Question: I have an app with an elaborated render callback that I doubt could do with AVAudioEngine. 

Anyway to use my AUGraph render callback ( with multiple buses ) with AVAudioEngine ?

Any sample code ?

Answer: 
The [Audio Unit API](https://developer.apple.com/documentation/audiounit) is not deprecated, only AUGraph which is presumably built on top of it. 

Make connections using [AudioUnitSetProperty](https://developer.apple.com/documentation/audiotoolbox/1440371-audiounitsetproperty) with `kAudioUnitProperty_MakeConnection` with an [AudioUnitConnection](https://developer.apple.com/documentation/audiotoolbox/audiounitconnection?language=objc) struct. 

Start and stop your output unit with [AudioOutputUnitStart](https://developer.apple.com/documentation/audiotoolbox/1439763-audiooutputunitstart?language=objc) and [AudioOutputUnitStop](https://developer.apple.com/documentation/audiotoolbox/1440513-audiooutputunitstop?language=objc). 

Set a render callback using [AudioUnitSetProperty](https://developer.apple.com/documentation/audiotoolbox/1440371-audiounitsetproperty) and `kAudioUnitProperty_SetRenderCallback` with an [AURenderCallbackStruct](https://developer.apple.com/documentation/audiotoolbox/aurendercallbackstruct?language=objc)

There's really not all that much that AUGraph gives you other than a little bookkeeping and a simpler connection syntax.
A (non-deprecated) [V3 AUAudioUnit](https://developer.apple.com/documentation/audiotoolbox/auaudiounit) subclass can still return an AUInternalRenderBlock which supports audio render callbacks. The AVAudioEngine API can then connect these Audio Units (for instance, to mixer nodes). Audio Unit V3 render functions still seem to allow short (buffers with less than 500 samples) for near-real-time audio synthesis and analysis on iOS.

See my V3 AU example app on GitHub (mixed Swift and Objective C): <https://github.com/hotpaw2/auv3test5>