Audiokit, how to playback a modified buffer in a tap? - ios

Question

I use Audiokit (in Objective-C) for realtime audio processing. I feed a C++ algorithm through a tap or lazy tap where the buffer is being modified.
I thought that would be obvious but...how can I playback the modified buffer in the output? Are taps only for analysis?
[self->microphoneGain.avAudioNode installTapOnBus:0 bufferSize:1024 format:format block:^(AVAudioPCMBuffer * _Nonnull buffer, AVAudioTime * _Nonnull when) {
if (buffer.frameLength == 0) {
return;
}
// Process data -> return modified buffer
processData(buffer.floatChannelData[0], buffer.floatChannelData[1], buffer.frameLength);
// -> How to play back buffer?
}];
Furthermore, I can't get taps buffer size lower than 4800 samples. What would be my best option to get a better latency? I read about AUAudioUnit subclassing, render callback or realtime mode for AudioEngine, but I'm quite lost when trying to implement one of these with AudioKit. Thanks!
EDIT:
I managed to set a render callback which has apparently solved both of my problems.
AURenderCallbackStruct processingCallback;
processingCallback.inputProc = processingCalbackProc;
processingCallback.inputProcRefCon = (__bridge void *)(self);
OSStatus status = AudioUnitSetProperty(AudioKit.engine.outputNode.audioUnit,
kAudioUnitProperty_SetRenderCallback,
kAudioUnitScope_Input,
0,
&processingCallback,
sizeof(processingCallback));
if(status != noErr) {
return false;
}
OSStatus processingCalbackProc (void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData)
{
__unsafe_unretained MyClass *self = (__bridge MyClass *)inRefCon;
printf("%u, ", (unsigned int)inNumberFrames); // -> low latency!
if (!ioData) ioData = self->audioBufferList;
OSStatus status = AudioUnitRender(AudioKit.engine.outputNode.audioUnit,
ioActionFlags,
inTimeStamp,
1,
inNumberFrames,
ioData);
if(status != noErr) { return status; }
// Get buffers
unsigned int inputChannels = 2;
float *buffer[inputChannels];
for (int i = 0; i < inputChannels; i++) {
buffer[i] = (float *)ioData->mBuffers[i].mData;
}
// Process data
processData(buffer[0], buffer[1], inNumberFrames);
return noErr;
}
Now I can easily get buffers as low as 256samples (probably even less but not needed in my case) and when buffer[n]are modified, it outputs the modified buffers.
Everything seems to be fine, I just hope this is the right approach.