Audio Engine failure when attachin… | Apple Developer Forums

According to the Apple docs, you should be able to connect audio nodes to an AVAudioEngine instance during runtime, but I'm getting a crash while trying to do so, in particular, when trying to connect instances of AVAudioUnitTimePitch or AVAudioUnitVarispeed to an AVAudioEngine with manual rendering mode enabled. The error message I get is:

Terminating app due to uncaught exception &#039;com.apple.coreaudio.avfaudio&#039;, reason:
&#039;player started when in a disconnected state&#039;

In my code, first, I configure the audio engine:

let engine = AVAudioEngine()
let format = AVAudioFormat(standardFormatWithSampleRate: 48000, channels: 2)!
try! engine.enableManualRenderingMode(.offline, format: format, maximumFrameCount: 1024)
try! engine.start()

Then, I try to attach the player to the engine:

let player = AVAudioPlayerNode()
configureEngine(player: player, useVarispeed: true)
player.play() &#x2F;&#x2F; this is the line that causes the crash

Finally, this is the function I use to configure the engine nodes graph:

func configureEngine(player: AVAudioPlayerNode, useVarispeed: Bool) {
    engine.attach(player)

    guard useVarispeed else {
        engine.connect(player, to: engine.mainMixerNode, format: format)
        return
    }

    let varispeed = AVAudioUnitVarispeed()
    engine.attach(varispeed)
    engine.connect(player, to: varispeed, format: format)
    engine.connect(varispeed, to: engine.mainMixerNode, format: format)
}

If I pass false as the value for the useVarispeed parameter, the crash goes away. What is even more interesting is that if I add a dummy player node before starting the engine, the crash goes away too 🤷‍♂️

Could anyone please add clarity on what's going on here? Is this a bug or a limitation of the framework that I'm not aware of?

Here's a simple project demonstrating the problem: https://github.com/rlaguilar/AVAudioEngineBug

Audio Engine failure when attaching AVAudioUnitTimeEffect