iOS 監聽App音量的變更

iOS 監聽App音量的變化
方法1：
     在applicationDidFinishLaunching函數裡添加
      [[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：當app進入後天后，依然會監聽到volume的變化

2. 對 AudioSession 添加volume變化的 listener, 可以放在startAudioSession函數裡
    //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. 擷取當前的volume
     float volume = 0.0;
    UInt32 dataSize = sizeof(float);
    OSStatus status = AudioSessionGetProperty (kAudioSessionProperty_CurrentHardwareOutputVolume,
                             &dataSize,
                             &volume);