在Objective-C中，同时录制和播放音频是相当简单的。在互联网上有大量的示例代码。但我想使用Swift中的Audio Unit/Core Audio同时录制和播放音频。使用Swift时，关于这方面的帮助和示例代码非常少。我找不到任何可以说明如何实现这一点的帮助。
我正在努力与下面的代码。

let preferredIOBufferDuration = 0.005 
let kInputBus  = AudioUnitElement(1)
let kOutputBus = AudioUnitElement(0)

init() {
    // This is my Audio Unit settings code.
    var status: OSStatus

    do {
        try AVAudioSession.sharedInstance().setPreferredIOBufferDuration(preferredIOBufferDuration)
    } catch let error as NSError {
        print(error)
    }

    var desc: AudioComponentDescription = AudioComponentDescription()
    desc.componentType = kAudioUnitType_Output
    desc.componentSubType = kAudioUnitSubType_VoiceProcessingIO
    desc.componentFlags = 0
    desc.componentFlagsMask = 0
    desc.componentManufacturer = kAudioUnitManufacturer_Apple

    let inputComponent: AudioComponent = AudioComponentFindNext(nil, &desc)

    status = AudioComponentInstanceNew(inputComponent, &audioUnit)
    checkStatus(status)

    var flag = UInt32(1)
    status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, kInputBus, &flag, UInt32(sizeof(UInt32)))
    checkStatus(status)

    status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Output, kOutputBus, &flag, UInt32(sizeof(UInt32)))
    checkStatus(status)

    var audioFormat: AudioStreamBasicDescription! = AudioStreamBasicDescription()
    audioFormat.mSampleRate = 8000
    audioFormat.mFormatID = kAudioFormatLinearPCM
    audioFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked
    audioFormat.mFramesPerPacket = 1
    audioFormat.mChannelsPerFrame = 1
    audioFormat.mBitsPerChannel = 16
    audioFormat.mBytesPerPacket = 2
    audioFormat.mBytesPerFrame = 2

    status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, kInputBus, &audioFormat, UInt32(sizeof(UInt32)))
    checkStatus(status)

    try! AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord)
    status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, kOutputBus, &audioFormat, UInt32(sizeof(UInt32)))
    checkStatus(status)

    // Set input/recording callback
    var inputCallbackStruct: AURenderCallbackStruct! = AURenderCallbackStruct(inputProc: recordingCallback, inputProcRefCon: UnsafeMutablePointer(unsafeAddressOf(self)))
    inputCallbackStruct.inputProc = recordingCallback
    inputCallbackStruct.inputProcRefCon = UnsafeMutablePointer(unsafeAddressOf(self))
    status = AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_SetInputCallback, kAudioUnitScope_Global, kInputBus, &inputCallbackStruct, UInt32(sizeof(UInt32)))
    checkStatus(status)

    // Set output/renderar/playback callback
    var renderCallbackStruct: AURenderCallbackStruct! = AURenderCallbackStruct(inputProc: playbackCallback, inputProcRefCon: UnsafeMutablePointer(unsafeAddressOf(self)))
    renderCallbackStruct.inputProc = playbackCallback
    renderCallbackStruct.inputProcRefCon = UnsafeMutablePointer(unsafeAddressOf(self))
    status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, kOutputBus, &renderCallbackStruct, UInt32(sizeof(UInt32)))
    checkStatus(status)

    flag = 0
    status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_ShouldAllocateBuffer, kAudioUnitScope_Output, kInputBus, &flag, UInt32(sizeof(UInt32)))
}

func recordingCallback(inRefCon: UnsafeMutablePointer<Void>,
                      ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>,
                      inTimeStamp: UnsafePointer<AudioTimeStamp>,
                      inBufNumber: UInt32,
                      inNumberFrames: UInt32,
                      ioData: UnsafeMutablePointer<AudioBufferList>) -> OSStatus {

    print("recordingCallback got fired  >>>")

    return noErr
}

func playbackCallback(inRefCon: UnsafeMutablePointer<Void>,
                      ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>,
                      inTimeStamp: UnsafePointer<AudioTimeStamp>,
                      inBufNumber: UInt32,
                      inNumberFrames: UInt32,
                      ioData: UnsafeMutablePointer<AudioBufferList>) -> OSStatus {

    print("playbackCallback got fired  <<<")

    return noErr
}

在这段代码中，只有recordingCallback方法被调用。而playbackCallback方法根本没有被触发。我确信我在这里出了问题。有人能帮我解决这个问题吗？我正在为这个问题绞尽脑汁。