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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);

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