Add OnDisplayMetricsChanged in DisplayObserver.

ui/gfx/display_observer.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_GFX_DISPLAY_OBSERVER_H_
 #define UI_GFX_DISPLAY_OBSERVER_H_
 
 #include "ui/gfx/gfx_export.h"
 
 namespace gfx {
 class Display;
 
 // Observers for display configuration changes.
 // TODO(oshima): consolidate |WorkAreaWatcherObserver| and
 // |DisplaySettingsProvier|. crbug.com/122863.
 class GFX_EXPORT DisplayObserver {
  public:
-  // Called when the |display|'s bound has changed.
-  virtual void OnDisplayBoundsChanged(const Display& display) = 0;
+  enum DisplayMetrics {
+    DISPLAY_METRICS_NONE = 0,
+    DISPLAY_METRICS_BOUNDS = 1 << 0,
+    DISPLAY_METRICS_WORK_AREA = 1 << 1,
+    DISPLAY_METRICS_DEVICE_SCALE_FACTOR = 1 << 2,
+    DISPLAY_METRICS_ROTATION = 1 << 3,
+  };
 
   // Called when |new_display| has been added.
   virtual void OnDisplayAdded(const Display& new_display) = 0;
 
   // Called when |old_display| has been removed.
   virtual void OnDisplayRemoved(const Display& old_display) = 0;
 
+  // Called when a |display| has one or more metrics changed. |changed_metrics|
+  // will contain the information about the change, see |DisplayMetrics|.
+  virtual void OnDisplayMetricsChanged(
+      const Display& display, DisplayMetrics changed_metrics) = 0;

[Elliot Glaysher, 2014/05/12 17:47:11]

Bitfields should be ints; enums imply mutual exclusivity.

[oshima, 2014/05/12 19:58:13]

Using enum for bitfield isn't uncommon (can be found gfx::Canvas, base::File and
many other places) and also I couldn't find this in the style guide. 

What's wrong is the argument type. It should be uint32 instead of
DisplayMetrics.

+
  protected:
   virtual ~DisplayObserver();
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_DISPLAY_OBSERVER_H_