Ensure delayed multidrag timer is stopped (#6340)

Previously we were relying on the gesture arena to call us back to
cancel our timer. However, in the case where we've already been
accepted, asking the gesture arena to reject us doesn't lead to a
callback and we fail to stop the timer (and hence trigger an assert).

Fixes #6156

packages/flutter/lib/src/gestures/multidrag.dart

@@ -72,6 +72,7 @@ abstract class MultiDragPointerState {
   }
 
   /// Resolve this pointer's entry in the [GestureArenaManager] with the given disposition.
+  @protected
   void resolve(GestureDisposition disposition) {
     _arenaEntry.resolve(disposition);
   }
@@ -93,17 +94,21 @@ abstract class MultiDragPointerState {
   /// Override this to call resolve() if the drag should be accepted or rejected.
   /// This is called when a pointer movement is received, but only if the gesture
   /// has not yet been resolved.
+  @protected
   void checkForResolutionAfterMove() { }
 
   /// Called when the gesture was accepted.
   ///
   /// Either immediately or at some future point before the gesture is disposed,
   /// call starter(), passing it initialPosition, to start the drag.
+  @protected
   void accepted(GestureMultiDragStartCallback starter);
 
   /// Called when the gesture was rejected.
   ///
   /// [dispose()] will be called immediately following this.
+  @protected
+  @mustCallSuper
   void rejected() {
     assert(_arenaEntry != null);
     assert(_client == null);
@@ -149,6 +154,7 @@ abstract class MultiDragPointerState {
   }
 
   /// Releases any resources used by the object.
+  @protected
   @mustCallSuper
   void dispose() {
     _arenaEntry?.resolve(GestureDisposition.rejected);
@@ -195,6 +201,7 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
 
   /// Subclasses should override this method to create per-pointer state
   /// objects to track the pointer associated with the given event.
+  @protected
   T createNewPointerState(PointerDownEvent event);
 
   void _handleEvent(PointerEvent event) {
@@ -215,7 +222,7 @@ abstract class MultiDragGestureRecognizer<T extends MultiDragPointerState> exten
       state._cancel();
       _removeState(event.pointer);
     } else if (event is! PointerDownEvent) {
-      // we get the PointerDownEvent that resulted in our addPointer gettig called since we
+      // we get the PointerDownEvent that resulted in our addPointer getting called since we
       // add ourselves to the pointer router then (before the pointer router has heard of
       // the event).
       assert(false);
@@ -408,6 +415,11 @@ class _DelayedPointerState extends MultiDragPointerState {
     assert(_starter == null);
   }
 
+  void _ensureTimerStopped() {
+    _timer?.cancel();
+    _timer = null;
+  }
+
   @override
   void accepted(GestureMultiDragStartCallback starter) {
     assert(_starter == null);
@@ -419,16 +431,25 @@ class _DelayedPointerState extends MultiDragPointerState {
 
   @override
   void checkForResolutionAfterMove() {
-    assert(_timer != null);
+    if (_timer == null) {
+      // If we've been accepted by the gesture arena but the pointer moves too
+      // much before the timer fires, we end up a state where the timer is
+      // stopped but we keep getting calls to this function because we never
+      // actually started the drag. In this case, _starter will be non-null
+      // because we're essentially waiting forever to start the drag.
+      assert(_starter != null);
+      return;
+    }
     assert(pendingDelta != null);
-    if (pendingDelta.distance > kTouchSlop)
+    if (pendingDelta.distance > kTouchSlop) {
       resolve(GestureDisposition.rejected);
+      _ensureTimerStopped();
+    }
   }
 
   @override
   void dispose() {
-    _timer?.cancel();
-    _timer = null;
+    _ensureTimerStopped();
     super.dispose();
   }
 }

packages/flutter/test/gestures/multidrag_test.dart

@@ -0,0 +1,40 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:flutter/gestures.dart';
+
+import 'gesture_tester.dart';
+
+class TestDrag extends Drag {
+}
+
+void main() {
+  setUp(ensureGestureBinding);
+
+  testGesture('Should recognize pan', (GestureTester tester) {
+    DelayedMultiDragGestureRecognizer drag = new DelayedMultiDragGestureRecognizer();
+
+    bool didStartDrag = false;
+    drag.onStart = (Point position) {
+      didStartDrag = true;
+      return new TestDrag();
+    };
+
+    TestPointer pointer = new TestPointer(5);
+    PointerDownEvent down = pointer.down(const Point(10.0, 10.0));
+    drag.addPointer(down);
+    tester.closeArena(5);
+    expect(didStartDrag, isFalse);
+    tester.async.flushMicrotasks();
+    expect(didStartDrag, isFalse);
+    tester.route(pointer.move(const Point(20.0, 20.0)));
+    expect(didStartDrag, isFalse);
+    tester.async.elapse(kLongPressTimeout * 2);
+    expect(didStartDrag, isFalse);
+    tester.route(pointer.move(const Point(30.0, 30.0)));
+    expect(didStartDrag, isFalse);
+    drag.dispose();
+  });
+}