如何在Swift中连接多个AudioUnits？

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

let component = AudioComponentFindNext(nil, &desc)

var audioUnit: AudioUnit?
AudioComponentInstanceNew(component, &audioUnit)

// 设置音频格式
let streamFormat = AudioStreamBasicDescription()
streamFormat.mSampleRate = 44100.0
streamFormat.mFormatID = kAudioFormatLinearPCM
streamFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked
streamFormat.mFramesPerPacket = 1
streamFormat.mChannelsPerFrame = 2
streamFormat.mBitsPerChannel = 16
streamFormat.mBytesPerFrame = streamFormat.mChannelsPerFrame * (streamFormat.mBitsPerChannel / 8)
streamFormat.mBytesPerPacket = streamFormat.mBytesPerFrame * streamFormat.mFramesPerPacket

AudioUnitSetProperty(audioUnit!, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &streamFormat, UInt32(MemoryLayout<AudioStreamBasicDescription>.size))

// 设置输入回调
var inputCallback = AURenderCallbackStruct(inputProc: inputRenderCallback, inputProcRefCon: nil)
AudioUnitSetProperty(audioUnit!, kAudioOutputUnitProperty_SetInputCallback, kAudioUnitScope_Global, 0, &inputCallback, UInt32(MemoryLayout<AURenderCallbackStruct>.size))

// 设置输出回调
var outputCallback = AURenderCallbackStruct(inputProc: outputRenderCallback, inputProcRefCon: nil)
AudioUnitSetProperty(audioUnit!, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Global, 0, &outputCallback, UInt32(MemoryLayout<AURenderCallbackStruct>.size))

var connection = AudioUnitConnection()
connection.sourceAudioUnit = sourceAudioUnit
connection.sourceOutputNumber = 0
connection.destInputNumber = 0

AudioUnitSetProperty(destinationAudioUnit, kAudioUnitProperty_MakeConnection, kAudioUnitScope_Input, 0, &connection, UInt32(MemoryLayout<AudioUnitConnection>.size))

AudioUnitInitialize(audioUnit!)
AudioOutputUnitStart(audioUnit!)