Support for showing/hiding status area volume controls in desktop devices

chrome/browser/chromeos/audio/audio_handler.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 CHROME_BROWSER_CHROMEOS_AUDIO_AUDIO_HANDLER_H_
 #define CHROME_BROWSER_CHROMEOS_AUDIO_AUDIO_HANDLER_H_
 #pragma once
 
 #include "base/basictypes.h"
+#include "base/observer_list.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/threading/thread.h"
 
 template <typename T> struct DefaultSingletonTraits;
 
 namespace chromeos {
 
 class AudioMixer;
 
 class AudioHandler {
  public:
+  class VolumeObserver {
+   public:
+    virtual void OnVolumeChanged() = 0;
+   protected:
+    // VolumeObserver registers/de-registers itself with AudioHandler in the
+    // ctor/dtor.
+    VolumeObserver();
+    virtual ~VolumeObserver();
+  };
+
   static void Initialize();
   static void Shutdown();
   // GetInstance returns NULL if not initialized or if already shutdown.

[Daniel Erat, 2012/01/27 18:03:57]

not your fault, but mind updating this comment to clarify that it returns NULL
if the AudioHandler hasn't been initialized, and that the mixer may still not be
initialized at this point?

[achuithb, 2012/01/31 00:58:43]

Done.

   static AudioHandler* GetInstance();
 
-  // Is the mixer initialized?
+  // GetInstanceIfInitialized returns NULL if GetInstance returns NULL or if
+  // the mixer has not finished initializing.
   // TODO(derat): All of the volume-percent methods will produce "interesting"
   // results before the mixer is initialized, since the driver's volume range
   // isn't known at that point.  This could be avoided if AudioMixer objects
   // instead took percentages and did their own conversions to decibels.
-  bool IsInitialized();
+  static AudioHandler* GetInstanceIfInitialized();
 
   // Gets volume level in our internal 0-100% range, 0 being pure silence.
   double GetVolumePercent();
 
   // Sets volume level from 0-100%.
   void SetVolumePercent(double volume_percent);
 
   // Adjusts volume up (positive percentage) or down (negative percentage).
   void AdjustVolumeByPercent(double adjust_by_percent);
 
   // Is the volume currently muted?
   bool IsMuted();
 
   // Mutes or unmutes all audio.
   void SetMuted(bool do_mute);
 
  private:
   // Defines the delete on exit Singleton traits we like.  Best to have this
   // and constructor/destructor private as recommended for Singletons.
   friend struct DefaultSingletonTraits<AudioHandler>;
+  friend class VolumeObserver;
 
   AudioHandler();
   virtual ~AudioHandler();
 
+  bool IsMixerInitialized();
+
   // Conversion between our internal scaling (0-100%) and decibels.
   double VolumeDbToPercent(double volume_db) const;
   double PercentToVolumeDb(double volume_percent) const;
 
+  void AddVolumeObserver(VolumeObserver* observer);
+  void RemoveVolumeObserver(VolumeObserver* observer);
+
   scoped_ptr<AudioMixer> mixer_;
 
+  ObserverList<VolumeObserver> volume_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(AudioHandler);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_AUDIO_AUDIO_HANDLER_H_