Added LayoutChangedNotifier. (#5304)

Added a simple widget that automatically dispatches a
LayoutChangedNotification when its child changes layout.

packages/flutter/lib/src/material/material.dart

@@ -656,5 +656,4 @@ class _InkHighlight extends InkFeature implements InkHighlight {
       _paintHighlight(canvas, rect.shift(originOffset), paint);
     }
   }
-
 }

packages/flutter/lib/src/widgets/size_changed_layout_notifier.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 'package:flutter/widgets.dart';
+
+/// Indicates that the size of one of the descendants of the object receiving
+/// this notification has changed, and that therefore any assumptions about that
+/// layout are no longer valid.
+///
+/// See [LayoutChangedNotification].
+class SizeChangedLayoutNotificaion extends LayoutChangedNotification {}
+
+/// A widget that automatically dispatches a [SizeChangedLayoutNotifier] when
+/// the layout of its child changes.
+///
+/// Useful especially when having some complex, layout-changing animation within
+/// [Material] that is also interactive.
+class SizeChangedLayoutNotifier extends SingleChildRenderObjectWidget {
+  /// Creates a [SizeChangedLayoutNotifier] that dispatches layout changed
+  /// notifications when [child] changes layout.
+  SizeChangedLayoutNotifier({
+    Key key,
+    Widget child
+  }) : super(key: key, child: child);
+
+  @override
+  _RenderSizeChangedWithCallback createRenderObject(BuildContext context) {
+    return new _RenderSizeChangedWithCallback(
+      onLayoutChangedCallback: () {
+        new SizeChangedLayoutNotificaion().dispatch(context);
+      }
+    );
+  }
+}
+
+class _RenderSizeChangedWithCallback extends RenderProxyBox {
+  _RenderSizeChangedWithCallback({
+    RenderBox child,
+    this.onLayoutChangedCallback
+  }) : super(child);
+
+  VoidCallback onLayoutChangedCallback;
+  Size _oldSize;
+
+  @override
+  void performLayout() {
+    super.performLayout();
+    if (onLayoutChangedCallback != null && size != _oldSize)
+      onLayoutChangedCallback();
+    _oldSize = size;
+  }
+}

packages/flutter/lib/widgets.dart

@@ -50,6 +50,7 @@ export 'src/widgets/scrollable_grid.dart';
 export 'src/widgets/scrollable_list.dart';
 export 'src/widgets/scrollable.dart';
 export 'src/widgets/semantics_debugger.dart';
+export 'src/widgets/size_changed_layout_notifier.dart';
 export 'src/widgets/status_transitions.dart';
 export 'src/widgets/table.dart';
 export 'src/widgets/text_selection.dart';

packages/flutter/test/widget/size_changed_layout_notification_test.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.
+
+import 'package:flutter/widgets.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+class NotifyMaterial extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    new LayoutChangedNotification().dispatch(context);
+    return new Container();
+  }
+}
+
+void main() {
+  testWidgets('SizeChangedLayoutNotification test', (WidgetTester tester) async {
+    bool notified = false;
+
+    await tester.pumpWidget(
+      new NotificationListener<LayoutChangedNotification>(
+        onNotification: (LayoutChangedNotification notification) {
+          notified = true;
+          return true;
+        },
+        child: new SizeChangedLayoutNotifier(
+          child: new SizedBox(
+            width: 100.0,
+            height: 100.0
+          )
+        )
+      )
+    );
+
+    await tester.pumpWidget(
+      new NotificationListener<LayoutChangedNotification>(
+        onNotification: (LayoutChangedNotification notification) {
+          notified = true;
+          return true;
+        },
+        child: new SizeChangedLayoutNotifier(
+          child: new SizedBox(
+            width: 200.0,
+            height: 100.0
+          )
+        )
+      )
+    );
+
+    expect(notified, isTrue);
+  });
+}