Refactor ScrollDragActivity into two pieces (#9629)

This will make it easier to implement nested scrolling.

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

@@ -20,9 +20,8 @@ import 'ticker_provider.dart';
 abstract class ScrollActivityDelegate {
   AxisDirection get axisDirection;
 
-  double get pixels;
   double setPixels(double pixels);
-  double applyUserOffset(double delta);
+  void applyUserOffset(double delta);
 
   void goIdle();
   void goBallistic(double velocity);
@@ -107,22 +106,29 @@ class IdleScrollActivity extends ScrollActivity {
   bool get isScrolling => false;
 }
 
-class DragScrollActivity extends ScrollActivity implements Drag {
-  DragScrollActivity(
+class ScrollDragController implements Drag {
+  ScrollDragController(
     ScrollActivityDelegate delegate,
     DragStartDetails details,
     this.onDragCanceled,
-  ) : _lastDetails = details, super(delegate);
+  ) : _delegate = delegate, _lastDetails = details;
 
-  final VoidCallback onDragCanceled;
+  ScrollActivityDelegate get delegate => _delegate;
+  ScrollActivityDelegate _delegate;
 
-  @override
-  void didTouch() {
-    assert(false);
-  }
+  final VoidCallback onDragCanceled;
 
   bool get _reversed => axisDirectionIsReversed(delegate.axisDirection);
 
+  /// Updates the controller's link to the [ScrollActivityDelegate].
+  ///
+  /// This should only be called when a controller is being moved from a defunct
+  /// (or about-to-be defunct) [ScrollActivityDelegate] object to a new one.
+  void updateDelegate(ScrollActivityDelegate value) {
+    assert(_delegate != value);
+    _delegate = value;
+  }
+
   @override
   void update(DragUpdateDetails details) {
     assert(details.primaryDelta != null);
@@ -133,8 +139,6 @@ class DragScrollActivity extends ScrollActivity implements Drag {
     if (_reversed) // e.g. an AxisDirection.up scrollable
       offset = -offset;
     delegate.applyUserOffset(offset);
-    // We ignore any reported overscroll returned by setPixels,
-    // because it gets reported via the reportOverscroll path.
   }
 
   @override
@@ -155,41 +159,59 @@ class DragScrollActivity extends ScrollActivity implements Drag {
     delegate.goBallistic(0.0);
   }
 
-  @override
+  @mustCallSuper
   void dispose() {
     _lastDetails = null;
     if (onDragCanceled != null)
       onDragCanceled();
-    super.dispose();
   }
 
+  dynamic get lastDetails => _lastDetails;
   dynamic _lastDetails;
+}
+
+class DragScrollActivity extends ScrollActivity {
+  DragScrollActivity(
+    ScrollActivityDelegate delegate,
+    ScrollDragController controller,
+  ) : _controller = controller, super(delegate);
+
+  ScrollDragController _controller;
+
+  @override
+  void didTouch() {
+    assert(false);
+  }
 
   @override
   void dispatchScrollStartNotification(ScrollMetrics metrics, BuildContext context) {
-    assert(_lastDetails is DragStartDetails);
-    new ScrollStartNotification(metrics: metrics, context: context, dragDetails: _lastDetails).dispatch(context);
+    final dynamic lastDetails = _controller.lastDetails;
+    assert(lastDetails is DragStartDetails);
+    new ScrollStartNotification(metrics: metrics, context: context, dragDetails: lastDetails).dispatch(context);
   }
 
   @override
   void dispatchScrollUpdateNotification(ScrollMetrics metrics, BuildContext context, double scrollDelta) {
-    assert(_lastDetails is DragUpdateDetails);
-    new ScrollUpdateNotification(metrics: metrics, context: context, scrollDelta: scrollDelta, dragDetails: _lastDetails).dispatch(context);
+    final dynamic lastDetails = _controller.lastDetails;
+    assert(lastDetails is DragUpdateDetails);
+    new ScrollUpdateNotification(metrics: metrics, context: context, scrollDelta: scrollDelta, dragDetails: lastDetails).dispatch(context);
   }
 
   @override
   void dispatchOverscrollNotification(ScrollMetrics metrics, BuildContext context, double overscroll) {
-    assert(_lastDetails is DragUpdateDetails);
-    new OverscrollNotification(metrics: metrics, context: context, overscroll: overscroll, dragDetails: _lastDetails).dispatch(context);
+    final dynamic lastDetails = _controller.lastDetails;
+    assert(lastDetails is DragUpdateDetails);
+    new OverscrollNotification(metrics: metrics, context: context, overscroll: overscroll, dragDetails: lastDetails).dispatch(context);
   }
 
   @override
   void dispatchScrollEndNotification(ScrollMetrics metrics, BuildContext context) {
     // We might not have DragEndDetails yet if we're being called from beginActivity.
+    final dynamic lastDetails = _controller.lastDetails;
     new ScrollEndNotification(
       metrics: metrics,
       context: context,
-      dragDetails: _lastDetails is DragEndDetails ? _lastDetails : null
+      dragDetails: lastDetails is DragEndDetails ? lastDetails : null
     ).dispatch(context);
   }
 
@@ -198,6 +220,12 @@ class DragScrollActivity extends ScrollActivity implements Drag {
 
   @override
   bool get isScrolling => true;
+
+  @override
+  void dispose() {
+    _controller = null;
+    super.dispose();
+  }
 }
 
 class BallisticScrollActivity extends ScrollActivity {

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

@@ -89,28 +89,28 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     assert(_pixels != null);
     assert(SchedulerBinding.instance.schedulerPhase.index <= SchedulerPhase.transientCallbacks.index);
     if (newPixels != pixels) {
-      final double overScroll = applyBoundaryConditions(newPixels);
+      final double overscroll = applyBoundaryConditions(newPixels);
       assert(() {
         final double delta = newPixels - pixels;
-        if (overScroll.abs() > delta.abs()) {
+        if (overscroll.abs() > delta.abs()) {
           throw new FlutterError(
             '$runtimeType.applyBoundaryConditions returned invalid overscroll value.\n'
             'setPixels() was called to change the scroll offset from $pixels to $newPixels.\n'
             'That is a delta of $delta units.\n'
-            '$runtimeType.applyBoundaryConditions reported an overscroll of $overScroll units.'
+            '$runtimeType.applyBoundaryConditions reported an overscroll of $overscroll units.'
           );
         }
         return true;
       });
       final double oldPixels = _pixels;
-      _pixels = newPixels - overScroll;
+      _pixels = newPixels - overscroll;
       if (_pixels != oldPixels) {
         notifyListeners();
         didUpdateScrollPositionBy(_pixels - oldPixels);
       }
-      if (overScroll != 0.0) {
-        didOverscrollBy(overScroll);
-        return overScroll;
+      if (overscroll != 0.0) {
+        didOverscrollBy(overscroll);
+        return overscroll;
       }
     }
     return 0.0;

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

@@ -112,6 +112,14 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     assert(other.context == context);
     super.absorb(other);
     _userScrollDirection = other._userScrollDirection;
+
+    assert(_currentDrag == null);
+    if (other._currentDrag != null) {
+      other._currentDrag.updateDelegate(this);
+      _currentDrag = other._currentDrag;
+      other._currentDrag = null;
+    }
+
     assert(activity == null);
     assert(other.activity != null);
     other.activity.updateDelegate(this);
@@ -176,6 +184,8 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
       oldIgnorePointer = false;
       wasScrolling = false;
     }
+    _currentDrag?.dispose();
+    _currentDrag = null;
     _activity = newActivity;
     isScrollingNotifier.value = activity.isScrolling;
     if (!activity.isScrolling)
@@ -187,9 +197,9 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
   }
 
   @override
-  double applyUserOffset(double delta) {
+  void applyUserOffset(double delta) {
     updateUserScrollDirection(delta > 0.0 ? ScrollDirection.forward : ScrollDirection.reverse);
-    return setPixels(pixels - physics.applyPhysicsToUserOffset(this, delta));
+    setPixels(pixels - physics.applyPhysicsToUserOffset(this, delta));
   }
 
   /// End the current [ScrollActivity], replacing it with an
@@ -334,20 +344,27 @@ class ScrollPositionWithSingleContext extends ScrollPosition implements ScrollAc
     activity.didTouch();
   }
 
+  ScrollDragController _currentDrag;
+
   /// Start a drag activity corresponding to the given [DragStartDetails].
   ///
-  /// The `dragCancelCallback` argument will be invoked if the drag is ended
+  /// The `onDragCanceled` argument will be invoked if the drag is ended
   /// prematurely (e.g. from another activity taking over). See
-  /// [DragScrollActivity.onDragCanceled] for details.
+  /// [ScrollDragController.onDragCanceled] for details.
   @override
-  DragScrollActivity drag(DragStartDetails details, VoidCallback dragCancelCallback) {
-    beginActivity(new DragScrollActivity(this, details, dragCancelCallback));
-    return activity;
+  Drag drag(DragStartDetails details, VoidCallback onDragCanceled) {
+    final ScrollDragController drag = new ScrollDragController(this, details, onDragCanceled);
+    beginActivity(new DragScrollActivity(this, drag));
+    assert(_currentDrag == null);
+    _currentDrag = drag;
+    return drag;
   }
 
   @override
   void dispose() {
     assert(pixels != null);
+    _currentDrag?.dispose();
+    _currentDrag = null;
     activity?.dispose(); // it will be null if it got absorbed by another ScrollPosition
     _activity = null;
     super.dispose();