Allow Programmatic Control of Draggable Sheet (#92440)

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

@@ -6,6 +6,7 @@ import 'dart:math' as math;
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
+import 'package:flutter/physics.dart';
 
 import 'basic.dart';
 import 'binding.dart';
@@ -34,6 +35,138 @@ typedef ScrollableWidgetBuilder = Widget Function(
   ScrollController scrollController,
 );
 
+/// Controls a [DraggableScrollableSheet].
+///
+/// Draggable scrollable controllers are typically stored as member variables in
+/// [State] objects and are reused in each [State.build]. Controllers can only
+/// be used to control one sheet at a time. A controller can be reused with a
+/// new sheet if the previous sheet has been disposed.
+///
+/// The controller's methods cannot be used until after the controller has been
+/// passed into a [DraggableScrollableSheet] and the sheet has run initState.
+class DraggableScrollableController {
+  _DraggableScrollableSheetScrollController? _attachedController;
+
+  /// Get the current size (as a fraction of the parent height) of the attached sheet.
+  double get size {
+    _assertAttached();
+    return _attachedController!.extent.currentSize;
+  }
+
+  /// Get the current pixel height of the attached sheet.
+  double get pixels {
+    _assertAttached();
+    return _attachedController!.extent.currentPixels;
+  }
+
+  /// Convert a sheet's size (fractional value of parent container height) to pixels.
+  double sizeToPixels(double size) {
+    _assertAttached();
+    return _attachedController!.extent.sizeToPixels(size);
+  }
+
+  /// Convert a sheet's pixel height to size (fractional value of parent container height).
+  double pixelsToSize(double pixels) {
+    _assertAttached();
+    return _attachedController!.extent.pixelsToSize(pixels);
+  }
+
+  /// Animates the attached sheet from its current size to [size] to the
+  /// provided new `size`, a fractional value of the parent container's height.
+  ///
+  /// Any active sheet animation is canceled. If the sheet's internal scrollable
+  /// is currently animating (e.g. responding to a user fling), that animation is
+  /// canceled as well.
+  ///
+  /// An animation will be interrupted whenever the user attempts to scroll
+  /// manually, whenever another activity is started, or when the sheet hits its
+  /// max or min size (e.g. if you animate to 1 but the max size is .8, the
+  /// animation will stop playing when it reaches .8).
+  ///
+  /// The duration must not be zero. To jump to a particular value without an
+  /// animation, use [jumpTo].
+  ///
+  /// When calling [animateTo] in widget tests, `await`ing the returned
+  /// [Future] may cause the test to hang and timeout. Instead, use
+  /// [WidgetTester.pumpAndSettle].
+  Future<void> animateTo(
+    double size, {
+    required Duration duration,
+    required Curve curve,
+  }) async {
+    _assertAttached();
+    assert(size >= 0 && size <= 1);
+    assert(duration != Duration.zero);
+    final AnimationController animationController = AnimationController.unbounded(
+      vsync: _attachedController!.position.context.vsync,
+      value: _attachedController!.extent.currentSize,
+    );
+    _attachedController!.position.goIdle();
+    // This disables any snapping until the next user interaction with the sheet.
+    _attachedController!.extent.hasDragged = false;
+    _attachedController!.extent.startActivity(onCanceled: () {
+      // Don't stop the controller if it's already finished and may have been disposed.
+      if (animationController.isAnimating) {
+        animationController.stop();
+      }
+    });
+    CurvedAnimation(parent: animationController, curve: curve).addListener(() {
+      _attachedController!.extent.updateSize(
+        animationController.value,
+        _attachedController!.position.context.notificationContext!,
+      );
+      if (animationController.value > _attachedController!.extent.maxSize ||
+          animationController.value < _attachedController!.extent.minSize) {
+        // Animation hit the max or min size, stop animating.
+        animationController.stop(canceled: false);
+      }
+    });
+    await animationController.animateTo(size, duration: duration);
+  }
+
+  /// Jumps the attached sheet from its current size to the given [size], a
+  /// fractional value of the parent container's height.
+  ///
+  /// If [size] is outside of a the attached sheet's min or max child size,
+  /// [jumpTo] will jump the sheet to the nearest valid size instead.
+  ///
+  /// Any active sheet animation is canceled. If the sheet's inner scrollable
+  /// is currently animating (e.g. responding to a user fling), that animation is
+  /// canceled as well.
+  void jumpTo(double size) {
+    _assertAttached();
+    assert(size >= 0 && size <= 1);
+    // Call start activity to interrupt any other playing activities.
+    _attachedController!.extent.startActivity(onCanceled: () {});
+    _attachedController!.position.goIdle();
+    _attachedController!.extent.hasDragged = false;
+    _attachedController!.extent.updateSize(size, _attachedController!.position.context.notificationContext!);
+  }
+
+  /// Reset the attached sheet to its initial size (see: [DraggableScrollableSheet.initialChildSize]).
+  void reset() {
+    _assertAttached();
+    _attachedController!.reset();
+  }
+
+  void _assertAttached() {
+    assert(
+      _attachedController != null,
+      'DraggableScrollableController is not attached to a sheet. A DraggableScrollableController '
+        'must be used in a DraggableScrollableSheet before any of its methods are called.',
+    );
+  }
+
+  void _attach(_DraggableScrollableSheetScrollController scrollController) {
+    assert(_attachedController == null, 'Draggable scrollable controller is already attached to a sheet.');
+    _attachedController = scrollController;
+  }
+
+  void _detach() {
+    _attachedController = null;
+  }
+}
+
 /// A container for a [Scrollable] that responds to drag gestures by resizing
 /// the scrollable until a limit is reached, and then scrolling.
 ///
@@ -118,6 +251,7 @@ class DraggableScrollableSheet extends StatefulWidget {
     this.expand = true,
     this.snap = false,
     this.snapSizes,
+    this.controller,
     required this.builder,
   })  : assert(initialChildSize != null),
         assert(minChildSize != null),
@@ -194,6 +328,9 @@ class DraggableScrollableSheet extends StatefulWidget {
   /// being built or since the last call to [DraggableScrollableActuator.reset].
   final List<double>? snapSizes;
 
+  /// A controller that can be used to programmatically control this sheet.
+  final DraggableScrollableController? controller;
+
   /// The builder that creates a child to display in this widget, which will
   /// use the provided [ScrollController] to enable dragging and scrolling
   /// of the contents.
@@ -293,7 +430,7 @@ class _DraggableSheetExtent {
     required this.initialSize,
     required this.onSizeChanged,
     ValueNotifier<double>? currentSize,
-    bool? hasChanged,
+    bool? hasDragged,
   })  : assert(minSize != null),
         assert(maxSize != null),
         assert(initialSize != null),
@@ -304,7 +441,9 @@ class _DraggableSheetExtent {
         _currentSize = (currentSize ?? ValueNotifier<double>(initialSize))
           ..addListener(onSizeChanged),
         availablePixels = double.infinity,
-        hasChanged = hasChanged ?? false;
+        hasDragged = hasDragged ?? false;
+
+  VoidCallback? _cancelActivity;
 
   final double minSize;
   final double maxSize;
@@ -315,19 +454,13 @@ class _DraggableSheetExtent {
   final VoidCallback onSizeChanged;
   double availablePixels;
 
-  // Used to disable snapping until the user interacts with the sheet. We set
-  // this to false on initialization and after programmatic interaction with the
-  // sheet to prevent snapping away from the initial size and after animateTo/jumpTo.
-  bool hasChanged;
+  // Used to disable snapping until the user has dragged on the sheet. We do
+  // this because we don't want to snap away from an initial or programmatically set size.
+  bool hasDragged;
 
   bool get isAtMin => minSize >= _currentSize.value;
   bool get isAtMax => maxSize <= _currentSize.value;
 
-  set currentSize(double value) {
-    assert(value != null);
-    hasChanged = true;
-    _currentSize.value = value.clamp(minSize, maxSize);
-  }
   double get currentSize => _currentSize.value;
   double get currentPixels => sizeToPixels(_currentSize.value);
 
@@ -335,18 +468,43 @@ class _DraggableSheetExtent {
   double get additionalMaxSize => isAtMax ? 0.0 : 1.0;
   List<double> get pixelSnapSizes => snapSizes.map(sizeToPixels).toList();
 
+  /// Start an activity that affects the sheet and register a cancel call back
+  /// that will be called if another activity starts.
+  ///
+  /// Note that `onCanceled` will get called even if the subsequent activity
+  /// started after this one finished so `onCanceled` should be safe to call at
+  /// any time.
+  void startActivity({required VoidCallback onCanceled}) {
+    _cancelActivity?.call();
+    _cancelActivity = onCanceled;
+  }
+
   /// The scroll position gets inputs in terms of pixels, but the size is
   /// expected to be expressed as a number between 0..1.
+  ///
+  /// This should only be called to respond to a user drag. To update the
+  /// size in response to a programmatic call, use [updateSize] directly.
   void addPixelDelta(double delta, BuildContext context) {
+    // Stop any playing sheet animations.
+    _cancelActivity?.call();
+    _cancelActivity = null;
+    // The user has interacted with the sheet, set `hasDragged` to true so that
+    // we'll snap if applicable.
+    hasDragged = true;
     if (availablePixels == 0) {
       return;
     }
     updateSize(currentSize + pixelsToSize(delta), context);
   }
 
-  /// Set the size to the new value. [newSize] should be a number between 0..1.
+  /// Set the size to the new value. [newSize] should be a number between
+  /// [minSize] and [maxSize].
+  ///
+  /// This can be triggered by a programmatic (e.g. controller triggered) change
+  /// or a user drag.
   void updateSize(double newSize, BuildContext context) {
-    currentSize = newSize;
+    assert(newSize != null);
+    _currentSize.value = newSize.clamp(minSize, maxSize);
     DraggableScrollableNotification(
       minExtent: minSize,
       maxExtent: maxSize,
@@ -360,8 +518,8 @@ class _DraggableSheetExtent {
     return pixels / availablePixels * maxSize;
   }
 
-  double sizeToPixels(double extent) {
-    return extent / maxSize * availablePixels;
+  double sizeToPixels(double size) {
+    return size / maxSize * availablePixels;
   }
 
   void dispose() {
@@ -383,11 +541,11 @@ class _DraggableSheetExtent {
       snapSizes: snapSizes,
       initialSize: initialSize,
       onSizeChanged: onSizeChanged,
-      // Use the possibly updated initialExtent if the user hasn't dragged yet.
-      currentSize: ValueNotifier<double>(hasChanged
+      // Use the possibly updated initialSize if the user hasn't dragged yet.
+      currentSize: ValueNotifier<double>(hasDragged
           ? _currentSize.value.clamp(minSize, maxSize)
           : initialSize),
-      hasChanged: hasChanged,
+      hasDragged: hasDragged,
     );
   }
 }
@@ -405,9 +563,10 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       snap: widget.snap,
       snapSizes: _impliedSnapSizes(),
       initialSize: widget.initialChildSize,
-      onSizeChanged: _setSize,
+      onSizeChanged: _setExtent,
     );
     _scrollController = _DraggableScrollableSheetScrollController(extent: _extent);
+    widget.controller?._attach(_scrollController);
   }
 
   List<double> _impliedSnapSizes() {
@@ -418,8 +577,6 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       assert(index == 0 || snapSize > widget.snapSizes![index - 1],
         '${_snapSizeErrorMessage(index)}\nSnap sizes must be in ascending order. ');
     }
-    widget.snapSizes?.asMap().forEach((int index, double snapSize) {
-    });
     // Ensure the snap sizes start and end with the min and max child sizes.
     if (widget.snapSizes == null || widget.snapSizes!.isEmpty) {
       return <double>[
@@ -444,22 +601,11 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
   void didChangeDependencies() {
     super.didChangeDependencies();
     if (_InheritedResetNotifier.shouldReset(context)) {
-      // jumpTo can result in trying to replace semantics during build.
-      // Just animate really fast.
-      // Avoid doing it at all if the offset is already 0.0.
-      if (_scrollController.offset != 0.0) {
-        _scrollController.animateTo(
-          0.0,
-          duration: const Duration(milliseconds: 1),
-          curve: Curves.linear,
-        );
-      }
-      _extent.hasChanged = false;
-      _extent._currentSize.value = _extent.initialSize;
+      _scrollController.reset();
     }
   }
 
-  void _setSize() {
+  void _setExtent() {
     setState(() {
       // _extent has been updated when this is called.
     });
@@ -482,6 +628,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
 
   @override
   void dispose() {
+    widget.controller?._detach();
     _scrollController.dispose();
     _extent.dispose();
     super.dispose();
@@ -495,7 +642,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
       snap: widget.snap,
       snapSizes: _impliedSnapSizes(),
       initialSize: widget.initialChildSize,
-      onSizeChanged: _setSize,
+      onSizeChanged: _setExtent,
     );
     // Modify the existing scroll controller instead of replacing it so that
     // developers listening to the controller do not have to rebuild their listeners.
@@ -513,7 +660,7 @@ class _DraggableScrollableSheetState extends State<DraggableScrollableSheet> {
 
   String _snapSizeErrorMessage(int invalidIndex) {
     final List<String> snapSizesWithIndicator = widget.snapSizes!.asMap().keys.map(
-          (int index) {
+      (int index) {
         final String snapSizeString = widget.snapSizes![index].toString();
         if (index == invalidIndex) {
           return '>>> $snapSizeString <<<';
@@ -576,6 +723,22 @@ class _DraggableScrollableSheetScrollController extends ScrollController {
   @override
   _DraggableScrollableSheetScrollPosition get position =>
       super.position as _DraggableScrollableSheetScrollPosition;
+
+  void reset() {
+    extent._cancelActivity?.call();
+    extent.hasDragged = false;
+    // jumpTo can result in trying to replace semantics during build.
+    // Just animate really fast.
+    // Avoid doing it at all if the offset is already 0.0.
+    if (offset != 0.0) {
+      animateTo(
+        0.0,
+        duration: const Duration(milliseconds: 1),
+        curve: Curves.linear,
+      );
+    }
+    extent.updateSize(extent.initialSize, position.context.notificationContext!);
+  }
 }
 
 /// A scroll position that manages scroll activities for
@@ -653,7 +816,7 @@ class _DraggableScrollableSheetScrollPosition
       },
     );
   }
-  bool get _shouldSnap => extent.snap && extent.hasChanged && !_isAtSnapSize;
+  bool get _shouldSnap => extent.snap && extent.hasDragged && !_isAtSnapSize;
 
   @override
   void dispose() {
@@ -742,6 +905,13 @@ class _DraggableScrollableSheetScrollPosition
 /// the user has tapped back if the sheet has started to cover more of the body
 /// than when at its initial position. This is important for users of assistive
 /// technology, where dragging may be difficult to communicate.
+///
+/// This is just a wrapper on top of [DraggableScrollableController]. It is
+/// primarily useful for controlling a sheet in a part of the widget tree that
+/// the current code does not control (e.g. library code trying to affect a sheet
+/// in library users' code). Generally, it's easier to control the sheet
+/// directly by creating a controller and passing the controller to the sheet in
+/// its constructor (see [DraggableScrollableSheet.controller]).
 class DraggableScrollableActuator extends StatelessWidget {
   /// Creates a widget that can notify descendent [DraggableScrollableSheet]s
   /// to reset to their initial position.

packages/flutter/test/widgets/draggable_scrollable_sheet_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 void main() {
   Widget _boilerplate(VoidCallback? onButtonPressed, {
+    DraggableScrollableController? controller,
     int itemCount = 100,
     double initialChildSize = .5,
     double maxChildSize = 1.0,
@@ -33,6 +34,7 @@ void main() {
             ),
             DraggableScrollableActuator(
               child: DraggableScrollableSheet(
+                controller: controller,
                 maxChildSize: maxChildSize,
                 minChildSize: minChildSize,
                 initialChildSize: initialChildSize,
@@ -346,33 +348,42 @@ void main() {
     ));
   }, variant: TargetPlatformVariant.all());
 
-  testWidgets('Does not snap away from initial child on reset', (WidgetTester tester) async {
-    const Key containerKey = ValueKey<String>('container');
-    const Key stackKey = ValueKey<String>('stack');
-    await tester.pumpWidget(_boilerplate(null,
-      snap: true,
-      containerKey: containerKey,
-      stackKey: stackKey,
-    ));
-    await tester.pumpAndSettle();
-    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+  for (final bool useActuator in <bool>[false, true]) {
+    testWidgets('Does not snap away from initial child on ${useActuator ? 'actuator' : 'controller'}.reset()', (WidgetTester tester) async {
+      const Key containerKey = ValueKey<String>('container');
+      const Key stackKey = ValueKey<String>('stack');
+      final DraggableScrollableController controller = DraggableScrollableController();
+      await tester.pumpWidget(_boilerplate(
+        null,
+        controller: controller,
+        snap: true,
+        containerKey: containerKey,
+        stackKey: stackKey,
+      ));
+      await tester.pumpAndSettle();
+      final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
 
-    await tester.drag(find.text('Item 1'), Offset(0, -.4 * screenHeight));
-    await tester.pumpAndSettle();
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(1.0, precisionErrorTolerance),
-    );
+      await tester.drag(find.text('Item 1'), Offset(0, -.4 * screenHeight));
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(1.0, precisionErrorTolerance),
+      );
 
-    DraggableScrollableActuator.reset(tester.element(find.byKey(containerKey)));
-    await tester.pumpAndSettle();
+      if (useActuator) {
+        DraggableScrollableActuator.reset(tester.element(find.byKey(containerKey)));
+      } else {
+        controller.reset();
+      }
+      await tester.pumpAndSettle();
 
-    // The sheet should have reset without snapping away from initial child.
-    expect(
-      tester.getSize(find.byKey(containerKey)).height / screenHeight,
-      closeTo(.5, precisionErrorTolerance),
-    );
-  }, variant: TargetPlatformVariant.all());
+      // The sheet should have reset without snapping away from initial child.
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.5, precisionErrorTolerance),
+      );
+    });
+  }
 
   testWidgets('Zero velocity drag snaps to nearest snap target', (WidgetTester tester) async {
     const Key stackKey = ValueKey<String>('stack');
@@ -695,4 +706,305 @@ void main() {
 
     expect(tester.takeException(), isNull);
   });
+
+  for (final bool shouldAnimate in <bool>[true, false]) {
+    testWidgets('Can ${shouldAnimate ? 'animate' : 'jump'} to arbitrary positions', (WidgetTester tester) async {
+      const Key stackKey = ValueKey<String>('stack');
+      const Key containerKey = ValueKey<String>('container');
+      final DraggableScrollableController controller = DraggableScrollableController();
+      await tester.pumpWidget(_boilerplate(
+        null,
+        controller: controller,
+        stackKey: stackKey,
+        containerKey: containerKey,
+      ));
+      await tester.pumpAndSettle();
+      final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+      // Use a local helper to animate so we can share code across a jumpTo test
+      // and an animateTo test.
+      void goTo(double size) => shouldAnimate
+          ? controller.animateTo(size, duration: const Duration(milliseconds: 200), curve: Curves.linear)
+          : controller.jumpTo(size);
+      // If we're animating, pump will call four times, two of which are for the
+      // animation duration.
+      final int expectedPumpCount = shouldAnimate ? 4 : 2;
+
+      goTo(.6);
+      expect(await tester.pumpAndSettle(), expectedPumpCount);
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.6, precisionErrorTolerance),
+      );
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 20'), findsOneWidget);
+      expect(find.text('Item 70'), findsNothing);
+
+      goTo(.4);
+      expect(await tester.pumpAndSettle(), expectedPumpCount);
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.4, precisionErrorTolerance),
+      );
+      expect(find.text('Item 1'), findsOneWidget);
+      expect(find.text('Item 20'), findsNothing);
+      expect(find.text('Item 70'), findsNothing);
+
+      await tester.fling(find.text('Item 1'), Offset(0, -screenHeight), 100);
+      await tester.pumpAndSettle();
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(1, precisionErrorTolerance),
+      );
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 20'), findsOneWidget);
+      expect(find.text('Item 70'), findsNothing);
+
+      // Programmatic control does not affect the inner scrollable's position.
+      goTo(.8);
+      expect(await tester.pumpAndSettle(), expectedPumpCount);
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.8, precisionErrorTolerance),
+      );
+      expect(find.text('Item 1'), findsNothing);
+      expect(find.text('Item 20'), findsOneWidget);
+      expect(find.text('Item 70'), findsNothing);
+
+      // Attempting to move to a size too big or too small instead moves to the
+      // min or max child size.
+      goTo(.5);
+      await tester.pumpAndSettle();
+      goTo(0);
+      // The animation was cut short by half, there should have been on less pumps
+      final int truncatedPumpCount = shouldAnimate ? expectedPumpCount - 1 : expectedPumpCount;
+      expect(await tester.pumpAndSettle(), truncatedPumpCount);
+      expect(
+        tester.getSize(find.byKey(containerKey)).height / screenHeight,
+        closeTo(.25, precisionErrorTolerance),
+      );
+    });
+  }
+
+  testWidgets('Can reuse a controller after the old controller is disposed', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+
+    // Pump a new sheet with the same controller. This will dispose of the old sheet first.
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+
+    controller.jumpTo(.6);
+    await tester.pumpAndSettle();
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.6, precisionErrorTolerance),
+    );
+  });
+
+  testWidgets('animateTo interrupts other animations', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: _boilerplate(
+        null,
+        controller: controller,
+        stackKey: stackKey,
+        containerKey: containerKey,
+      ),
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+
+    await tester.flingFrom(Offset(0, .5*screenHeight), Offset(0, -.5*screenHeight), 2000);
+    // Wait until `flinFrom` finished dragging, but before the scrollable goes ballistic.
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(1, precisionErrorTolerance),
+    );
+    expect(find.text('Item 1'), findsOneWidget);
+
+    controller.animateTo(.9, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pumpAndSettle();
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.9, precisionErrorTolerance),
+    );
+    // The ballistic animation should have been canceled so item 1 should still be visible.
+    expect(find.text('Item 1'), findsOneWidget);
+  });
+
+  testWidgets('Other animations interrupt animateTo', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: _boilerplate(
+        null,
+        controller: controller,
+        stackKey: stackKey,
+        containerKey: containerKey,
+      ),
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+
+    controller.animateTo(1, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.75, precisionErrorTolerance),
+    );
+
+    // Interrupt animation and drag downward.
+    await tester.drag(find.text('Item 1'), Offset(0, .1 * screenHeight));
+    await tester.pumpAndSettle();
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.65, precisionErrorTolerance),
+    );
+  });
+
+  testWidgets('animateTo can be interrupted by other animateTo or jumpTo', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: _boilerplate(
+        null,
+        controller: controller,
+        stackKey: stackKey,
+        containerKey: containerKey,
+      ),
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+
+    controller.animateTo(1, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.75, precisionErrorTolerance),
+    );
+
+    // Interrupt animation with a new `animateTo`.
+    controller.animateTo(.25, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 100));
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.5, precisionErrorTolerance),
+    );
+
+    // Interrupt animation with a jump.
+    controller.jumpTo(.6);
+    await tester.pumpAndSettle();
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.6, precisionErrorTolerance),
+    );
+  });
+
+  testWidgets('Can get size and pixels', (WidgetTester tester) async {
+    const Key stackKey = ValueKey<String>('stack');
+    const Key containerKey = ValueKey<String>('container');
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+      stackKey: stackKey,
+      containerKey: containerKey,
+    ));
+    await tester.pumpAndSettle();
+    final double screenHeight = tester.getSize(find.byKey(stackKey)).height;
+
+    expect(controller.sizeToPixels(.25), .25*screenHeight);
+    expect(controller.pixelsToSize(.25*screenHeight), .25);
+
+    controller.animateTo(.6, duration: const Duration(milliseconds: 200), curve: Curves.linear);
+    await tester.pumpAndSettle();
+    expect(
+      tester.getSize(find.byKey(containerKey)).height / screenHeight,
+      closeTo(.6, precisionErrorTolerance),
+    );
+    expect(controller.size, closeTo(.6, precisionErrorTolerance));
+    expect(controller.pixels, closeTo(.6*screenHeight, precisionErrorTolerance));
+
+    await tester.drag(find.text('Item 5'), Offset(0, .2*screenHeight));
+    expect(controller.size, closeTo(.4, precisionErrorTolerance));
+    expect(controller.pixels, closeTo(.4*screenHeight, precisionErrorTolerance));
+  });
+
+  testWidgets('Cannot attach a controller to multiple sheets', (WidgetTester tester) async {
+    final DraggableScrollableController controller = DraggableScrollableController();
+    await tester.pumpWidget(Directionality(
+      textDirection: TextDirection.ltr,
+      child: Stack(
+        children: <Widget>[
+          _boilerplate(
+            null,
+            controller: controller,
+          ),
+          _boilerplate(
+            null,
+            controller: controller,
+          )
+        ],
+      ),
+    ), null, EnginePhase.build);
+    expect(tester.takeException(), isAssertionError);
+  });
+
+  testWidgets('Invalid controller interactions throw assertion errors', (WidgetTester tester) async {
+    final DraggableScrollableController controller = DraggableScrollableController();
+    // Can't use a controller before attaching it.
+    expect(() => controller.jumpTo(.1), throwsAssertionError);
+
+    expect(() => controller.pixels, throwsAssertionError);
+    expect(() => controller.size, throwsAssertionError);
+    expect(() => controller.pixelsToSize(0), throwsAssertionError);
+    expect(() => controller.sizeToPixels(0), throwsAssertionError);
+
+    await tester.pumpWidget(_boilerplate(
+      null,
+      controller: controller,
+    ));
+
+
+    // Can't jump or animate to invalid sizes.
+    expect(() => controller.jumpTo(-1), throwsAssertionError);
+    expect(() => controller.jumpTo(1.1), throwsAssertionError);
+    expect(
+      () => controller.animateTo(-1, duration: const Duration(milliseconds: 1), curve: Curves.linear),
+      throwsAssertionError,
+    );
+    expect(
+      () => controller.animateTo(1.1, duration: const Duration(milliseconds: 1), curve: Curves.linear),
+      throwsAssertionError,
+    );
+
+    // Can't use animateTo with a zero duration.
+    expect(() => controller.animateTo(.5, duration: Duration.zero, curve: Curves.linear), throwsAssertionError);
+  });
 }