Gallery animation demo back button update (#9853)

examples/flutter_gallery/lib/demo/animation/home.dart

@@ -15,6 +15,8 @@ import 'sections.dart';
 import 'widgets.dart';
 
 const Color _kAppBackgroundColor = const Color(0xFF353662);
+const Duration _kScrollDuration = const Duration(milliseconds: 400);
+const Curve _kScrollCurve = Curves.fastOutSlowIn;
 
 // This app's contents start out at _kHeadingMaxHeight and they function like
 // an appbar. Initially the appbar occupies most of the screen and its section
@@ -449,6 +451,13 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
     );
   }
 
+  void _handleBackButton(double midScrollOffset) {
+    if (_scrollController.offset >= midScrollOffset)
+      _scrollController.animateTo(0.0, curve: _kScrollCurve, duration: _kScrollDuration);
+    else
+      Navigator.of(context).maybePop();
+  }
+
   // Only enable paging for the heading when the user has scrolled to midScrollOffset.
   // Paging is enabled/disabled by setting the heading's PageView scroll physics.
   bool _handleScrollNotification(ScrollNotification notification, double midScrollOffset) {
@@ -466,18 +475,16 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
   }
 
   void _maybeScroll(double midScrollOffset, int pageIndex, double xOffset) {
-    const Duration duration = const Duration(milliseconds: 400);
-    const Curve curve = Curves.fastOutSlowIn;
     if (_scrollController.offset < midScrollOffset) {
       // Scroll the overall list to the point where only one section card shows.
       // At the same time scroll the PageViews to the page at pageIndex.
-      _headingPageController.animateToPage(pageIndex, curve: curve, duration: duration);
-      _scrollController.animateTo(midScrollOffset, curve: curve, duration: duration);
+      _headingPageController.animateToPage(pageIndex, curve: _kScrollCurve, duration: _kScrollDuration);
+      _scrollController.animateTo(midScrollOffset, curve: _kScrollCurve, duration: _kScrollDuration);
     } else {
       // One one section card is showing: scroll one page forward or back.
       final double centerX = _headingPageController.position.viewportDimension / 2.0;
       final int newPageIndex = xOffset > centerX ? pageIndex + 1 : pageIndex - 1;
-      _headingPageController.animateToPage(newPageIndex, curve: curve, duration: duration);
+      _headingPageController.animateToPage(newPageIndex, curve: _kScrollCurve, duration: _kScrollDuration);
     }
   }
 
@@ -605,9 +612,15 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
           new Positioned(
             top: statusBarHeight,
             left: 0.0,
-            child: const IconTheme(
+            child: new IconTheme(
               data: const IconThemeData(color: Colors.white),
-              child: const BackButton(),
+              child: new IconButton(
+                icon: const BackButtonIcon(),
+                tooltip: 'Back',
+                onPressed: () {
+                  _handleBackButton(appBarMidScrollOffset);
+                }
+              ),
             ),
           ),
         ],

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

@@ -9,6 +9,35 @@ import 'icon_button.dart';
 import 'icons.dart';
 import 'theme.dart';
 
+/// A "back" icon that's appropriate for the current [TargetPlatform].
+///
+/// See also:
+///
+///  * [BackButton], an [IconButton] with a [BackButtonIcon] that calls
+///    [Navigator.maybePop] to return to the previous route.
+///  * [IconButton], which is a more general widget for creating buttons
+///    with icons.
+///  * [Icon], a material design icon.
+class BackButtonIcon extends StatelessWidget {
+  const BackButtonIcon({ Key key }) : super(key: key);
+
+  /// Returns tha appropriate "back" icon for the given `platform`.
+  static IconData _getIconData(TargetPlatform platform) {
+    switch (platform) {
+      case TargetPlatform.android:
+      case TargetPlatform.fuchsia:
+        return Icons.arrow_back;
+      case TargetPlatform.iOS:
+        return Icons.arrow_back_ios;
+    }
+    assert(false);
+    return null;
+  }
+
+  @override
+  Widget build(BuildContext context) => new Icon(_getIconData(Theme.of(context).platform));
+}
+
 /// A material design back button.
 ///
 /// A [BackButton] is an [IconButton] with a "back" icon appropriate for the
@@ -27,6 +56,8 @@ import 'theme.dart';
 ///
 ///  * [AppBar], which automatically uses a [BackButton] in its
 ///    [AppBar.leading] slot when appropriate.
+///  * [BackButtonIcon], which is useful if you need to create a back button
+///    that responds differently to being pressed.
 ///  * [IconButton], which is a more general widget for creating buttons with
 ///    icons.
 ///  * [CloseButton], an alternative which may be more appropriate for leaf
@@ -36,27 +67,14 @@ class BackButton extends StatelessWidget {
   /// target platform.
   const BackButton({ Key key }) : super(key: key);
 
-  /// Returns tha appropriate "back" icon for the given `platform`.
-  static IconData getIconData(TargetPlatform platform) {
-    switch (platform) {
-      case TargetPlatform.android:
-      case TargetPlatform.fuchsia:
-        return Icons.arrow_back;
-      case TargetPlatform.iOS:
-        return Icons.arrow_back_ios;
-    }
-    assert(false);
-    return null;
-  }
-
   @override
   Widget build(BuildContext context) {
     return new IconButton(
-      icon: new Icon(getIconData(Theme.of(context).platform)),
+      icon: const BackButtonIcon(),
       tooltip: 'Back', // TODO(ianh): Figure out how to localize this string
       onPressed: () {
         Navigator.of(context).maybePop();
-      },
+      }
     );
   }
 }

packages/flutter/test/material/back_button_test.dart

@@ -32,4 +32,31 @@ void main() {
 
     expect(find.text('Home'), findsOneWidget);
   });
+
+  testWidgets('BackButton icon', (WidgetTester tester) async {
+    final Key iOSKey = new UniqueKey();
+    final Key androidKey = new UniqueKey();
+
+
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Column(
+          children: <Widget>[
+            new Theme(
+              data: new ThemeData(platform: TargetPlatform.iOS),
+              child: new BackButtonIcon(key: iOSKey),
+            ),
+            new Theme(
+              data: new ThemeData(platform: TargetPlatform.android),
+              child: new BackButtonIcon(key: androidKey),
+            ),
+          ],
+        ),
+      ),
+    );
+
+    final Icon iOSIcon = tester.widget(find.descendant(of: find.byKey(iOSKey), matching: find.byType(Icon)));
+    final Icon androidIcon = tester.widget(find.descendant(of: find.byKey(androidKey), matching: find.byType(Icon)));
+    expect(iOSIcon == androidIcon, false);
+  });
 }