Mozart: Move frame lag compensation to new class.

mojo/services/gfx/composition/cpp/frame_tracker.h

+// Copyright 2016 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.
+
+#ifndef MOJO_SERVICES_GFX_COMPOSITION_CPP_SCHEDULING_H_
+#define MOJO_SERVICES_GFX_COMPOSITION_CPP_SCHEDULING_H_
+
+#include "mojo/services/gfx/composition/interfaces/scheduling.mojom.h"
+
+namespace mojo {
+namespace gfx {
+namespace composition {
+
+// Tracks frame scheduling information.
+class FrameTracker {
+ public:
+  FrameTracker();
+  ~FrameTracker();
+
+  // Returns the number of frames that have been tracked.
+  uint64_t frame_count() const { return frame_count_; }
+
+  // Returns the current frame info.
+  // This value is not meaningful when |frame_count()| is zero.
+  const mojo::gfx::composition::FrameInfo& frame_info() const {
+    return frame_info_;
+  }
+
+  // Clears the frame tracker's state such that the next update will be
+  // treated as if it were the first.
+  void Clear();
+
+  // Updates |frame_info()| with new frame scheduling information
+  // from |raw_frame_info| and applies compensation for lag.
+  //
+  // |now| should come from a recent call to |mojo::GetTimeTicksNow()|.
+  //
+  // Whenever an application receives new frame scheduling information from the
+  // system, it should call this function before using it.
+  //
+  // Returns the time delta between the previous frame and the current frame
+  // in microseconds, or 0 if this is the first frame.
+  uint64_t Update(const mojo::gfx::composition::FrameInfo& raw_frame_info,
+                  MojoTimeTicks now);
+
+ private:
+  uint64_t frame_count_ = 0u;
+  mojo::gfx::composition::FrameInfo frame_info_;
+
+  MOJO_DISALLOW_COPY_AND_ASSIGN(FrameTracker);
+};
+
+}  // namespace composition
+}  // namespace gfx
+}  // namespace mojo
+
+#endif  // MOJO_SERVICES_GFX_COMPOSITION_CPP_SCHEDULING_H_