Snap scrolling: additional tests, cleanup

examples/widgets/card_collection.dart

@@ -70,6 +70,36 @@ class CardCollectionAppState extends State<CardCollectionApp> {
     _initCardModels();
   }
 
+  double _variableSizeToSnapOffset(double scrollOffset) {
+    double cumulativeHeight = 0.0;
+    double  margins = 8.0;
+    List<double> cumulativeHeights = _cardModels.map((card) {
+      cumulativeHeight += card.height + margins;
+      return cumulativeHeight;
+    })
+    .toList();
+
+    double offsetForIndex(int i) {
+      return 12.0 + (margins + _cardModels[i].height) / 2.0 + ((i == 0) ? 0.0 : cumulativeHeights[i - 1]);
+    }
+
+    for (int i = 0; i <  cumulativeHeights.length; i++) {
+      if (cumulativeHeights[i] >= scrollOffset)
+        return offsetForIndex(i);
+    }
+    return offsetForIndex(cumulativeHeights.length - 1);
+  }
+
+  double _fixedSizeToSnapOffset(double scrollOffset) {
+    double cardHeight = _cardModels[0].height;
+    int cardIndex = (scrollOffset.clamp(0.0, cardHeight * (_cardModels.length - 1)) / cardHeight).floor();
+    return 12.0 + cardIndex * cardHeight + cardHeight * 0.5;
+  }
+
+  double _toSnapOffset(double scrollOffset) {
+    return _fixedSizeCards ? _fixedSizeToSnapOffset(scrollOffset) : _variableSizeToSnapOffset(scrollOffset);
+  }
+
   void dismissCard(CardModel card) {
     if (_cardModels.contains(card)) {
       setState(() {
@@ -155,7 +185,7 @@ class CardCollectionAppState extends State<CardCollectionApp> {
           new DrawerHeader(child: new Text('Options')),
           buildDrawerCheckbox("Snap fling scrolls to center", _snapToCenter, _toggleSnapToCenter),
           buildDrawerCheckbox("Fixed size cards", _fixedSizeCards, _toggleFixedSizeCards),
-          new DrawerDivider(),//buildDrawerSpacerItem(),
+          new DrawerDivider(),
           buildDrawerRadioItem(DismissDirection.horizontal, 'action/code'),
           buildDrawerRadioItem(DismissDirection.left, 'navigation/arrow_back'),
           buildDrawerRadioItem(DismissDirection.right, 'navigation/arrow_forward'),
@@ -255,33 +285,6 @@ class CardCollectionAppState extends State<CardCollectionApp> {
     });
   }
 
-  double _variableSizeToSnapOffset(double scrollOffset) {
-    double cumulativeHeight = 0.0;
-    double  margins = 8.0;
-    List<double> cumulativeHeights = _cardModels.map((card) {
-      cumulativeHeight += card.height + margins;
-      return cumulativeHeight;
-    })
-    .toList();
-
-    for (int i = 0; i <  cumulativeHeights.length; i++) {
-      if (cumulativeHeights[i] >= scrollOffset)
-        return 12.0 + (margins + _cardModels[i].height) / 2.0 + ((i == 0) ? 0.0 : cumulativeHeights[i - 1]);
-    }
-
-    assert(false);
-    return 0.0;
-  }
-
-  double _fixedSizeToSnapOffset(double scrollOffset) {
-    double cardHeight = _cardModels[0].height;
-    return 12.0 + (scrollOffset / cardHeight).floor() * cardHeight + cardHeight * 0.5;
-  }
-
-  double _toSnapOffset(double scrollOffset) {
-    return _fixedSizeCards ? _fixedSizeToSnapOffset(scrollOffset) : _variableSizeToSnapOffset(scrollOffset);
-  }
-
   Widget build(BuildContext context) {
 
     Widget cardCollection;

packages/flutter/lib/src/fn3/scrollable.dart

@@ -147,8 +147,9 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
       if (!endScrollOffset.isNaN) {
         double alignedScrollOffset = _alignedScrollSnapOffset(endScrollOffset);
         if (_scrollOffsetIsInBounds(alignedScrollOffset)) {
+          double snapVelocity = velocity.abs() * (alignedScrollOffset - scrollOffset).sign;
           Simulation toSnapSimulation = scrollBehavior.createSnapScrollSimulation(
-              scrollOffset, alignedScrollOffset, velocity);
+              scrollOffset, alignedScrollOffset, snapVelocity);
           _toEndAnimation.start(toSnapSimulation);
           return;
         }

packages/unit/test/widget/snap_scrolling_test.dart

@@ -25,9 +25,10 @@ double snapOffsetCallback(double offset) {
   return (offset / itemExtent).floor() * itemExtent;
 }
 
-Widget buildScrollableList() {
+Widget buildFrame() {
   scrollableListKey = new GlobalKey();
-  return new Container(
+  return new Center(
+    child: new Container(
       height: itemExtent * 2.0,
       child: new ScrollableList<int>(
         key: scrollableListKey,
@@ -37,6 +38,7 @@ Widget buildScrollableList() {
         itemBuilder: buildItem,
         itemExtent: itemExtent
       )
+    )
   );
 }
 
@@ -55,33 +57,71 @@ void fling(double velocity) {
 }
 
 void main() {
-  test('ScrollableList snap scrolling, fling(-800)', () {
   WidgetTester tester = new WidgetTester();
+  tester.pumpFrame(buildFrame());
 
-    tester.pumpFrame(new Center(child: buildScrollableList()));
+  test('ScrollableList snap scrolling, fling(-800)', () {
+    scrollOffset = 0.0;
+    tester.pumpFrameWithoutChange();
     expect(scrollOffset, 0.0);
 
+    double t0 = 0.0;
+    int dt = 1000;
     new FakeAsync().run((async) {
       fling(-800.0);
-      tester.pumpFrameWithoutChange(); // Start the scheduler at 0.0
-      tester.pumpFrameWithoutChange(1000.0);
-      async.elapse(new Duration(seconds: 1));
+      tester.pumpFrameWithoutChange(t0); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(t0 + dt);
+      async.elapse(new Duration(milliseconds: dt));
       expect(scrollOffset, closeTo(200.0, 1.0));
     });
   });
 
   test('ScrollableList snap scrolling, fling(-2000)', () {
-    WidgetTester tester = new WidgetTester();
-
-    tester.pumpFrame(new Center(child: buildScrollableList()));
+    scrollOffset = 0.0;
+    tester.pumpFrameWithoutChange();
     expect(scrollOffset, 0.0);
 
+    double t0 = 0.0;
+    int dt = 1000;
     new FakeAsync().run((async) {
       fling(-2000.0);
-      tester.pumpFrameWithoutChange(); // Start the scheduler at 0.0
-      tester.pumpFrameWithoutChange(1000.0);
-      async.elapse(new Duration(seconds: 1));
+      tester.pumpFrameWithoutChange(t0); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(t0 + dt);
+      async.elapse(new Duration(milliseconds: dt));
       expect(scrollOffset, closeTo(400.0, 1.0));
     });
   });
+
+  test('ScrollableList snap scrolling, fling(800)', () {
+    scrollOffset = 400.0;
+    tester.pumpFrameWithoutChange(1000.0);
+    expect(scrollOffset, 400.0);
+
+    double t0 = 0.0;
+    int dt = 2000;
+    new FakeAsync().run((async) {
+      fling(800.0);
+      tester.pumpFrameWithoutChange(t0); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(t0 + dt);
+      async.elapse(new Duration(milliseconds: dt));
+      expect(scrollOffset, closeTo(0.0, 1.0));
+    });
+  });
+
+  test('ScrollableList snap scrolling, fling(2000)', () {
+    scrollOffset = 800.0;
+    tester.pumpFrameWithoutChange(1000.0);
+    expect(scrollOffset, 800.0);
+
+    double t0 = 0.0;
+    int dt = 1500;
+    new FakeAsync().run((async) {
+      fling(2000.0);
+      tester.pumpFrameWithoutChange(t0); // Start the scheduler at 0.0
+      tester.pumpFrameWithoutChange(t0 + dt);
+      async.elapse(new Duration(milliseconds: dt));
+      expect(scrollOffset, closeTo(200.0, 1.0));
+    });
+  });
+
 }