Added ExpandIcon widget. (#5463)

Added a widget that automaitcally rotates the expansion icon and
offers a callback interface that captures presses.

packages/flutter/lib/material.dart

@@ -34,6 +34,7 @@ export 'src/material/drawer.dart';
 export 'src/material/drawer_header.dart';
 export 'src/material/drawer_item.dart';
 export 'src/material/drop_down.dart';
+export 'src/material/expand_icon.dart';
 export 'src/material/flat_button.dart';
 export 'src/material/flexible_space_bar.dart';
 export 'src/material/floating_action_button.dart';

packages/flutter/lib/src/material/expand_icon.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/widgets.dart';
+import 'package:meta/meta.dart';
+
+import 'colors.dart';
+import 'debug.dart';
+import 'icon.dart';
+import 'icons.dart';
+import 'icon_button.dart';
+import 'theme.dart';
+
+/// A widget representing a rotating expand/collapse button. The icon rotates
+/// 180 deg when pressed, then reverts the animation on a second press.
+/// The underlying icon is [Icons.expand_more].
+///
+/// See [IconButton] for a more general implementation of a pressable button
+/// with an icon.
+class ExpandIcon extends StatefulWidget {
+  /// Creates an [ExpandIcon] with the given padding, and a callback that is
+  /// triggered when the icon is pressed.
+  ExpandIcon({
+    Key key,
+    this.size: 24.0,
+    @required this.onPressed,
+    this.padding: const EdgeInsets.all(8.0)
+  }) : super(key: key) {
+    assert(this.size != null);
+    assert(this.padding != null);
+  }
+
+  /// The size of the icon.
+  ///
+  /// This property must not be null. It defaults to 24.0.
+  final double size;
+
+  /// The callback triggered when the icon is pressed and the state changes
+  /// between expanded and collapsed.
+  ///
+  /// If this is set to null, the button will be disabled.
+  final ValueChanged<bool> onPressed;
+
+  /// The padding around the icon. The entire padded icon will reactb to input
+  /// gestures.
+  ///
+  /// This property must not be null. It defaults to 8.0 padding on all sides.
+  final EdgeInsets padding;
+
+  @override
+  _ExpandIconState createState() => new _ExpandIconState();
+}
+
+class _ExpandIconState extends State<ExpandIcon> {
+  AnimationController _controller;
+  Animation<double> _iconTurns;
+  bool _isExpanded = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _controller = new AnimationController(duration: kThemeAnimationDuration);
+    _iconTurns = new Tween<double>(begin: 0.0, end: 0.5).animate(
+      new CurvedAnimation(
+        parent: _controller,
+        curve: Curves.fastOutSlowIn
+      )
+    );
+  }
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  void _handlePressed() {
+    setState(() {
+      _isExpanded = !_isExpanded;
+      if (_isExpanded)
+        _controller.forward();
+      else
+        _controller.reverse();
+    });
+
+    if (config.onPressed != null)
+      config.onPressed(_isExpanded);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    assert(debugCheckHasMaterial(context));
+    return new IconButton(
+      padding: config.padding,
+      color: Colors.black38,
+      onPressed: config.onPressed == null ? null : _handlePressed,
+      icon: new RotationTransition(
+        turns: _iconTurns,
+        child: new Icon(Icons.expand_more)
+      )
+    );
+  }
+}

packages/flutter/test/material/expand_icon_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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('ExpandIcon test', (WidgetTester tester) async {
+    bool expanded = false;
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Center(
+          child: new ExpandIcon(
+            onPressed: (bool isExpanded) {
+              expanded = !expanded;
+            }
+          )
+        )
+      )
+    );
+
+    expect(expanded, isFalse);
+    await tester.tap(find.byType(ExpandIcon));
+    expect(expanded, isTrue);
+    await tester.pump(const Duration(milliseconds: 100));
+    await tester.tap(find.byType(ExpandIcon));
+    expect(expanded, isFalse);
+  });
+
+  testWidgets('ExpandIcon disabled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      new Material(
+        child: new Center(
+          child: new ExpandIcon(
+            onPressed: null
+          )
+        )
+      )
+    );
+
+    IconTheme iconTheme = tester.firstWidget(find.byType(IconTheme));
+    expect(iconTheme.data.color, equals(Colors.black26));
+  });
+}