Changed ExpandIcon to accept default state. (#5521)

Changed the ExpandIcon constructor to take a default expansion state.
If the widget gets rebuilt with a different expansion value, the
animation will trigger but the callback will not.

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

@@ -23,21 +23,29 @@ class ExpandIcon extends StatefulWidget {
   /// triggered when the icon is pressed.
   ExpandIcon({
     Key key,
+    this.isExpanded: false,
     this.size: 24.0,
     @required this.onPressed,
     this.padding: const EdgeInsets.all(8.0)
   }) : super(key: key) {
+    assert(this.isExpanded != null);
     assert(this.size != null);
     assert(this.padding != null);
   }
 
+  /// Whether the icon is in an expanded state.
+  ///
+  /// Rebuilding the widget with a different [isExpanded] value will trigger
+  /// the animation, but will not trigger the [onPressed] callback.
+  final bool isExpanded;
+
   /// 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.
+  /// between expanded and collapsed. The value passed to the current state.
   ///
   /// If this is set to null, the button will be disabled.
   final ValueChanged<bool> onPressed;
@@ -55,7 +63,6 @@ class ExpandIcon extends StatefulWidget {
 class _ExpandIconState extends State<ExpandIcon> {
   AnimationController _controller;
   Animation<double> _iconTurns;
-  bool _isExpanded = false;
 
   @override
   void initState() {
@@ -75,17 +82,20 @@ class _ExpandIconState extends State<ExpandIcon> {
     super.dispose();
   }
 
-  void _handlePressed() {
-    setState(() {
-      _isExpanded = !_isExpanded;
-      if (_isExpanded)
+  @override
+  void didUpdateConfig(ExpandIcon oldConfig) {
+    if (config.isExpanded != oldConfig.isExpanded) {
+      if (config.isExpanded) {
         _controller.forward();
-      else
+      } else {
         _controller.reverse();
-    });
+      }
+    }
+  }
 
+  void _handlePressed() {
     if (config.onPressed != null)
-      config.onPressed(_isExpanded);
+      config.onPressed(config.isExpanded);
   }
 
   @override

packages/flutter/test/material/expand_icon_test.dart

@@ -43,4 +43,36 @@ void main() {
     IconTheme iconTheme = tester.firstWidget(find.byType(IconTheme));
     expect(iconTheme.data.color, equals(Colors.black26));
   });
+
+  testWidgets('ExpandIcon test isExpanded does not trigger callback', (WidgetTester tester) async {
+    bool expanded = false;
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Center(
+          child: new ExpandIcon(
+            isExpanded: false,
+            onPressed: (bool isExpanded) {
+              expanded = !expanded;
+            }
+          )
+        )
+      )
+    );
+
+    await tester.pumpWidget(
+      new Material(
+        child: new Center(
+          child: new ExpandIcon(
+            isExpanded: true,
+            onPressed: (bool isExpanded) {
+              expanded = !expanded;
+            }
+          )
+        )
+      )
+    );
+
+    expect(expanded, isFalse);
+  });
 }