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() {}
+    virtual ~VolumeObserver() {}
+    DISALLOW_COPY_AND_ASSIGN(VolumeObserver);
+  };
+
   static void Initialize();
   static void Shutdown();
   // GetInstance returns NULL if not initialized or if already shutdown.
+  // The mixer may be uninitialized, so use GetInstanceIfInitialized
+  // for volume control until the TODO below is resolved.
   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);
 
+  void AddVolumeObserver(VolumeObserver* observer);
+  void RemoveVolumeObserver(VolumeObserver* observer);
+
  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>;
 
   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;
 
   scoped_ptr<AudioMixer> mixer_;
 
+  ObserverList<VolumeObserver> volume_observers_;
+
   DISALLOW_COPY_AND_ASSIGN(AudioHandler);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_AUDIO_AUDIO_HANDLER_H_