Animation Demo performance tweaks (#8586)

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

@@ -161,12 +161,14 @@ class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
 // until they're all visible.
 class _AllSectionsLayout extends MultiChildLayoutDelegate {
   _AllSectionsLayout({
+    this.translation,
     this.tColumnToRow,
     this.tCollapsed,
     this.cardCount,
     this.selectedIndex,
   });
 
+  final FractionalOffset translation;
   final double tColumnToRow;
   final double tCollapsed;
   final int cardCount;
@@ -186,6 +188,7 @@ class _AllSectionsLayout extends MultiChildLayoutDelegate {
     final double columnCardWidth = size.width - columnCardX;
     final double columnCardHeight = size.height / cardCount;
     final double rowCardWidth = size.width;
+    final Offset offset = translation.alongSize(size);
     double columnCardY = 0.0;
     double rowCardX = -(selectedIndex * rowCardWidth);
 
@@ -209,7 +212,7 @@ class _AllSectionsLayout extends MultiChildLayoutDelegate {
       // Layout the card for index.
       final Rect columnCardRect = new Rect.fromLTWH(columnCardX, columnCardY, columnCardWidth, columnCardHeight);
       final Rect rowCardRect = new Rect.fromLTWH(rowCardX, 0.0, rowCardWidth, size.height);
-      final Rect cardRect = _interpolateRect(columnCardRect, rowCardRect);
+      final Rect cardRect = _interpolateRect(columnCardRect, rowCardRect).shift(offset);
       final String cardId = 'card$index';
       if (hasChild(cardId)) {
         // Add a small horizontal gap between the cards.
@@ -228,7 +231,7 @@ class _AllSectionsLayout extends MultiChildLayoutDelegate {
       final Point columnTitleOrigin = new Point(columnTitleX, columnTitleY);
       final Point rowTitleOrigin = new Point(centeredRowTitleX, rowTitleY);
       final Point titleOrigin = _interpolatePoint(columnTitleOrigin, rowTitleOrigin);
-      positionChild('title$index', titleOrigin.toOffset());
+      positionChild('title$index', titleOrigin.toOffset() + offset);
 
       // Layout the selection indicator for index.
       final Size indicatorSize = layoutChild('indicator$index', new BoxConstraints.loose(cardRect.size));
@@ -240,7 +243,7 @@ class _AllSectionsLayout extends MultiChildLayoutDelegate {
       final double rowIndicatorY = titleRect.bottomCenter.y + 16.0;
       final Point rowIndicatorOrigin = new Point(centeredRowIndicatorX, rowIndicatorY);
       final Point indicatorOrigin = _interpolatePoint(columnIndicatorOrigin, rowIndicatorOrigin);
-      positionChild('indicator$index', indicatorOrigin.toOffset());
+      positionChild('indicator$index', indicatorOrigin.toOffset() + offset);
 
       columnCardY += columnCardHeight;
       rowCardX += rowCardWidth;
@@ -257,31 +260,35 @@ class _AllSectionsLayout extends MultiChildLayoutDelegate {
   }
 }
 
-class _AllSectionsView extends StatelessWidget {
+class _AllSectionsView extends AnimatedWidget {
   _AllSectionsView({
     Key key,
+    this.sectionIndex,
     this.sections,
     this.selectedIndex,
     this.minHeight,
     this.midHeight,
     this.maxHeight,
     this.sectionCards: const <Widget>[],
-  }) : super(key: key) {
+  }) : super(key: key, listenable: selectedIndex) {
     assert(sections != null);
     assert(sectionCards != null);
     assert(sectionCards.length == sections.length);
-    assert(selectedIndex >= 0.0 && selectedIndex < sections.length.toDouble());
+    assert(sectionIndex >= 0 && sectionIndex < sections.length);
+    assert(selectedIndex != null);
+    assert(selectedIndex.value >= 0.0 && selectedIndex.value < sections.length.toDouble());
   }
 
+  final int sectionIndex;
   final List<Section> sections;
-  final double selectedIndex;
+  final ValueNotifier<double> selectedIndex;
   final double minHeight;
   final double midHeight;
   final double maxHeight;
   final List<Widget> sectionCards;
 
   double _selectedIndexDelta(int index) {
-    return (index.toDouble() - selectedIndex).abs().clamp(0.0, 1.0);
+    return (index.toDouble() - selectedIndex.value).abs().clamp(0.0, 1.0);
   }
 
   Widget _build(BuildContext context, BoxConstraints constraints) {
@@ -342,10 +349,11 @@ class _AllSectionsView extends StatelessWidget {
 
     return new CustomMultiChildLayout(
       delegate: new _AllSectionsLayout(
+        translation: new FractionalOffset(selectedIndex.value - sectionIndex, 0.0),
         tColumnToRow: tColumnToRow,
         tCollapsed: tCollapsed,
         cardCount: sections.length,
-        selectedIndex: selectedIndex,
+        selectedIndex: selectedIndex.value,
       ),
       children: children,
     );
@@ -370,7 +378,7 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
   final ScrollController _scrollController = new ScrollController();
   final PageController _headingPageController = new PageController();
   final PageController _detailsPageController = new PageController();
-  double _selectedIndex = 0.0;
+  ValueNotifier<double> selectedIndex = new ValueNotifier<double>(0.0);
 
   @override
   Widget build(BuildContext context) {
@@ -401,9 +409,7 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
 
   bool _handlePageNotification(ScrollNotification notification, PageController leader, PageController follower) {
     if (notification.depth == 0 && notification is ScrollUpdateNotification) {
-      setState(() {
-        _selectedIndex = leader.page;
-      });
+      selectedIndex.value = leader.page;
       if (follower.page != leader.page)
         follower.position.jumpTo(leader.position.pixels, settle: false);
     }
@@ -441,19 +447,17 @@ class _AnimationDemoHomeState extends State<AnimationDemoHome> {
     for (int index = 0; index < allSections.length; index++) {
       headings.add(new Container(
           decoration: new BoxDecoration(backgroundColor: _kAppBackgroundColor),
-          child: new FractionalTranslation(
-            translation: new FractionalOffset(_selectedIndex - index, 0.0),
           child: new ClipRect(
             child: new _AllSectionsView(
+              sectionIndex: index,
               sections: allSections,
-                selectedIndex: _selectedIndex,
+              selectedIndex: selectedIndex,
               minHeight: _kAppBarMinHeight,
               midHeight: _kAppBarMidHeight,
               maxHeight: maxHeight,
               sectionCards: sectionCards,
             ),
           ),
-          ),
         )
       );
     }

packages/flutter/lib/src/foundation/change_notifier.dart

@@ -178,4 +178,7 @@ class ValueNotifier<T> extends ChangeNotifier {
     _value = newValue;
     notifyListeners();
   }
+
+  @override
+  String toString() => '<$runtimeType>(value: $value)';
 }