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diff --git a/widget/SwipeTracker.h b/widget/SwipeTracker.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef SwipeTracker_h
+#define SwipeTracker_h
+
+#include "EventForwards.h"
+#include "mozilla/layers/AxisPhysicsMSDModel.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "nsRefreshObservers.h"
+#include "Units.h"
+
+class nsIWidget;
+class nsRefreshDriver;
+
+namespace mozilla {
+
+class PanGestureInput;
+
+/**
+ * SwipeTracker turns PanGestureInput events into swipe events
+ * (WidgetSimpleGestureEvent) and dispatches them into Gecko.
+ * The swiping behavior mirrors the behavior of the Cocoa API
+ * -[NSEvent
+ *     trackSwipeEventWithOptions:dampenAmountThresholdMin:max:usingHandler:].
+ * The advantage of using this class over the Cocoa API is that this class
+ * properly supports submitting queued up events to it, and that it hopefully
+ * doesn't intermittently break scrolling the way the Cocoa API does (bug
+ * 927702).
+ *
+ * The swipe direction is either left or right. It is determined before the
+ * SwipeTracker is created and stays fixed during the swipe.
+ * During the swipe, the swipe has a current "value" which is between 0 and the
+ * target value. The target value is either 1 (swiping left) or -1 (swiping
+ * right) - see SwipeSuccessTargetValue().
+ * A swipe can either succeed or fail. If it succeeds, the swipe animation
+ * animates towards the success target value; if it fails, it animates back to
+ * a value of 0. A swipe can only succeed if the user is swiping in an allowed
+ * direction. (Since both the allowed directions and the swipe direction are
+ * known at swipe start time, it's clear from the beginning whether a swipe is
+ * doomed to fail. In that case, the purpose of the SwipeTracker is to simulate
+ * a bounce-back animation.)
+ */
+class SwipeTracker final : public nsARefreshObserver {
+ public:
+  NS_INLINE_DECL_REFCOUNTING(SwipeTracker, override)
+
+  SwipeTracker(nsIWidget& aWidget, const PanGestureInput& aSwipeStartEvent,
+               uint32_t aAllowedDirections, uint32_t aSwipeDirection);
+
+  void Destroy();
+
+  nsEventStatus ProcessEvent(const PanGestureInput& aEvent,
+                             bool aProcessingFirstEvent = false);
+  void CancelSwipe(const TimeStamp& aTimeStamp);
+
+  static WidgetSimpleGestureEvent CreateSwipeGestureEvent(
+      EventMessage aMsg, nsIWidget* aWidget,
+      const LayoutDeviceIntPoint& aPosition, const TimeStamp& aTimeStamp);
+
+  // nsARefreshObserver
+  void WillRefresh(mozilla::TimeStamp aTime) override;
+
+  static bool CanTriggerSwipe(const PanGestureInput& aPanInput);
+
+ protected:
+  ~SwipeTracker();
+
+  bool SwipingInAllowedDirection() const {
+    return mAllowedDirections & mSwipeDirection;
+  }
+  double SwipeSuccessTargetValue() const;
+  double ClampToAllowedRange(double aGestureAmount) const;
+  bool ComputeSwipeSuccess() const;
+  void StartAnimating(double aStartValue, double aTargetValue);
+  void SwipeFinished(const TimeStamp& aTimeStamp);
+  void UnregisterFromRefreshDriver();
+  bool SendSwipeEvent(EventMessage aMsg, uint32_t aDirection, double aDelta,
+                      const TimeStamp& aTimeStamp);
+
+  nsIWidget& mWidget;
+  RefPtr<nsRefreshDriver> mRefreshDriver;
+  layers::AxisPhysicsMSDModel mAxis;
+  const LayoutDeviceIntPoint mEventPosition;
+  TimeStamp mLastEventTimeStamp;
+  TimeStamp mLastAnimationFrameTime;
+  const uint32_t mAllowedDirections;
+  const uint32_t mSwipeDirection;
+  double mGestureAmount;
+  double mCurrentVelocity;
+  bool mEventsAreControllingSwipe;
+  bool mEventsHaveStartedNewGesture;
+  bool mRegisteredWithRefreshDriver;
+};
+
+struct SwipeEventQueue {
+  SwipeEventQueue(uint32_t aAllowedDirections, uint64_t aInputBlockId)
+      : allowedDirections(aAllowedDirections), inputBlockId(aInputBlockId) {}
+
+  nsTArray<PanGestureInput> queuedEvents;
+  uint32_t allowedDirections;
+  uint64_t inputBlockId;
+};
+
+}  // namespace mozilla
+
+#endif  // SwipeTracker_h