How to connect multiple AudioUnits in Swift?

Question

I currently have a RemoteIO Audio Unit configured and working, it simply takes input, and passes it to output, so I can hear myself through the headphones of my iPhone when I speak into its microphone.

The next step in what I want to do is add optional effects and create a chain. I understand that AUGraph has been deprecated and that I need to use kAudioUnitProperty_MakeConnection to connect things together, but I have a few key questions and I'm unable to get audio out just yet.

Firstly: If I want to go RemoteIO Input -> Reverb -> RemoteIO Output, do I need two instances of the RemoteIO Audio Unit? Or can I use the same one? I am guessing just one, but to connect different things to its input and output scopes, but I'm having trouble making this happen.

Secondly: how do render callbacks play into this? I implemented a single render callback (an AURenderCallbackStruct and set that as the kAudioUnitProperty_SetRenderCallback property on my RemoteIO Audio Unit, and in the implementation of the callback, I do this:

    func performRender(
        _ ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>,
        inTimeStamp: UnsafePointer<AudioTimeStamp>,
        inBufNumber: UInt32,
        inNumberFrames: UInt32,
        ioData: UnsafeMutablePointer<AudioBufferList>
    ) -> OSStatus {
        guard let unit = audioUnit else { crash("Asked to render before the AURemoteIO was created.") }
        return AudioUnitRender(unit, ioActionFlags, inTimeStamp, 1, inNumberFrames, ioData)
    }

Do I need a render callback at all to make this work? Do I need two, one to render from RemoteIO -> Reverb, and another to render back to Reverb -> RemoteIO?

The CoreAudio documentation is notoriously sketchy but I'm having trouble finding any up-to-date info on how to do this without AUGraph which is deprecated.

Any advice hugely appreciated!

Answer 1

You only need one RemoteIO (apps only get one), don't need any explicit render callbacks (unless you are synthesizing samples in code), and if you add kAudioUnitProperty_MakeConnection input connections on your full chain of Audio Units, starting the output unit will pull data from the rest of the chain of units, all the way back to the microphone (or from whatever the OS has connected to the mic input).