Read compositor VSync information from platform, when possible

ui/compositor/compositor_vsync_manager.h

+// Copyright (c) 2014 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 UI_COMPOSITOR_COMPOSITOR_VSYNC_MANAGER_H_
+#define UI_COMPOSITOR_COMPOSITOR_VSYNC_MANAGER_H_
+
+#include "base/memory/ref_counted.h"
+#include "base/observer_list_threadsafe.h"
+#include "base/synchronization/lock.h"
+#include "base/time/time.h"
+#include "ui/compositor/compositor_export.h"
+
+namespace ui {
+
+// This class manages vsync parameters for a compositor. It merges updates of
+// the parameters from different sources and sends the merged updates to
+// observers which register to it. This class is explicitly synchronized and is
+// safe to use and update from any thread. Observers of the manager will be
+// notified on the thread they have registered from, and should be removed from
+// the same thread.
+class COMPOSITOR_EXPORT CompositorVSyncManager
+    : public base::RefCountedThreadSafe<CompositorVSyncManager> {
+ public:
+  class Observer {
+   public:
+    virtual void OnUpdateVSyncParameters(base::TimeTicks timebase,
+                                         base::TimeDelta interval) = 0;
+  };
+
+  CompositorVSyncManager();
+
+  // The "authoritative" vsync interval, if provided, will override |interval|
+  // as reported by UpdateVSyncParameters() whenever it is called.  This is
+  // typically the value reported by a more reliable source, e.g. the platform
+  // display configuration.  In the particular case of ChromeOS -- this is the
+  // value queried through XRandR, which is more reliable than the value
+  // queried through the 3D context.
+  void SetAuthoritativeVSyncInterval(base::TimeDelta interval);
+
+  // The vsync parameters consist of |timebase|, which is the platform timestamp
+  // of the last vsync, and |interval|, which is the interval between vsyncs.
+  // |interval| may be overriden by SetAuthoritativeVSyncInterval() above.
+  void UpdateVSyncParameters(base::TimeTicks timebase,
+                             base::TimeDelta interval);
+
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+ private:
+  friend class base::RefCountedThreadSafe<CompositorVSyncManager>;
+
+  ~CompositorVSyncManager();
+
+  void NotifyObservers(base::TimeTicks timebase, base::TimeDelta interval);
+
+  // List of observers.
+  scoped_refptr<ObserverListThreadSafe<Observer> > observer_list_;
+
+  // Protects the cached vsync parameters below.
+  base::Lock vsync_parameters_lock_;
+  base::TimeTicks last_timebase_;
+  base::TimeDelta last_interval_;
+  base::TimeDelta authoritative_vsync_interval_;
+
+  DISALLOW_COPY_AND_ASSIGN(CompositorVSyncManager);
+};
+
+}  // namespace ui
+
+#endif  // UI_COMPOSITOR_COMPOSITOR_VSYNC_MANAGER_H_