Merge pull request #2858 from abarth/physics_with_changing_behavior

Scrollable physics should be reasonable when sizes change

packages/flutter/lib/src/animation/animation_controller.dart

@@ -124,6 +124,12 @@ class AnimationController extends Animation<double>
     _checkStatusChanged();
   }
 
+  /// The amount of time that has passed between the time the animation started and the most recent tick of the animation.
+  ///
+  /// If the controller is not animating, the last elapsed duration is null;
+  Duration get lastElapsedDuration => _lastElapsedDuration;
+  Duration _lastElapsedDuration;
+
   /// Whether this animation is currently animating in either the forward or reverse direction.
   bool get isAnimating => _ticker.isTicking;
 
@@ -205,6 +211,7 @@ class AnimationController extends Animation<double>
     assert(simulation != null);
     assert(!isAnimating);
     _simulation = simulation;
+    _lastElapsedDuration = const Duration();
     _value = simulation.x(0.0).clamp(lowerBound, upperBound);
     Future<Null> result = _ticker.start();
     _checkStatusChanged();
@@ -214,6 +221,7 @@ class AnimationController extends Animation<double>
   /// Stops running this animation.
   void stop() {
     _simulation = null;
+    _lastElapsedDuration = null;
     _ticker.stop();
   }
 
@@ -233,6 +241,7 @@ class AnimationController extends Animation<double>
   }
 
   void _tick(Duration elapsed) {
+    _lastElapsedDuration = elapsed;
     double elapsedInSeconds = elapsed.inMicroseconds.toDouble() / Duration.MICROSECONDS_PER_SECOND;
     _value = _simulation.x(elapsedInSeconds).clamp(lowerBound, upperBound);
     if (_simulation.isDone(elapsedInSeconds))

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

@@ -768,7 +768,7 @@ class _TabBarState<T> extends ScrollableState<TabBar<T>> implements TabBarSelect
   }
 
   void _updateScrollBehavior() {
-    scrollTo(scrollBehavior.updateExtents(
+    didUpdateScrollBehavior(scrollBehavior.updateExtents(
       containerExtent: config.scrollDirection == Axis.vertical ? _viewportSize.height : _viewportSize.width,
       contentExtent: _tabWidths.reduce((double sum, double width) => sum + width),
       scrollOffset: scrollOffset
@@ -943,11 +943,11 @@ class _TabBarViewState<T> extends PageableListState<TabBarView<T>> implements Ta
 
   void _updateScrollBehaviorForSelectedIndex(int selectedIndex) {
     if (selectedIndex == 0) {
-      scrollTo(scrollBehavior.updateExtents(contentExtent: 2.0, containerExtent: 1.0, scrollOffset: 0.0));
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(contentExtent: 2.0, containerExtent: 1.0, scrollOffset: 0.0));
     } else if (selectedIndex == _tabCount - 1) {
-      scrollTo(scrollBehavior.updateExtents(contentExtent: 2.0, containerExtent: 1.0, scrollOffset: 1.0));
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(contentExtent: 2.0, containerExtent: 1.0, scrollOffset: 1.0));
     } else {
-      scrollTo(scrollBehavior.updateExtents(contentExtent: 3.0, containerExtent: 1.0, scrollOffset: 1.0));
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(contentExtent: 3.0, containerExtent: 1.0, scrollOffset: 1.0));
     }
   }
 

packages/flutter/lib/src/widgets/editable.dart

@@ -235,7 +235,7 @@ class RawInputLineState extends ScrollableState<RawInputLine> {
     // render object via our return value.
     _containerWidth = dimensions.containerSize.width;
     _contentWidth = dimensions.contentSize.width;
-    scrollTo(scrollBehavior.updateExtents(
+    didUpdateScrollBehavior(scrollBehavior.updateExtents(
       contentExtent: _contentWidth,
       containerExtent: _containerWidth,
       // Set the scroll offset to match the content width so that the

packages/flutter/lib/src/widgets/pageable_list.dart

@@ -147,7 +147,7 @@ class PageableListState<T extends PageableList> extends ScrollableState<T> {
 
   void _updateScrollBehavior() {
     config.scrollableListPainter?.contentExtent = _itemCount.toDouble();
-    scrollTo(scrollBehavior.updateExtents(
+    didUpdateScrollBehavior(scrollBehavior.updateExtents(
       contentExtent: _itemCount.toDouble(),
       containerExtent: 1.0,
       scrollOffset: scrollOffset

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

@@ -226,10 +226,12 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
     _scrollOffset = PageStorage.of(context)?.readState(context) ?? config.initialScrollOffset ?? 0.0;
   }
 
+  Simulation _simulation;
   AnimationController _controller;
 
   @override
   void dispose() {
+    _simulation = null;
     _controller.stop();
     super.dispose();
   }
@@ -358,6 +360,7 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
       return new Future<Null>.value();
 
     if (duration == null) {
+      _simulation = null;
       _controller.stop();
       _setScrollOffset(newScrollOffset);
       return new Future<Null>.value();
@@ -368,12 +371,27 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   Future<Null> _animateTo(double newScrollOffset, Duration duration, Curve curve) {
+    _simulation = null;
     _controller.stop();
     _controller.value = scrollOffset;
     _startScroll();
     return _controller.animateTo(newScrollOffset, duration: duration, curve: curve).then(_endScroll);
   }
 
+  void didUpdateScrollBehavior(double newScrollOffset) {
+    if (newScrollOffset == _scrollOffset)
+      return;
+    if (_numberOfInProgressScrolls > 0) {
+      if (_simulation != null) {
+        double dx = _simulation.dx(_controller.lastElapsedDuration.inMicroseconds / Duration.MICROSECONDS_PER_SECOND);
+        // TODO(abarth): We should be consistent about the units we use for velocity (i.e., per second).
+        _startToEndAnimation(dx / Duration.MILLISECONDS_PER_SECOND);
+      }
+      return;
+    }
+    scrollTo(newScrollOffset);
+  }
+
   /// Fling the scroll offset with the given velocity.
   ///
   /// Calling this function starts a physics-based animation of the scroll
@@ -390,17 +408,18 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   /// Calling this function starts a physics-based animation of the scroll
   /// offset either to a snap point or to within the scrolling bounds. The
   /// physics simulation used is determined by the scroll behavior.
-      Future<Null> settleScrollOffset() {
+  Future<Null> settleScrollOffset() {
     return _startToEndAnimation(0.0);
   }
 
   Future<Null> _startToEndAnimation(double scrollVelocity) {
+    _simulation = null;
     _controller.stop();
-    Simulation simulation = _createSnapSimulation(scrollVelocity) ?? _createFlingSimulation(scrollVelocity);
-    if (simulation == null)
+    _simulation = _createSnapSimulation(scrollVelocity) ?? _createFlingSimulation(scrollVelocity);
+    if (_simulation == null)
       return new Future<Null>.value();
     _startScroll();
-    return _controller.animateWith(simulation).then(_endScroll);
+    return _controller.animateWith(_simulation).then(_endScroll);
   }
 
   /// Whether this scrollable should attempt to snap scroll offsets.
@@ -471,6 +490,7 @@ abstract class ScrollableState<T extends Scrollable> extends State<T> {
   }
 
   void _handleDragDown(_) {
+    _simulation = null;
     _controller.stop();
   }
 
@@ -653,7 +673,7 @@ class _ScrollableViewportState extends ScrollableState<ScrollableViewport> {
     // render object via our return value.
     _viewportSize = config.scrollDirection == Axis.vertical ? dimensions.containerSize.height : dimensions.containerSize.width;
     _childSize = config.scrollDirection == Axis.vertical ? dimensions.contentSize.height : dimensions.contentSize.width;
-    scrollTo(scrollBehavior.updateExtents(
+    didUpdateScrollBehavior(scrollBehavior.updateExtents(
       contentExtent: _childSize,
       containerExtent: _viewportSize,
       scrollOffset: scrollOffset
@@ -819,7 +839,7 @@ class ScrollableMixedWidgetListState extends ScrollableState<ScrollableMixedWidg
     // setState() callback because we are called during layout and all
     // we're updating is the new offset, which we are providing to the
     // render object via our return value.
-    scrollTo(scrollBehavior.updateExtents(
+    didUpdateScrollBehavior(scrollBehavior.updateExtents(
       contentExtent: dimensions.contentSize.height,
       containerExtent: dimensions.containerSize.height,
       scrollOffset: scrollOffset

packages/flutter/lib/src/widgets/scrollable_grid.dart

@@ -50,7 +50,7 @@ class _ScrollableGridState extends ScrollableState<ScrollableGrid> {
 
   void _handleExtentsChanged(double contentExtent, double containerExtent) {
     setState(() {
-      scrollTo(scrollBehavior.updateExtents(
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(
         contentExtent: contentExtent,
         containerExtent: containerExtent,
         scrollOffset: scrollOffset

packages/flutter/lib/src/widgets/scrollable_list.dart

@@ -55,7 +55,7 @@ class _ScrollableListState extends ScrollableState<ScrollableList> {
   void _handleExtentsChanged(double contentExtent, double containerExtent) {
     config.scrollableListPainter?.contentExtent = contentExtent;
     setState(() {
-      scrollTo(scrollBehavior.updateExtents(
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(
         contentExtent: config.itemsWrap ? double.INFINITY : contentExtent,
         containerExtent: containerExtent,
         scrollOffset: scrollOffset
@@ -338,7 +338,7 @@ class _ScrollableLazyListState extends ScrollableState<ScrollableLazyList> {
   void _handleExtentsChanged(double contentExtent, double containerExtent) {
     config.scrollableListPainter?.contentExtent = contentExtent;
     setState(() {
-      scrollTo(scrollBehavior.updateExtents(
+      didUpdateScrollBehavior(scrollBehavior.updateExtents(
         contentExtent: contentExtent,
         containerExtent: containerExtent,
         scrollOffset: scrollOffset