Exposing inner controller of NestedScrollView (#49004)

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

@@ -120,9 +120,10 @@ typedef NestedScrollViewHeaderSliversBuilder = List<Widget> Function(BuildContex
 ///           top: false,
 ///           bottom: false,
 ///           child: Builder(
-///             // This Builder is needed to provide a BuildContext that is "inside"
+///             // This Builder is needed to provide a BuildContext that is
-///             // the NestedScrollView, so that sliverOverlapAbsorberHandleFor() can
+///             // "inside" the NestedScrollView, so that
-///             // find the NestedScrollView.
+///             // sliverOverlapAbsorberHandleFor() can find the
+///             // NestedScrollView.
 ///             builder: (BuildContext context) {
 ///               return CustomScrollView(
 ///                 // The "controller" and "primary" members should be left
@@ -136,7 +137,8 @@ typedef NestedScrollViewHeaderSliversBuilder = List<Widget> Function(BuildContex
 ///                 key: PageStorageKey<String>(name),
 ///                 slivers: <Widget>[
 ///                   SliverOverlapInjector(
-///                     // This is the flip side of the SliverOverlapAbsorber above.
+///                     // This is the flip side of the SliverOverlapAbsorber
+///                     // above.
 ///                     handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
 ///                   ),
 ///                   SliverPadding(
@@ -268,7 +270,10 @@ class NestedScrollView extends StatefulWidget {
   /// documentation.
   static SliverOverlapAbsorberHandle sliverOverlapAbsorberHandleFor(BuildContext context) {
     final _InheritedNestedScrollView target = context.dependOnInheritedWidgetOfExactType<_InheritedNestedScrollView>();
-    assert(target != null, 'NestedScrollView.sliverOverlapAbsorberHandleFor must be called with a context that contains a NestedScrollView.');
+    assert(
+      target != null,
+      'NestedScrollView.sliverOverlapAbsorberHandleFor must be called with a context that contains a NestedScrollView.',
+    );
     return target.state._absorberHandle;
   }
 
@@ -285,18 +290,93 @@ class NestedScrollView extends StatefulWidget {
   }
 
   @override
-  _NestedScrollViewState createState() => _NestedScrollViewState();
+  NestedScrollViewState createState() => NestedScrollViewState();
 }
 
-class _NestedScrollViewState extends State<NestedScrollView> {
+/// The [State] for a [NestedScrollView].
+///
+/// The [ScrollController]s, [innerController] and [outerController], of the
+/// [NestedScrollView]'s children may be accessed through its state. This is
+/// useful for obtaining respective scroll positions in the [NestedScrollView].
+///
+/// If you want to access the inner or outer scroll controller of a
+/// [NestedScrollView], you can get its [NestedScrollViewState] by supplying a
+/// `GlobalKey<NestedScrollViewState>` to the [NestedScrollView.key] parameter).
+///
+/// {@tool sample --template=stateless_widget_material}
+/// [NestedScrollViewState] can be obtained using a [GlobalKey].
+/// Using the following setup, you can access the inner scroll controller
+/// using `globalKey.currentState.innerController`.
+///
+/// ```dart preamble
+/// final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+/// ```
+/// ```dart
+/// @override
+/// Widget build(BuildContext context) {
+///   return NestedScrollView(
+///     key: globalKey,
+///     headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+///       return <Widget>[
+///         SliverAppBar(
+///           title: Text('NestedScrollViewState Demo!'),
+///         ),
+///       ];
+///     },
+///     body: CustomScrollView(
+///       // Body slivers go here!
+///     ),
+///   );
+/// }
+///
+/// ScrollController get innerController {
+///   return globalKey.currentState.innerController;
+/// }
+/// ```
+/// {@end-tool}
+class NestedScrollViewState extends State<NestedScrollView> {
   final SliverOverlapAbsorberHandle _absorberHandle = SliverOverlapAbsorberHandle();
 
+  /// The [ScrollController] provided to the [ScrollView] in
+  /// [NestedScrollView.body].
+  ///
+  /// Manipulating the [ScrollPosition] of this controller pushes the outer
+  /// header sliver(s) up and out of view. The position of the [outerController]
+  /// will be set to [ScrollPosition.maxScrollExtent], unless you use
+  /// [ScrollPosition.setPixels].
+  ///
+  /// See also:
+  ///
+  ///  * [outerController], which exposes the [ScrollController] used by the
+  ///    the sliver(s) contained in [NestedScrollView.headerSliverBuilder].
+  ScrollController get innerController => _coordinator._innerController;
+
+  /// The [ScrollController] provided to the [ScrollView] in
+  /// [NestedScrollView.headerSliverBuilder].
+  ///
+  /// This is equivalent to [NestedScrollView.controller], if provided.
+  ///
+  /// Manipulating the [ScrollPosition] of this controller pushes the inner body
+  /// sliver(s) down. The position of the [innerController] will be set to
+  /// [ScrollPosition.minScrollExtent], unless you use
+  /// [ScrollPosition.setPixels]. Visually, the inner body will be scrolled to
+  /// its beginning.
+  ///
+  /// See also:
+  ///
+  ///  * [innerController], which exposes the [ScrollController] used by the
+  ///    [ScrollView] contained in [NestedScrollView.body].
+  ScrollController get outerController => _coordinator._outerController;
+
   _NestedScrollCoordinator _coordinator;
 
   @override
   void initState() {
     super.initState();
-    _coordinator = _NestedScrollCoordinator(this, widget.controller, _handleHasScrolledBodyChanged);
+    _coordinator = _NestedScrollCoordinator(
+      this, widget.controller,
+      _handleHasScrolledBodyChanged,
+    );
   }
 
   @override
@@ -348,8 +428,8 @@ class _NestedScrollViewState extends State<NestedScrollView> {
             scrollDirection: widget.scrollDirection,
             reverse: widget.reverse,
             physics: widget.physics != null
-                ? widget.physics.applyTo(const ClampingScrollPhysics())
+              ? widget.physics.applyTo(const ClampingScrollPhysics())
-                : const ClampingScrollPhysics(),
+              : const ClampingScrollPhysics(),
             controller: _coordinator._outerController,
             slivers: widget._buildSlivers(
               context,
@@ -410,7 +490,7 @@ class _InheritedNestedScrollView extends InheritedWidget {
        assert(child != null),
        super(key: key, child: child);
 
-  final _NestedScrollViewState state;
+  final NestedScrollViewState state;
 
   @override
   bool updateShouldNotify(_InheritedNestedScrollView old) => state != old.state;
@@ -469,11 +549,19 @@ typedef _NestedScrollActivityGetter = ScrollActivity Function(_NestedScrollPosit
 class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldController {
   _NestedScrollCoordinator(this._state, this._parent, this._onHasScrolledBodyChanged) {
     final double initialScrollOffset = _parent?.initialScrollOffset ?? 0.0;
-    _outerController = _NestedScrollController(this, initialScrollOffset: initialScrollOffset, debugLabel: 'outer');
+    _outerController = _NestedScrollController(
-    _innerController = _NestedScrollController(this, initialScrollOffset: 0.0, debugLabel: 'inner');
+      this,
+      initialScrollOffset: initialScrollOffset,
+      debugLabel: 'outer',
+    );
+    _innerController = _NestedScrollController(
+      this,
+      initialScrollOffset: 0.0,
+      debugLabel: 'inner',
+    );
   }
 
-  final _NestedScrollViewState _state;
+  final NestedScrollViewState _state;
   ScrollController _parent;
   final VoidCallback _onHasScrolledBodyChanged;
 
@@ -550,14 +638,22 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   @override
   void goIdle() {
-    beginActivity(_createIdleScrollActivity(_outerPosition), _createIdleScrollActivity);
+    beginActivity(
+      _createIdleScrollActivity(_outerPosition),
+      _createIdleScrollActivity,
+    );
   }
 
   @override
   void goBallistic(double velocity) {
     beginActivity(
       createOuterBallisticScrollActivity(velocity),
-      (_NestedScrollPosition position) => createInnerBallisticScrollActivity(position, velocity),
+      (_NestedScrollPosition position) {
+        return createInnerBallisticScrollActivity(
+          position,
+          velocity,
+        );
+      },
     );
   }
 
@@ -593,7 +689,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
     if (innerPosition == null) {
       // It's either just us or a velocity=0 situation.
       return _outerPosition.createBallisticScrollActivity(
-        _outerPosition.physics.createBallisticSimulation(_outerPosition, velocity),
+        _outerPosition.physics.createBallisticSimulation(
+          _outerPosition,
+          velocity,
+        ),
         mode: _NestedBallisticScrollActivityMode.independent,
       );
     }
@@ -611,7 +710,9 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
   ScrollActivity createInnerBallisticScrollActivity(_NestedScrollPosition position, double velocity) {
     return position.createBallisticScrollActivity(
       position.physics.createBallisticSimulation(
-        velocity == 0 ? position as ScrollMetrics : _getMetrics(position, velocity),
+        velocity == 0
+          ? position as ScrollMetrics
+          : _getMetrics(position, velocity),
         velocity,
       ),
       mode: _NestedBallisticScrollActivityMode.inner,
@@ -622,7 +723,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
     assert(innerPosition != null);
     double pixels, minRange, maxRange, correctionOffset, extra;
     if (innerPosition.pixels == innerPosition.minScrollExtent) {
-      pixels = _outerPosition.pixels.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent) as double; // TODO(ianh): gracefully handle out-of-range outer positions
+      pixels = _outerPosition.pixels.clamp(
+        _outerPosition.minScrollExtent,
+        _outerPosition.maxScrollExtent,
+      ) as double; // TODO(ianh): gracefully handle out-of-range outer positions
       minRange = _outerPosition.minScrollExtent;
       maxRange = _outerPosition.maxScrollExtent;
       assert(minRange <= maxRange);
@@ -688,7 +792,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   double unnestOffset(double value, _NestedScrollPosition source) {
     if (source == _outerPosition)
-      return value.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent) as double;
+      return value.clamp(
+        _outerPosition.minScrollExtent,
+        _outerPosition.maxScrollExtent,
+      ) as double;
     if (value < source.minScrollExtent)
       return value - source.minScrollExtent + _outerPosition.minScrollExtent;
     return value - source.minScrollExtent + _outerPosition.maxScrollExtent;
@@ -696,7 +803,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   double nestOffset(double value, _NestedScrollPosition target) {
     if (target == _outerPosition)
-      return value.clamp(_outerPosition.minScrollExtent, _outerPosition.maxScrollExtent) as double;
+      return value.clamp(
+        _outerPosition.minScrollExtent,
+        _outerPosition.maxScrollExtent,
+      ) as double;
     if (value < _outerPosition.minScrollExtent)
       return value - _outerPosition.minScrollExtent + target.minScrollExtent;
     if (value > _outerPosition.maxScrollExtent)
@@ -711,7 +821,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
     for (final _NestedScrollPosition position in _innerPositions) {
       if (!position.haveDimensions)
         return;
-      maxInnerExtent = math.max(maxInnerExtent, position.maxScrollExtent - position.minScrollExtent);
+      maxInnerExtent = math.max(
+        maxInnerExtent,
+        position.maxScrollExtent - position.minScrollExtent,
+      );
     }
     _outerPosition.updateCanDrag(maxInnerExtent);
   }
@@ -758,7 +871,10 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   ScrollHoldController hold(VoidCallback holdCancelCallback) {
     beginActivity(
-      HoldScrollActivity(delegate: _outerPosition, onHoldCanceled: holdCancelCallback),
+      HoldScrollActivity(
+        delegate: _outerPosition,
+        onHoldCanceled: holdCancelCallback,
+      ),
       (_NestedScrollPosition position) => HoldScrollActivity(delegate: position),
     );
     return this;
@@ -786,7 +902,9 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   @override
   void applyUserOffset(double delta) {
-    updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse);
+    updateUserScrollDirection(
+      delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse
+    );
     assert(delta != 0.0);
     if (_innerPositions.isEmpty) {
       _outerPosition.applyFullDragUpdate(delta);
@@ -805,7 +923,8 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
       }
     } else {
       // dragging "down" - delta is positive
-      // prioritize the inner views, so that the inner content will move before the app bar grows
+      // prioritize the inner views, so that the inner content will move before
+      // the app bar grows
       double outerDelta = 0.0; // it will go positive if it changes
       final List<double> overscrolls = <double>[];
       final List<_NestedScrollPosition> innerPositions = _innerPositions.toList();
@@ -831,7 +950,9 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
   }
 
   void updateParent() {
-    _outerPosition?.setParent(_parent ?? PrimaryScrollController.of(_state.context));
+    _outerPosition?.setParent(
+      _parent ?? PrimaryScrollController.of(_state.context)
+    );
   }
 
   @mustCallSuper
@@ -984,9 +1105,13 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
     // One is if the physics allow it, via applyFullDragUpdate (see below). An
     // overscroll situation can also be forced, e.g. if the scroll position is
     // artificially set using the scroll controller.
-    final double min = delta < 0.0 ? -double.infinity : math.min(minScrollExtent, pixels);
+    final double min = delta < 0.0
+      ? -double.infinity
+      : math.min(minScrollExtent, pixels);
     // The logic for max is equivalent but on the other side.
-    final double max = delta > 0.0 ? double.infinity : math.max(maxScrollExtent, pixels);
+    final double max = delta > 0.0
+      ? double.infinity
+      : math.max(maxScrollExtent, pixels);
     final double oldPixels = pixels;
     final double newPixels = (pixels - delta).clamp(min, max) as double;
     final double clampedDelta = newPixels - pixels;
@@ -1007,7 +1132,10 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
     assert(delta != 0.0);
     final double oldPixels = pixels;
     // Apply friction:
-    final double newPixels = pixels - physics.applyPhysicsToUserOffset(this, delta);
+    final double newPixels = pixels - physics.applyPhysicsToUserOffset(
+      this,
+      delta,
+    );
     if (oldPixels == newPixels)
       return 0.0; // delta must have been so small we dropped it during floating point addition
     // Check for overscroll:
@@ -1050,7 +1178,8 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
     beginActivity(IdleScrollActivity(this));
   }
 
-  // This is called by activities when they finish their work and want to go ballistic.
+  // This is called by activities when they finish their work and want to go
+  // ballistic.
   @override
   void goBallistic(double velocity) {
     Simulation simulation;
@@ -1075,9 +1204,20 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
         assert(metrics != null);
         if (metrics.minRange == metrics.maxRange)
           return IdleScrollActivity(this);
-        return _NestedOuterBallisticScrollActivity(coordinator, this, metrics, simulation, context.vsync);
+        return _NestedOuterBallisticScrollActivity(
+          coordinator,
+          this,
+          metrics,
+          simulation,
+          context.vsync,
+        );
       case _NestedBallisticScrollActivityMode.inner:
-        return _NestedInnerBallisticScrollActivity(coordinator, this, simulation, context.vsync);
+        return _NestedInnerBallisticScrollActivity(
+          coordinator,
+          this,
+          simulation,
+          context.vsync,
+        );
       case _NestedBallisticScrollActivityMode.independent:
         return BallisticScrollActivity(this, simulation, context.vsync);
     }
@@ -1090,7 +1230,11 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
     @required Duration duration,
     @required Curve curve,
   }) {
-    return coordinator.animateTo(coordinator.unnestOffset(to, this), duration: duration, curve: curve);
+    return coordinator.animateTo(
+      coordinator.unnestOffset(to, this),
+      duration: duration,
+      curve: curve,
+    );
   }
 
   @override
@@ -1157,12 +1301,18 @@ class _NestedInnerBallisticScrollActivity extends BallisticScrollActivity {
 
   @override
   void resetActivity() {
-    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(delegate, velocity));
+    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(
+      delegate,
+      velocity,
+    ));
   }
 
   @override
   void applyNewDimensions() {
-    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(delegate, velocity));
+    delegate.beginActivity(coordinator.createInnerBallisticScrollActivity(
+      delegate,
+      velocity,
+    ));
   }
 
   @override
@@ -1190,12 +1340,16 @@ class _NestedOuterBallisticScrollActivity extends BallisticScrollActivity {
 
   @override
   void resetActivity() {
-    delegate.beginActivity(coordinator.createOuterBallisticScrollActivity(velocity));
+    delegate.beginActivity(
+      coordinator.createOuterBallisticScrollActivity(velocity)
+    );
   }
 
   @override
   void applyNewDimensions() {
-    delegate.beginActivity(coordinator.createOuterBallisticScrollActivity(velocity));
+    delegate.beginActivity(
+      coordinator.createOuterBallisticScrollActivity(velocity)
+    );
   }
 
   @override
@@ -1290,7 +1444,10 @@ class SliverOverlapAbsorberHandle extends ChangeNotifier {
   double _scrollExtent;
 
   void _setExtents(double layoutValue, double scrollValue) {
-    assert(_writers == 1, 'Multiple RenderSliverOverlapAbsorbers have been provided the same SliverOverlapAbsorberHandle.');
+    assert(
+      _writers == 1,
+      'Multiple RenderSliverOverlapAbsorbers have been provided the same SliverOverlapAbsorberHandle.',
+    );
     _layoutExtent = layoutValue;
     _scrollExtent = scrollValue;
   }
@@ -1434,7 +1591,10 @@ class RenderSliverOverlapAbsorber extends RenderSliver with RenderObjectWithChil
 
   @override
   void performLayout() {
-    assert(handle._writers == 1, 'A SliverOverlapAbsorberHandle cannot be passed to multiple RenderSliverOverlapAbsorber objects at the same time.');
+    assert(
+      handle._writers == 1,
+      'A SliverOverlapAbsorberHandle cannot be passed to multiple RenderSliverOverlapAbsorber objects at the same time.',
+    );
     if (child == null) {
       geometry = const SliverGeometry();
       return;
@@ -1453,7 +1613,10 @@ class RenderSliverOverlapAbsorber extends RenderSliver with RenderObjectWithChil
       hasVisualOverflow: childLayoutGeometry.hasVisualOverflow,
       scrollOffsetCorrection: childLayoutGeometry.scrollOffsetCorrection,
     );
-    handle._setExtents(childLayoutGeometry.maxScrollObstructionExtent, childLayoutGeometry.maxScrollObstructionExtent);
+    handle._setExtents(
+      childLayoutGeometry.maxScrollObstructionExtent,
+      childLayoutGeometry.maxScrollObstructionExtent,
+    );
   }
 
   @override
@@ -1464,7 +1627,11 @@ class RenderSliverOverlapAbsorber extends RenderSliver with RenderObjectWithChil
   @override
   bool hitTestChildren(SliverHitTestResult result, { @required double mainAxisPosition, @required double crossAxisPosition }) {
     if (child != null)
-      return child.hitTest(result, mainAxisPosition: mainAxisPosition, crossAxisPosition: crossAxisPosition);
+      return child.hitTest(
+        result,
+        mainAxisPosition: mainAxisPosition,
+        crossAxisPosition: crossAxisPosition,
+      );
     return false;
   }
 
@@ -1599,7 +1766,10 @@ class RenderSliverOverlapInjector extends RenderSliver {
   void performLayout() {
     _currentLayoutExtent = handle.layoutExtent;
     _currentMaxExtent = handle.layoutExtent;
-    final double clampedLayoutExtent = math.min(_currentLayoutExtent - constraints.scrollOffset, constraints.remainingPaintExtent);
+    final double clampedLayoutExtent = math.min(
+      _currentLayoutExtent - constraints.scrollOffset,
+      constraints.remainingPaintExtent,
+    );
     geometry = SliverGeometry(
       scrollExtent: _currentLayoutExtent,
       paintExtent: math.max(0.0, clampedLayoutExtent),
@@ -1631,7 +1801,14 @@ class RenderSliverOverlapInjector extends RenderSliver {
             break;
         }
         for (int index = -2; index <= 2; index += 1) {
-          paintZigZag(context.canvas, paint, start - delta * index.toDouble(), end - delta * index.toDouble(), 10, 10.0);
+          paintZigZag(
+            context.canvas,
+            paint,
+            start - delta * index.toDouble(),
+            end - delta * index.toDouble(),
+            10,
+            10.0,
+          );
         }
       }
       return true;
@@ -1680,7 +1857,10 @@ class NestedScrollViewViewport extends Viewport {
   RenderNestedScrollViewViewport createRenderObject(BuildContext context) {
     return RenderNestedScrollViewViewport(
       axisDirection: axisDirection,
-      crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection),
+      crossAxisDirection: crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(
+        context,
+        axisDirection,
+      ),
       anchor: anchor,
       offset: offset,
       handle: handle,
@@ -1691,7 +1871,10 @@ class NestedScrollViewViewport extends Viewport {
   void updateRenderObject(BuildContext context, RenderNestedScrollViewViewport renderObject) {
     renderObject
       ..axisDirection = axisDirection
-      ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(context, axisDirection)
+      ..crossAxisDirection = crossAxisDirection ?? Viewport.getDefaultCrossAxisDirection(
+        context,
+        axisDirection,
+      )
       ..anchor = anchor
       ..offset = offset
       ..handle = handle;
@@ -1709,7 +1892,8 @@ class NestedScrollViewViewport extends Viewport {
 /// This viewport takes a [SliverOverlapAbsorberHandle] and notifies it any time
 /// the viewport needs to recompute its layout (e.g. when it is scrolled).
 class RenderNestedScrollViewViewport extends RenderViewport {
-  /// Create a variant of [RenderViewport] that has a [SliverOverlapAbsorberHandle].
+  /// Create a variant of [RenderViewport] that has a
+  /// [SliverOverlapAbsorberHandle].
   ///
   /// The [handle] must not be null.
   RenderNestedScrollViewViewport({

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -22,7 +22,12 @@ class _CustomPhysics extends ClampingScrollPhysics {
   }
 }
 
-Widget buildTest({ ScrollController controller, String title = 'TTTTTTTT' }) {
+Widget buildTest({
+  ScrollController controller,
+  String title = 'TTTTTTTT',
+  Key key,
+  bool expanded = true,
+}) {
   return Localizations(
     locale: const Locale('en', 'US'),
     delegates: const <LocalizationsDelegate<dynamic>>[
@@ -38,6 +43,7 @@ Widget buildTest({ ScrollController controller, String title = 'TTTTTTTT' }) {
           body: DefaultTabController(
             length: 4,
             child: NestedScrollView(
+              key: key,
               dragStartBehavior: DragStartBehavior.down,
               controller: controller,
               headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
@@ -45,7 +51,7 @@ Widget buildTest({ ScrollController controller, String title = 'TTTTTTTT' }) {
                   SliverAppBar(
                     title: Text(title),
                     pinned: true,
-                    expandedHeight: 200.0,
+                    expandedHeight: expanded ? 200.0 : 0.0,
                     forceElevated: innerBoxIsScrolled,
                     bottom: const TabBar(
                       tabs: <Tab>[
@@ -119,7 +125,10 @@ void main() {
     final Offset point1 = tester.getCenter(find.text('aaa1'));
     await tester.dragFrom(point1, const Offset(0.0, 200.0));
     await tester.pump();
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
     await tester.flingFrom(point1, const Offset(0.0, -80.0), 50000.0);
     await tester.pump(const Duration(milliseconds: 20));
     final Offset point2 = tester.getCenter(find.text('aaa1'));
@@ -128,6 +137,7 @@ void main() {
     // the following expectation should switch to 200.0.
     expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 120.0);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
+
   testWidgets('NestedScrollView overscroll and release and hold', (WidgetTester tester) async {
     await tester.pumpWidget(buildTest());
     expect(find.text('aaa2'), findsOneWidget);
@@ -147,11 +157,14 @@ void main() {
     expect(find.text('aaa2'), findsNothing);
     await tester.pump(const Duration(milliseconds: 1000));
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
+
   testWidgets('NestedScrollView overscroll and release', (WidgetTester tester) async {
     await tester.pumpWidget(buildTest());
     expect(find.text('aaa2'), findsOneWidget);
     await tester.pump(const Duration(milliseconds: 500));
-    final TestGesture gesture1 = await tester.startGesture(tester.getCenter(find.text('aaa1')));
+    final TestGesture gesture1 = await tester.startGesture(
+      tester.getCenter(find.text('aaa1'))
+    );
     await gesture1.moveBy(const Offset(0.0, 200.0));
     await tester.pumpAndSettle();
     expect(find.text('aaa2'), findsNothing);
@@ -169,19 +182,31 @@ void main() {
     expect(find.text('aaa3'), findsNothing);
     expect(find.text('bbb1'), findsNothing);
     await tester.pump(const Duration(milliseconds: 250));
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
 
     await tester.drag(find.text('AA'), const Offset(0.0, -20.0));
     await tester.pump(const Duration(milliseconds: 250));
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 180.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      180.0,
+    );
 
     await tester.drag(find.text('AA'), const Offset(0.0, -20.0));
     await tester.pump(const Duration(milliseconds: 250));
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 160.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      160.0,
+    );
 
     await tester.drag(find.text('AA'), const Offset(0.0, -20.0));
     await tester.pump(const Duration(milliseconds: 250));
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 140.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      140.0,
+    );
 
     expect(find.text('aaa4'), findsNothing);
     await tester.pump(const Duration(milliseconds: 250));
@@ -203,17 +228,25 @@ void main() {
     await tester.pumpAndSettle(const Duration(milliseconds: 250));
     expect(find.text('bbb1'), findsNothing);
     expect(find.text('ccc1'), findsOneWidget);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, minHeight);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      minHeight,
+    );
 
     await tester.pump(const Duration(milliseconds: 250));
     await tester.fling(find.text('AA'), const Offset(0.0, 50.0), 10000.0);
     await tester.pumpAndSettle(const Duration(milliseconds: 250));
     expect(find.text('ccc1'), findsOneWidget);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
   });
 
   testWidgets('NestedScrollView with a ScrollController', (WidgetTester tester) async {
-    final ScrollController controller = ScrollController(initialScrollOffset: 50.0);
+    final ScrollController controller = ScrollController(
+      initialScrollOffset: 50.0,
+    );
 
     double scrollOffset;
     controller.addListener(() {
@@ -226,26 +259,45 @@ void main() {
     expect(controller.position.maxScrollExtent, 200.0);
 
     // The appbar's expandedHeight - initialScrollOffset = 150.
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 150.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      150.0,
+    );
 
     // Fully expand the appbar by scrolling (no animation) to 0.0.
     controller.jumpTo(0.0);
     await tester.pumpAndSettle();
     expect(scrollOffset, 0.0);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
 
     // Scroll back to 50.0 animating over 100ms.
-    controller.animateTo(50.0, duration: const Duration(milliseconds: 100), curve: Curves.linear);
+    controller.animateTo(
+      50.0,
+      duration: const Duration(milliseconds: 100),
+      curve: Curves.linear,
+    );
     await tester.pump();
     await tester.pump();
     expect(scrollOffset, 0.0);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
     await tester.pump(const Duration(milliseconds: 50)); // 50ms - halfway to scroll offset = 50.0.
     expect(scrollOffset, 25.0);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 175.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      175.0,
+    );
     await tester.pump(const Duration(milliseconds: 50)); // 100ms - all the way to scroll offset = 50.0.
     expect(scrollOffset, 50.0);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 150.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      150.0,
+    );
 
     // Scroll to the end, (we're not scrolling to the end of the list that contains aaa1,
     // just to the end of the outer scrollview). Verify that the first item in each tab
@@ -288,12 +340,18 @@ void main() {
     expect(find.text('Page0'), findsOneWidget);
     expect(find.text('Page1'), findsNothing);
     expect(find.text('Page2'), findsNothing);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
 
     // A scroll collapses Page0's appbar to 150.0.
     controller.jumpTo(50.0);
     await tester.pumpAndSettle();
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 150.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      150.0,
+    );
 
     // Fling to Page1. Page1's appbar height is the same as the appbar for Page0.
     await tester.fling(find.text('Page0'), const Offset(-100.0, 0.0), 10000.0);
@@ -301,19 +359,28 @@ void main() {
     expect(find.text('Page0'), findsNothing);
     expect(find.text('Page1'), findsOneWidget);
     expect(find.text('Page2'), findsNothing);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 150.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      150.0,
+    );
 
     // Expand Page1's appbar and then fling to Page2. Page2's appbar appears
     // fully expanded.
     controller.jumpTo(0.0);
     await tester.pumpAndSettle();
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
     await tester.fling(find.text('Page1'), const Offset(-100.0, 0.0), 10000.0);
     await tester.pumpAndSettle();
     expect(find.text('Page0'), findsNothing);
     expect(find.text('Page1'), findsNothing);
     expect(find.text('Page2'), findsOneWidget);
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
+    expect(
+      tester.renderObject<RenderBox>(find.byType(AppBar)).size.height,
+      200.0,
+    );
   });
 
   testWidgets('NestedScrollViews with custom physics', (WidgetTester tester) async {
@@ -347,7 +414,10 @@ void main() {
     final Offset point1 = tester.getCenter(find.text('AA'));
     await tester.dragFrom(point1, const Offset(0.0, 200.0));
     await tester.pump(const Duration(milliseconds: 20));
-    final Offset point2 = tester.getCenter(find.text('AA', skipOffstage: false));
+    final Offset point2 = tester.getCenter(find.text(
+      'AA',
+      skipOffstage: false,
+    ));
     expect(point1.dy, greaterThan(point2.dy));
   });
 
@@ -368,29 +438,31 @@ void main() {
               // These are the slivers that show up in the "outer" scroll view.
               return <Widget>[
                 SliverOverlapAbsorber(
-                  // This widget takes the overlapping behavior of the SliverAppBar,
+                  // This widget takes the overlapping behavior of the
-                  // and redirects it to the SliverOverlapInjector below. If it is
+                  // SliverAppBar, and redirects it to the SliverOverlapInjector
-                  // missing, then it is possible for the nested "inner" scroll view
+                  // below. If it is missing, then it is possible for the nested
-                  // below to end up under the SliverAppBar even when the inner
+                  // "inner" scroll view below to end up under the SliverAppBar
-                  // scroll view thinks it has not been scrolled.
+                  // even when the inner scroll view thinks it has not been
-                  // This is not necessary if the "headerSliverBuilder" only builds
+                  // scrolled. This is not necessary if the
-                  // widgets that do not overlap the next sliver.
+                  // "headerSliverBuilder" only builds widgets that do not
+                  // overlap the next sliver.
                   handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                   sliver: SliverAppBar(
                     title: const Text('Books'), // This is the title in the app bar.
                     pinned: true,
                     expandedHeight: 150.0,
-                    // The "forceElevated" property causes the SliverAppBar to show
+                    // The "forceElevated" property causes the SliverAppBar to
-                    // a shadow. The "innerBoxIsScrolled" parameter is true when the
+                    // show a shadow. The "innerBoxIsScrolled" parameter is true
-                    // inner scroll view is scrolled beyond its "zero" point, i.e.
+                    // when the inner scroll view is scrolled beyond its "zero"
-                    // when it appears to be scrolled below the SliverAppBar.
+                    // point, i.e. when it appears to be scrolled below the
-                    // Without this, there are cases where the shadow would appear
+                    // SliverAppBar. Without this, there are cases where the
-                    // or not appear inappropriately, because the SliverAppBar is
+                    // shadow would appear or not appear inappropriately,
-                    // not actually aware of the precise position of the inner
+                    // because the SliverAppBar is not actually aware of the
-                    // scroll views.
+                    // precise position of the inner scroll views.
                     forceElevated: innerBoxIsScrolled,
                     bottom: TabBar(
-                      // These are the widgets to put in each tab in the tab bar.
+                      // These are the widgets to put in each tab in the tab
+                      // bar.
                       tabs: _tabs.map<Widget>((String name) => Tab(text: name)).toList(),
                       dragStartBehavior: DragStartBehavior.down,
                     ),
@@ -406,24 +478,27 @@ void main() {
                   top: false,
                   bottom: false,
                   child: Builder(
-                    // This Builder is needed to provide a BuildContext that is "inside"
+                    // This Builder is needed to provide a BuildContext that is
-                    // the NestedScrollView, so that sliverOverlapAbsorberHandleFor() can
+                    // "inside" the NestedScrollView, so that
-                    // find the NestedScrollView.
+                    // sliverOverlapAbsorberHandleFor() can find the
+                    // NestedScrollView.
                     builder: (BuildContext context) {
                       return CustomScrollView(
                         // The "controller" and "primary" members should be left
                         // unset, so that the NestedScrollView can control this
                         // inner scroll view.
                         // If the "controller" property is set, then this scroll
-                        // view will not be associated with the NestedScrollView.
+                        // view will not be associated with the
-                        // The PageStorageKey should be unique to this ScrollView;
+                        // NestedScrollView. The PageStorageKey should be unique
-                        // it allows the list to remember its scroll position when
+                        // to this ScrollView; it allows the list to remember
-                        // the tab view is not on the screen.
+                        // its scroll position when the tab view is not on the
+                        // screen.
                         key: PageStorageKey<String>(name),
                         dragStartBehavior: DragStartBehavior.down,
                         slivers: <Widget>[
                           SliverOverlapInjector(
-                            // This is the flip side of the SliverOverlapAbsorber above.
+                            // This is the flip side of the
+                            // SliverOverlapAbsorber above.
                             handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
                           ),
                           SliverPadding(
@@ -431,24 +506,27 @@ void main() {
                             // In this example, the inner scroll view has
                             // fixed-height list items, hence the use of
                             // SliverFixedExtentList. However, one could use any
-                            // sliver widget here, e.g. SliverList or SliverGrid.
+                            // sliver widget here, e.g. SliverList or
+                            // SliverGrid.
                             sliver: SliverFixedExtentList(
-                              // The items in this example are fixed to 48 pixels
+                              // The items in this example are fixed to 48
-                              // high. This matches the Material Design spec for
+                              // pixels high. This matches the Material Design
-                              // ListTile widgets.
+                              // spec for ListTile widgets.
                               itemExtent: 48.0,
                               delegate: SliverChildBuilderDelegate(
                                 (BuildContext context, int index) {
                                   // This builder is called for each child.
-                                  // In this example, we just number each list item.
+                                  // In this example, we just number each list
+                                  // item.
                                   return ListTile(
                                     title: Text('Item $index'),
                                   );
                                 },
-                                // The childCount of the SliverChildBuilderDelegate
+                                // The childCount of the
-                                // specifies how many children this inner list
+                                // SliverChildBuilderDelegate specifies how many
-                                // has. In this example, each tab has a list of
+                                // children this inner list has. In this
-                                // exactly 30 items, but this is arbitrary.
+                                // example, each tab has a list of exactly 30
+                                // items, but this is arbitrary.
                                 childCount: 30,
                               ),
                             ),
@@ -498,7 +576,9 @@ void main() {
     expect(find.text('Item 18'), findsNothing);
     _checkPhysicalLayer(elevation: 0);
     // scroll down
-    final TestGesture gesture0 = await tester.startGesture(tester.getCenter(find.text('Item 2')));
+    final TestGesture gesture0 = await tester.startGesture(
+      tester.getCenter(find.text('Item 2'))
+    );
     await gesture0.moveBy(const Offset(0.0, -120.0)); // tiny bit more than the pinned app bar height (56px * 2)
     await tester.pump();
     expect(buildCount, expectedBuildCount);
@@ -515,7 +595,9 @@ void main() {
     expect(buildCount, expectedBuildCount);
     _checkPhysicalLayer(elevation: 4);
     // scroll down
-    final TestGesture gesture1 = await tester.startGesture(tester.getCenter(find.text('Item 2')));
+    final TestGesture gesture1 = await tester.startGesture(
+      tester.getCenter(find.text('Item 2'))
+    );
     await gesture1.moveBy(const Offset(0.0, -800.0));
     await tester.pump();
     expect(buildCount, expectedBuildCount);
@@ -527,15 +609,22 @@ void main() {
     expect(buildCount, expectedBuildCount);
     _checkPhysicalLayer(elevation: 4);
     // swipe left to bring in tap on the right
-    final TestGesture gesture2 = await tester.startGesture(tester.getCenter(find.byType(NestedScrollView)));
+    final TestGesture gesture2 = await tester.startGesture(
+      tester.getCenter(find.byType(NestedScrollView))
+    );
     await gesture2.moveBy(const Offset(-400.0, 0.0));
     await tester.pump();
     expect(buildCount, expectedBuildCount);
     expect(find.text('Item 18'), findsOneWidget);
     expect(find.text('Item 2'), findsOneWidget);
     expect(find.text('Item 0'), findsOneWidget);
-    expect(tester.getTopLeft(find.ancestor(of: find.text('Item 0'), matching: find.byType(ListTile))).dy,
+    expect(tester.getTopLeft(
-           tester.getBottomLeft(find.byType(AppBar)).dy + 8.0);
+      find.ancestor(
+        of: find.text('Item 0'),
+        matching: find.byType(ListTile),
+      )).dy,
+      tester.getBottomLeft(find.byType(AppBar)).dy + 8.0,
+    );
     _checkPhysicalLayer(elevation: 4);
     await gesture2.up();
     await tester.pump(); // start sideways scroll
@@ -552,7 +641,9 @@ void main() {
     await tester.pump(const Duration(seconds: 1)); // just checking we don't rebuild...
     expect(buildCount, expectedBuildCount);
     // peek left to see it's still in the right place
-    final TestGesture gesture3 = await tester.startGesture(tester.getCenter(find.byType(NestedScrollView)));
+    final TestGesture gesture3 = await tester.startGesture(
+      tester.getCenter(find.byType(NestedScrollView))
+    );
     await gesture3.moveBy(const Offset(400.0, 0.0));
     await tester.pump(); // bring the left page into view
     expect(buildCount, expectedBuildCount);
@@ -577,7 +668,9 @@ void main() {
     expect(buildCount, expectedBuildCount);
     _checkPhysicalLayer(elevation: 0);
     // scroll back up
-    final TestGesture gesture4 = await tester.startGesture(tester.getCenter(find.byType(NestedScrollView)));
+    final TestGesture gesture4 = await tester.startGesture(
+      tester.getCenter(find.byType(NestedScrollView))
+    );
     await gesture4.moveBy(const Offset(0.0, 200.0)); // expands the appbar again
     await tester.pump();
     expect(buildCount, expectedBuildCount);
@@ -589,7 +682,9 @@ void main() {
     expect(buildCount, expectedBuildCount);
     _checkPhysicalLayer(elevation: 0);
     // peek left to see it's now back at zero
-    final TestGesture gesture5 = await tester.startGesture(tester.getCenter(find.byType(NestedScrollView)));
+    final TestGesture gesture5 = await tester.startGesture(
+      tester.getCenter(find.byType(NestedScrollView))
+    );
     await gesture5.moveBy(const Offset(400.0, 0.0));
     await tester.pump(); // bring the left page into view
     await tester.pump(); // shadow would come back starting here, but there's no shadow to show
@@ -640,39 +735,467 @@ void main() {
         ),
       ),
     );
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(
-    expect(tester.getRect(find.byKey(key2)), const Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0));
+      tester.getRect(find.byKey(key1)),
-    final TestGesture gesture = await tester.startGesture(const Offset(10.0, 10.0));
+      const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0),
+    );
+    expect(
+      tester.getRect(find.byKey(key2)),
+      const Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0),
+    );
+    final TestGesture gesture = await tester.startGesture(
+      const Offset(10.0, 10.0)
+    );
     await gesture.moveBy(const Offset(0.0, -10.0)); // scroll up
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, -10.0, 800.0, 100.0));
+    expect(
-    expect(tester.getRect(find.byKey(key2)), const Rect.fromLTWH(0.0, 90.0, 800.0, 1000.0));
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, -10.0, 800.0, 100.0),
+    );
+    expect(
+      tester.getRect(find.byKey(key2)),
+      const Rect.fromLTWH(0.0, 90.0, 800.0, 1000.0),
+    );
     await gesture.moveBy(const Offset(0.0, 10.0)); // scroll back to origin
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(
-    expect(tester.getRect(find.byKey(key2)), const Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0));
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0),
+    );
+    expect(
+      tester.getRect(find.byKey(key2)),
+      const Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0),
+    );
     await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
     await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
     await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0),
+    );
     expect(tester.getRect(find.byKey(key2)).top, greaterThan(100.0));
     expect(tester.getRect(find.byKey(key2)).top, lessThan(130.0));
     await gesture.moveBy(const Offset(0.0, -1.0)); // scroll back a little
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, -1.0, 800.0, 100.0));
+    expect(
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, -1.0, 800.0, 100.0),
+    );
     expect(tester.getRect(find.byKey(key2)).top, greaterThan(100.0));
     expect(tester.getRect(find.byKey(key2)).top, lessThan(129.0));
     await gesture.moveBy(const Offset(0.0, -10.0)); // scroll back a lot
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, -11.0, 800.0, 100.0));
+    expect(
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, -11.0, 800.0, 100.0),
+    );
     await gesture.moveBy(const Offset(0.0, 20.0)); // overscroll again
     await tester.pump();
-    expect(tester.getRect(find.byKey(key1)), const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(
+      tester.getRect(find.byKey(key1)),
+      const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0),
+    );
     await gesture.up();
     debugDefaultTargetPlatformOverride = null;
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
+  group('NestedScrollViewState exposes inner and outer controllers', () {
+    testWidgets('Scrolling by less than the outer extent does not scroll the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(
+        key: globalKey,
+        expanded: false,
+      ));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 104.0);
+      final double scrollExtent = appBarHeight - 50.0;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is not an expanded AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, 54.0);
+      // the inner scroll controller should not have scrolled.
+      expect(globalKey.currentState.innerController.offset, 0.0);
+    });
+
+    testWidgets('Scrolling by exactly the outer extent does not scroll the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(
+        key: globalKey,
+        expanded: false,
+      ));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 104.0);
+      final double scrollExtent = appBarHeight;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is not an expanded AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, 104.0);
+      // the inner scroll controller should not have scrolled.
+      expect(globalKey.currentState.innerController.offset, 0.0);
+    });
+
+    testWidgets('Scrolling by greater than the outer extent scrolls the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(
+        key: globalKey,
+        expanded: false,
+      ));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 104.0);
+      final double scrollExtent = appBarHeight + 50.0;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is not an expanded AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, appBarHeight);
+      // the inner scroll controller should have scrolled equivalent to the
+      // difference between the applied scrollExtent and the outer extent.
+      expect(
+        globalKey.currentState.innerController.offset,
+        scrollExtent - appBarHeight,
+      );
+    });
+
+    testWidgets('scrolling by less than the expanded outer extent does not scroll the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(key: globalKey));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 200.0);
+      final double scrollExtent = appBarHeight - 50.0;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is an expanding AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, 150.0);
+      // the inner scroll controller should not have scrolled.
+      expect(globalKey.currentState.innerController.offset, 0.0);
+    });
+
+    testWidgets('scrolling by exactly the expanded outer extent does not scroll the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(key: globalKey));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 200.0);
+      final double scrollExtent = appBarHeight;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is an expanding AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, 200.0);
+      // the inner scroll controller should not have scrolled.
+      expect(globalKey.currentState.innerController.offset, 0.0);
+    });
+
+    testWidgets('scrolling by greater than the expanded outer extent scrolls the inner body', (WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      await tester.pumpWidget(buildTest(key: globalKey));
+
+      double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      expect(appBarHeight, 200.0);
+      final double scrollExtent = appBarHeight + 50.0;
+      expect(globalKey.currentState.outerController.offset, 0.0);
+      expect(globalKey.currentState.innerController.offset, 0.0);
+
+      // The scroll gesture should occur in the inner body, so the whole
+      // scroll view is scrolled.
+      final TestGesture gesture = await tester.startGesture(Offset(
+        0.0,
+        appBarHeight + 1.0,
+      ));
+      await gesture.moveBy(Offset(0.0, -scrollExtent));
+      await tester.pump();
+
+      appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+      // This is an expanding AppBar.
+      expect(appBarHeight, 104.0);
+      // The outer scroll controller should show an offset of the applied
+      // scrollExtent.
+      expect(globalKey.currentState.outerController.offset, 200.0);
+      // the inner scroll controller should have scrolled equivalent to the
+      // difference between the applied scrollExtent and the outer extent.
+      expect(globalKey.currentState.innerController.offset, 50.0);
+    });
+
+    testWidgets('NestedScrollViewState.outerController should correspond to NestedScrollView.controller', (
+      WidgetTester tester) async {
+      final GlobalKey<NestedScrollViewState> globalKey = GlobalKey();
+      final ScrollController scrollController = ScrollController();
+
+      await tester.pumpWidget(buildTest(
+        controller: scrollController,
+        key: globalKey,
+      ));
+
+      // Scroll to compare offsets between controllers.
+      final TestGesture gesture = await tester.startGesture(const Offset(
+        0.0,
+        100.0,
+      ));
+      await gesture.moveBy(const Offset(0.0, -100.0));
+      await tester.pump();
+
+      expect(
+        scrollController.offset,
+        globalKey.currentState.outerController.offset,
+      );
+      expect(
+        tester.widget<NestedScrollView>(find.byType(NestedScrollView)).controller.offset,
+        globalKey.currentState.outerController.offset,
+      );
+    });
+
+    group('manipulating controllers when', () {
+      testWidgets('outer: not scrolled, inner: not scrolled', (WidgetTester tester) async {
+        final GlobalKey<NestedScrollViewState> globalKey1 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey1,
+          expanded: false,
+        ));
+        expect(globalKey1.currentState.outerController.position.pixels, 0.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        final double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+
+        // Manipulating Inner
+        globalKey1.currentState.innerController.jumpTo(100.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 100.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+        globalKey1.currentState.innerController.jumpTo(0.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+
+        // Reset
+        final GlobalKey<NestedScrollViewState> globalKey2 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey2,
+          expanded: false,
+        ));
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+
+        // Manipulating Outer
+        globalKey2.currentState.outerController.jumpTo(100.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 100.0);
+        globalKey2.currentState.outerController.jumpTo(0.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+      });
+
+      testWidgets('outer: not scrolled, inner: scrolled', (WidgetTester tester) async {
+        final GlobalKey<NestedScrollViewState> globalKey1 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey1,
+          expanded: false,
+        ));
+        expect(globalKey1.currentState.outerController.position.pixels, 0.0);
+        globalKey1.currentState.innerController.position.setPixels(10.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 10.0);
+        final double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+
+        // Manipulating Inner
+        globalKey1.currentState.innerController.jumpTo(100.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 100.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+        globalKey1.currentState.innerController.jumpTo(0.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+
+        // Reset
+        final GlobalKey<NestedScrollViewState> globalKey2 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey2,
+          expanded: false,
+        ));
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+        globalKey2.currentState.innerController.position.setPixels(10.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 10.0);
+
+        // Manipulating Outer
+        globalKey2.currentState.outerController.jumpTo(100.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 100.0);
+        globalKey2.currentState.outerController.jumpTo(0.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+      });
+
+      testWidgets('outer: scrolled, inner: not scrolled', (WidgetTester tester) async {
+        final GlobalKey<NestedScrollViewState> globalKey1 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey1,
+          expanded: false,
+        ));
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        globalKey1.currentState.outerController.position.setPixels(10.0);
+        expect(globalKey1.currentState.outerController.position.pixels, 10.0);
+        final double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+
+        // Manipulating Inner
+        globalKey1.currentState.innerController.jumpTo(100.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 100.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+        globalKey1.currentState.innerController.jumpTo(0.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+
+        // Reset
+        final GlobalKey<NestedScrollViewState> globalKey2 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey2,
+          expanded: false,
+        ));
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        globalKey2.currentState.outerController.position.setPixels(10.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 10.0);
+
+        // Manipulating Outer
+        globalKey2.currentState.outerController.jumpTo(100.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 100.0);
+        globalKey2.currentState.outerController.jumpTo(0.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+      });
+
+      testWidgets('outer: scrolled, inner: scrolled', (WidgetTester tester) async {
+        final GlobalKey<NestedScrollViewState> globalKey1 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey1,
+          expanded: false,
+        ));
+        globalKey1.currentState.innerController.position.setPixels(10.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 10.0);
+        globalKey1.currentState.outerController.position.setPixels(10.0);
+        expect(globalKey1.currentState.outerController.position.pixels, 10.0);
+        final double appBarHeight = tester.renderObject<RenderBox>(find.byType(AppBar)).size.height;
+
+        // Manipulating Inner
+        globalKey1.currentState.innerController.jumpTo(100.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 100.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+        globalKey1.currentState.innerController.jumpTo(0.0);
+        expect(globalKey1.currentState.innerController.position.pixels, 0.0);
+        expect(
+          globalKey1.currentState.outerController.position.pixels,
+          appBarHeight,
+        );
+
+        // Reset
+        final GlobalKey<NestedScrollViewState> globalKey2 = GlobalKey();
+        await tester.pumpWidget(buildTest(
+          key: globalKey2,
+          expanded: false,
+        ));
+        globalKey2.currentState.innerController.position.setPixels(10.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 10.0);
+        globalKey2.currentState.outerController.position.setPixels(10.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 10.0);
+
+        // Manipulating Outer
+        globalKey2.currentState.outerController.jumpTo(100.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 100.0);
+        globalKey2.currentState.outerController.jumpTo(0.0);
+        expect(globalKey2.currentState.innerController.position.pixels, 0.0);
+        expect(globalKey2.currentState.outerController.position.pixels, 0.0);
+      });
+    });
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/39963.
   testWidgets('NestedScrollView with SliverOverlapAbsorber in or out of the first screen', (WidgetTester tester) async {
     await tester.pumpWidget(const _TestLayoutExtentIsNegative(1));