ui: Prefer +/- operators to apply offsets.

ui/gl/gl_surface.h

 // Copyright (c) 2012 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_GL_GL_SURFACE_H_
 #define UI_GL_GL_SURFACE_H_
 
 #include <string>
 
+#include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "build/build_config.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/gfx/size.h"
 #include "ui/gl/gl_export.h"
 
 namespace base {
 class TimeDelta;
 class TimeTicks;
 }
@@ skipping 80 matching lines @@
   // Get the platform specific display on which this surface resides, if
   // available.
   virtual void* GetDisplay();
 
   // Get the platfrom specific configuration for this surface, if available.
   virtual void* GetConfig();
 
   // Get the GL pixel format of the surface, if available.
   virtual unsigned GetFormat();
 
+  typedef base::Callback<void(const base::TimeTicks timebase,
+                              const base::TimeDelta interval)>
+      UpdateVSyncCallback;
+
   // Get the time of the most recent screen refresh, along with the time
-  // between consecutive refreshes. Returns false when these values are
-  // unavailable.
-  virtual bool GetVSyncParameters(base::TimeTicks* timebase,
-                                  base::TimeDelta* interval);
+  // between consecutive refreshes. The callback is called as soon as
+  // the data is available: it could be immediately from this method,
+  // later via a PostTask to the current MessageLoop, or never (if we have
+  // no data source). We provide the strong guarantee that the callback will
+  // not be called once the instance of this class is destroyed.
+  virtual void GetVSyncParameters(const UpdateVSyncCallback& callback);
 
   // Create a GL surface that renders directly to a view.
   static scoped_refptr<GLSurface> CreateViewGLSurface(
       bool software,
       gfx::AcceleratedWidget window);
 
   // Create a GL surface used for offscreen rendering.
   static scoped_refptr<GLSurface> CreateOffscreenGLSurface(
       bool software,
       const gfx::Size& size);
@@ skipping 32 matching lines @@
   virtual gfx::Size GetSize() OVERRIDE;
   virtual void* GetHandle() OVERRIDE;
   virtual unsigned int GetBackingFrameBufferObject() OVERRIDE;
   virtual bool OnMakeCurrent(GLContext* context) OVERRIDE;
   virtual void SetBackbufferAllocation(bool allocated) OVERRIDE;
   virtual void SetFrontbufferAllocation(bool allocated) OVERRIDE;
   virtual void* GetShareHandle() OVERRIDE;
   virtual void* GetDisplay() OVERRIDE;
   virtual void* GetConfig() OVERRIDE;
   virtual unsigned GetFormat() OVERRIDE;
-  virtual bool GetVSyncParameters(base::TimeTicks* timebase,
-                                  base::TimeDelta* interval) OVERRIDE;
+  virtual void GetVSyncParameters(const UpdateVSyncCallback& callback) OVERRIDE;
 
   GLSurface* surface() const { return surface_.get(); }
 
  protected:
   virtual ~GLSurfaceAdapter();
 
  private:
   scoped_refptr<GLSurface> surface_;
 
   DISALLOW_COPY_AND_ASSIGN(GLSurfaceAdapter);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_SURFACE_H_