1 files changed, 59 insertions, 0 deletions
diff --git a/gfx/layers/apz/src/FlingAccelerator.h b/gfx/layers/apz/src/FlingAccelerator.h
new file mode 100644
index 0000000000..49e9a7b257
--- /dev/null
+++ b/gfx/layers/apz/src/FlingAccelerator.h
@@ -0,0 +1,59 @@
+/* -*- 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 mozilla_layers_FlingAccelerator_h
+#define mozilla_layers_FlingAccelerator_h
+
+#include "mozilla/layers/SampleTime.h"
+#include "Units.h"
+
+namespace mozilla {
+namespace layers {
+
+struct FlingHandoffState;
+
+/**
+ * This class is used to track state that is used when determining whether a
+ * fling should be accelerated.
+ */
+class FlingAccelerator final {
+ public:
+  FlingAccelerator() {}
+
+  // Resets state so that the next fling will not be accelerated.
+  void Reset();
+
+  // Returns false after a reset or before the first fling.
+  bool IsTracking() const { return mIsTracking; }
+
+  // Starts a new fling, and returns the (potentially accelerated) velocity that
+  // should be used for that fling.
+  ParentLayerPoint GetFlingStartingVelocity(
+      const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+      const FlingHandoffState& aHandoffState);
+
+  void ObserveFlingCanceled(const ParentLayerPoint& aVelocity) {
+    mPreviousFlingCancelVelocity = aVelocity;
+  }
+
+ protected:
+  bool ShouldAccelerate(const SampleTime& aNow,
+                        const ParentLayerPoint& aVelocity,
+                        const FlingHandoffState& aHandoffState) const;
+
+  // The initial velocity of the most recent fling.
+  ParentLayerPoint mPreviousFlingStartingVelocity;
+  // The velocity that the previous fling animation had at the point it was
+  // interrupted.
+  ParentLayerPoint mPreviousFlingCancelVelocity;
+  // Whether the upcoming fling is eligible for acceleration.
+  bool mIsTracking = false;
+};
+
+}  // namespace layers
+}  // namespace mozilla
+
+#endif  // mozilla_layers_FlingAccelerator_h