How do I subclass AVAudioUnit? - core-audio

Question

Since the way to instantiate an AVAudioUnit is this:
[AVAudioUnit instantiateWithComponentDescription:componentDescription options:0 completionHandler:^(__kindof AVAudioUnit * _Nullable audioUnit, NSError * _Nullable error) {
}];
How am I supposed to subclass AVAudioUnit? I have tried this:
[MySubclassOfAVAudioUnit instantiateWithComponentDescription:componentDescription options:0 completionHandler:^(__kindof AVAudioUnit * _Nullable audioUnit, NSError * _Nullable error) {
}];
But the audioUnit that is returned in the block is still of type AVAudioUnit and NOT MySubclassOfAVAudioUnit.
Per Rhythmic Fistman's response, I am registering my custom AUAudioUnit subclass with Apple's example code:
componentDescription.componentType = kAudioUnitType_Effect;
componentDescription.componentSubType = 0x666c7472; /*'fltr'*/
componentDescription.componentManufacturer = 0x44656d6f; /*'Demo'*/
componentDescription.componentFlags = 0;
componentDescription.componentFlagsMask = 0;
I want my AVAudioUnit subclass to always use my AUAudioUnit.

Answer 1

From instantiateWithComponentDescription:completionHandler:
The returned AVAudioUnit instance normally will be of a subclass (AVAudioUnitEffect,
AVAudioUnitGenerator, AVAudioUnitMIDIInstrument, or AVAudioUnitTimeEffect), selected
according to the component's type.
UPDATE
I got this wrong - you can't instantiate your own AVAudioUnit subclass, you can only instantiate your AUAudioUnit, wrapped in the relevant built-in AVFoundation AVAudioUnit subclass (e.g. AVAudioUnitEffect, etc).
The following code causes MyAUAudioUnit, a subclass of AUAudioUnit to be instantiated:
#import <AVFoundation/AVFoundation.h>
#interface MyAUAudioUnit : AUAudioUnit {
}
#end
#implementation MyAUAudioUnit
// implement it here
#end
// later
- (void)instantiateMyAUAudioUnitWrappedInAVAudioUnit {
// register it (need only be done once)
AudioComponentDescription desc;
desc.componentType = kAudioUnitType_Effect;
desc.componentSubType = 0x666c7472; /*'fltr'*/
desc.componentManufacturer = 0x44656d6f; /*'Demo'*/
desc.componentFlags = 0;
desc.componentFlagsMask = 0;
[AUAudioUnit registerSubclass:MyAUAudioUnit.class asComponentDescription:desc name:#"MyAU" version:1];
// Instantiate as many times as you like:
[AVAudioUnit instantiateWithComponentDescription:desc options:0 completionHandler:^(AVAudioUnit * audioUnit, NSError *error) {
NSLog(#"AVAudioUnit: %#, error: %#", audioUnit, error);
}];
}
WRONG BIT
So to have your AVAudioUnit subclass instantiated, you must first register it with the AUAudioUnit method:
+[AUAudioUnit registerSubclass:asComponentDescription:name:version:]
There is a code snippet and some possible gotchas in this devforum thread.