AVAudioEngine obtains channel audio data

Question

When a Mac is connected to an audio interface, I can use AVAudioEngine to obtain audio data from the MIC and OTG channels of the audio interface. Part of the code is as follows:

buffer = CircularBuffer<Float>(channelCount: 2, capacity: Int(framesPerSample!) * 512)
engine.setInputDevice(device)
format = AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: device.nominalSampleRate!, channels: 2, interleaved: false)!
engine.connect(engine.inputNode, to: engine.mainMixerNode, format: format)
let channels = device.channels(scope: .input)
var channelArray:Array<Int32> = Array(repeating: -1, count: channels.intValue)
if index % 2 == 0{
 channelArray[0] = index.int32Value
}else{
 channelArray[1] = index.int32Value
}
guard let audioUnit = engine.inputNode.audioUnit else {
 return
}
audioUnit.setPropertyChannelMap(&channelArray, .input, 1)
checkErr(AudioUnitAddRenderNotify(engine.inputNode.audioUnit!,
 renderCallback,
 Unmanaged.passUnretained(self).toOpaque()))
let renderCallback: AURenderCallback = {
 (inRefCon: UnsafeMutableRawPointer,
 ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>,
 inTimeStamp: UnsafePointer<AudioTimeStamp>,
 inBusNumber: UInt32,
 inNumberFrames: UInt32,
 ioData: UnsafeMutablePointer<AudioBufferList>?) -> OSStatus in
 
 if ioActionFlags.pointee == AudioUnitRenderActionFlags.unitRenderAction_PostRender {
 let inSelf = Unmanaged<Engine>.fromOpaque(inRefCon).takeUnretainedValue()
 
 let sampleTime = inTimeStamp.pointee.mSampleTime
 
 let start = sampleTime.int64Value
 let end = start + Int64(inNumberFrames)
 if inSelf.buffer.write(from: ioData!, start: start, end: end) != .noError {
 return noErr
 }
 inSelf.getVolumeIntensity(from: ioData, inNumberFrames: inNumberFrames)
 inSelf.lastSampleTime = sampleTime
 }
 
 return noErr
}

If the audio interface is used as an output device to play audio, How can I use AVAudioEngine to obtain audio data from each output channel of the audio interface?

• Tim Roberts: Are you asking how to capture the audio output from a different application?
• user22997196: @TimRoberts When using the audio device as an output device, directly obtain the audio from output channels 1 and 2 of the audio interface, rather than targeting a specific application.
• Gordon Childs: If you want the output sent to the device, you can use a CoreAudio tap, see AudioHardwareCreateProcessTap().
• user22997196: @GordonChilds For example, I have two playback software, A and B. When I open the software, A is set to play to channel 1 of the device, and B is set to play to channel 2. In this case, how can I obtain the audio data from output channel 1?