MultiRoute Audio Input in iOS

Question

We've been working with AudioUnits in Core Audio. It is simultaniously a very powerful audio framework, and one of the worst documented which makes it both a joy and a frustration to work with.

We want to accomplish something we know iPads had been able to do since iOS 6.0 - Multiple audio inputs.

So far - from the 2012 Developer Talk - It appears you have to set the audio session to MultiRoute. We've done this. If I plug in an a soundcard from a keyboard. I can see that there are two inputs. Great. We're then told that we need to set a ChannelMap on a Remote I/O unit.

To what? Well... here's where it gets vague. We need to set all the channels we don't want to -1 and the channels we want to 0 and 1 (for stereo input or for mono?).

We attempt this and... nothing. Sound still plays through on the 'last in wins' principle. Microphone if everything plugged out, soundcard if that's the one plugged in. But we can't switch between them.

This setup code is always run before the other function listed

func setupAudioSession() {
    self.audioSession = AVAudioSession.sharedInstance()

    do {
        try audioSession.setCategory(AVAudioSessionCategoryMultiRoute, with: [.mixWithOthers])
        try audioSession.setActive(true)

        audioSessionWasSetup = true
    } catch let error {
        //TODO: Implement something here
        print(error)
        audioSessionWasSetup = false
    }
}

We then have a remote I/O with an associated audiograph set up. This has been tested and works beautifully. But we need to be able to set where it's pulling sound from.

I've attempted to do it with this, but not only doesn't it have any effect... nothing happens.

Am I missing something?

private func setChannelMap(onAudioUnit audioUnit: AudioUnit?, toChannel channelIndex: Int = 0) {
    var channelMap: [Int32] = []

    if audioUnit == nil {
        return
    }

    var numberOfInputChannels: UInt32 = 4 // Two stereo inputs? - I'm just guessing here

    let mapSize: UInt32 = numberOfInputChannels * UInt32(MemoryLayout<Int32>.size);
    for _ in 0...(numberOfInputChannels) {
        channelMap.append(-1)
    }

    channelMap[2 * channelIndex] = 0;
    channelMap[2 * channelIndex + 1] = 1;

    let status = AudioUnitSetProperty(audioUnit!,
                                      kAudioOutputUnitProperty_ChannelMap,
                                      kAudioUnitScope_Input,
                                      0,
                                      &channelMap,
                                      mapSize);
    self.checkError(status, "Failed to set Channel Map on input unit")
}

There isn't any documentation on this at all as far as I've been able to find. Nor any code examples.

I hope you can help us.