iOS 监听App音量的变化

方法1：
     在applicationDidFinishLaunching函数里添加
      [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(volumeChanged:)
                                             name:@"AVSystemController_SystemVolumeDidChangeNotification"
                                             object:nil];

  - (void)volumeChanged:(NSNotification *)notification
{
    float volume =
    [[[notification userInfo]
      objectForKey:@"AVSystemController_AudioVolumeNotificationParameter"]
     floatValue];
   
    DDLogVerbose(@"current volume = %f", volume);  
}

   弊端：当app进入后天后，依然会监听到volume的变化

2. 对 AudioSession 添加volume变化的 listener, 可以放在startAudioSession函数里
    //add a listener for Outputvolume
    AudioSessionAddPropertyListener(kAudioSessionProperty_CurrentHardwareOutputVolume ,
                                    volumeListenerCallback,
                                    self
                                    );


void volumeListenerCallback (
                             void                      *inClientData,
                             AudioSessionPropertyID    inID,
                             UInt32                    inDataSize,
                             const void                *inData
                             ){
    const float *volumePointer = inData;
    float volume = *volumePointer;
    NSLog(@"volumeListenerCallback %f", volume);
}

3. 获取当前的volume
     float volume = 0.0;
    UInt32 dataSize = sizeof(float);
    OSStatus status = AudioSessionGetProperty (kAudioSessionProperty_CurrentHardwareOutputVolume,
                             &dataSize,
                             &volume);