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);