    AutoLayoutDelegate;
+
+class AutoLayout extends MultiChildRenderObjectWidget {
+  AutoLayout({
+    Key key,
+    this.delegate,
+    List<Widget> children: const <Widget>[]
+  }) : super(key: key, children: children);
+
+  final AutoLayoutDelegate delegate;
+
+  RenderAutoLayout createRenderObject() => new RenderAutoLayout(delegate: delegate);
+
+  void updateRenderObject(RenderAutoLayout renderObject, AutoLayout oldWidget) {
+    renderObject.delegate = delegate;
+  }
+}
+
+class AutoLayoutChild extends ParentDataWidget<AutoLayout> {
+  AutoLayoutChild({ Key key, this.params, Widget child })
+    : super(key: key, child: child);
+
+  final AutoLayoutParams params;
+
+  void applyParentData(RenderObject renderObject) {
+    assert(renderObject.parentData is AutoLayoutParentData);
+    final AutoLayoutParentData parentData = renderObject.parentData;
+    // AutoLayoutParentData filters out redundant writes and marks needs layout
+    // as appropriate.
+    parentData.params = params;
+  }
+}

packages/flutter/lib/widgets.dart

@@ -6,6 +6,7 @@
 library widgets;
 
 export 'src/widgets/asset_vendor.dart';
+export 'src/widgets/auto_layout.dart';
 export 'src/widgets/basic.dart';
 export 'src/widgets/binding.dart';
 export 'src/widgets/checked_mode_banner.dart';