公司项目使用的音视频库需要在用户切换麦克风时收到通知，在网上找了很久半天，最后在苹果的官方示例代码中发现了解决方案

AudioObjectPropertyAddress theAddress = { kAudioHardwarePropertyDefaultInputDevice,
        kAudioObjectPropertyScopeGlobal,
        kAudioObjectPropertyElementMaster };
    
AudioObjectAddPropertyListener(kAudioObjectSystemObject, &theAddress, AOPropertyListenerProc, (__bridge void * _Nullable)(self));

使用设备属性设置一个监听器，这里我需要监听麦克风设备的切换，所以选择了kAudioHardwarePropertyDefaultInputDevice，AOPropertyListenerProc为一个函数指针，作为通知后的回调
之后再实现AOPropertyListenerProc函数

OSStatus AOPropertyListenerProc(AudioObjectID inObjectID, UInt32 inNumberAddresses, const AudioObjectPropertyAddress inAddresses[], void* inClientData)
{
    
    for (UInt32 x=0; x<inNumberAddresses; x++) {
        
        switch (inAddresses[x].mSelector)
        {
                /*
                 * These are the other types of notifications we might receive, however, they are beyond
                 * the scope of this sample and we ignore them.
                 *
                 case kAudioHardwarePropertyDefaultInputDevice:
                 fprintf(stderr, "AOPropertyListenerProc: default input device changed\n");
                 break;
                 
                 case kAudioHardwarePropertyDefaultOutputDevice:
                 fprintf(stderr, "AOPropertyListenerProc: default output device changed\n");
                 break;
                 
                 case kAudioHardwarePropertyDefaultSystemOutputDevice:
                 fprintf(stderr, "AOPropertyListenerProc: default system output device changed\n");
                 break;
                 */
            case kAudioHardwarePropertyDefaultInputDevice:{
                fprintf(stderr, "AOPropertyListenerProc: default input device changed\n");
            }
                break;
//            case kAudioHardwarePropertyDevices:
//            {
//                fprintf(stderr, "AOPropertyListenerProc: kAudioHardwarePropertyDevices\n");
//            }
//                break;
                
            default:
                fprintf(stderr, "AOPropertyListenerProc: unknown message\n");
                break;
        }
    }
    
    return noErr;
}