I'm working on a CoreAudio plugin based on Apple's NullAudio example (https://developer.apple.com/library/archive/samplecode/AudioDriverExamples/Introduction/Intro.html).
I currently need to disable audio output functionality. The minimum I need is to make the plugin not show up as an available audio output device. It's not at all obvious to me from the code how to do this. Does anyone else have advice?
I also wanted to disable the output of the NullAudio device some time ago and the only way I managed to do it is by making the following changes in the NullAudio.c:
In NullAudio_HasDeviceProperty() function disable the output scope in the preferred channel layout case, like this:
case kAudioDevicePropertyPreferredChannelLayout:
theAnswer = inAddress->mScope == kAudioObjectPropertyScopeInput;
In NullAudio_GetDevicePropertyDataSize() function return 1 object size for the global scope and zero size for the output scope for the stream property case, and leave the input scope case as is:
case kAudioDevicePropertyStreams:
switch(inAddress->mScope)
{
case kAudioObjectPropertyScopeGlobal:
*outDataSize = 1 * sizeof(AudioObjectID);
break;
case kAudioObjectPropertyScopeOutput:
*outDataSize = 0 * sizeof(AudioObjectID);
break;
};
break;
I haven't fully tested this, though. I assumed it worked, because I could see that the NullAudio device had a format of 2ins/0outs in Audio MIDI Setup app. Maybe only one of these changes is needed. Maybe more changes are need.
You should probably use it as a starting point rather as a solution, because I think that in order to make this fully and correctly functional you would need to do more changes like this throughout the code. Find where the output scope/stream is referenced (if it's active, how many owned objects the device has etc.) and try to make logical changes to disable the output stream. At least that's how I tried to find my way through this code base.
Good luck and let me know if it works, or if you found another/better (or the correct) way!