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I am trying to write a MIDIPlayer class which is a wrapper for an AVAudioEngine and an AVAudioUnitMIDIInstrument. I have written a loop that gets the names and ASBDs for all AudioComponents of type MusicDevice and then chooses the most desired unit according to a list which works quite similar to font substitution, with Apples DLS MusicDevice as the ultimate fallback. Here's my sample code:import AVKit fileprivate func setupAVAudioEngine(engine: inout AVAudioEngine, instrumentAU: inout AVAudioUnitMIDIInstrument?) { var instrumentACD = AudioComponentDescription(componentType: kAudioUnitType_MusicDevice, componentSubType: 0, componentManufacturer: 0, componentFlags: 0, componentFlagsMask: 0) var instrumentComponents: [(AudioComponentDescription, String)] = [] var instrumentComponent: AudioComponent? = nil repeat { instrumentComponent = AudioComponentFindNext(instrumentComponent, &instrumentACD) if instrumentComponent == nil { break } var compDescr = AudioComponentDescription() var name: Unmanaged<CFString>? AudioComponentCopyName(instrumentComponent!, &name) AudioComponentGetDescription(instrumentComponent!, &compDescr) let nameString = name!.takeRetainedValue() as String instrumentComponents.append((compDescr, nameString)) name?.release() } while true let instrumentComponentSubstitutionList = ["MakeMusic: SmartMusicSoftSynth","Apple: AUMIDISynth","Apple: DLSMusicDevice"] var found = false for instrument in instrumentComponentSubstitutionList { for member in instrumentComponents { if member.1 == instrument { instrumentACD = member.0 print("\(member.1) found") found = true break } if found {break} } } print("Try to create InstrumentNode with ACD: \(instrumentACD)") instrumentAU = AVAudioUnitMIDIInstrument(audioComponentDescription: instrumentACD) print("InstrumentNode created: \(instrumentAU!.name)") print() engine.attach(instrumentAU!) engine.connect(instrumentAU!, to: engine.mainMixerNode, format: nil) } open class MIDIPlayer { private var audioEngine: AVAudioEngine private var instrumentUnit: AVAudioUnitMIDIInstrument private var mainMixer: AVAudioMixerNode public init() { self.audioEngine = AVAudioEngine() self.mainMixer = audioEngine.mainMixerNode var instrumentAU: AVAudioUnitMIDIInstrument? setupAVAudioEngine(engine: &audioEngine, instrumentAU: &instrumentAU) self.instrumentUnit = instrumentAU! try! audioEngine.start() } public func playMIDINote(_ note: UInt8) { print("Playing MIDI Note \(note)") instrumentUnit.startNote(note, withVelocity: 70, onChannel: 0) sleep(1) instrumentUnit.stopNote(note, onChannel: 0) } } let midiPlayer = MIDIPlayer() midiPlayer.playMIDINote(60) midiPlayer.playMIDINote(62) midiPlayer.playMIDINote(64) midiPlayer.playMIDINote(65) midiPlayer.playMIDINote(67)The code works perfectly fine in an Xcode 11.3.1 playground, howewer, when I use the exact same code outside the playground I get different kinds of errors: When I copy the same code into a command line project and run it from XCode it still works but the console gives me the following error:[AudioHAL_Client] AudioHardware.cpp:666:AudioObjectGetPropertyData: AudioObjectGetPropertyData: no object with given ID 0When I run the executable without Xcode no error is reported.When I create a single View app, put the class and its setup func in its own source file and create an instance in the applicationDidFinishLaunching method of the AppDelegate I get the following errors:[AudioHAL_Client] HALC_ShellDriverPlugIn.cpp:104:Open: HALC_ShellDriverPlugIn::Open: opening the plug-in failed, Error: 2003329396 (what)[AudioHAL_Client] AudioHardware.cpp:666:AudioObjectGetPropertyData: AudioObjectGetPropertyData: no object with given ID 0These errors are written to the console even before my setup function is called. However, the code still works (I hear notes playing), but only if I change the instrumentComponentSubstitutionListso that one of the Apple AUs will be found (in other words: not the SmartMusicSoftSynth). When I keep the SoftSynth the preferred device the code crashes and I get the additional error:[avae] AVAEInternal.h:103:_AVAE_CheckNoErr: [AUInterface.mm:461:AUInterfaceBaseV3: (AudioComponentInstanceNew(comp, &_auv2)): error -3000Note: In the playground and the command line app the SoftSynth works.Some observations which may or may not relate to the issue:In this blogpost http://www.rockhoppertech.com/blog/multi-timbral-avaudiounitmidiinstrument/ Gene DeLisa mentions that the AVAudioUnitSampler is the only subclass of the abstract class AVAudioUnitMIDIInstrument. This post is from 2016, but I do not find any further information about that. But, obviously, the DLS MusicDevice as well as the 3rd party SoftSynth work – at least in some environments.The ASBD taken from the found AudioComponents for both of the Apple Units have their componentFlags property set to 2. The documentation says: must be set to zero unless a known specific value is requested. The componentFlags property of the SoftSynth is 0.My Question(s):Why do I get these errors?Why does the 3rd party plugIn work in the playground and the command line app but not in a single view app?Is AVAudioEngine ready to host other Instrument Units than the monotimbral SamplerUnit?Or do I have to step down and use the instrumentAUs directly and not the AVAudioUnitMIDIInstrument wrapper?