Currently a Juce host does not listen to a hosted AU plugin’s events. Parameter changes do not get propagated to the host, which prevents us from tracking the “dirty” status of a hosted plugin (among other useful applications). This feature is nicely implemented for VST already, but is still lacking from AU.
As I need it for my host, I will attempt to implement it. If anyone else is interested in having this feature implemented too, please chime in! It will be easier if we could join forces, share code and hand it over to Jules when we are done.
This is what I found out so far: Currently a call to AUEventListenerCreate (…) is missing from juce_AudioUnitPluginFormat.mm. From what I’ve read in the Apple docs, that function seems to be key to the feature. I found more docs on this here: http://developer.apple.com/library/mac/#technotes/tn2104/_index.html . As far as I understood this at the moment, the listener callback installed with AUEventListenerCreate basically just needs to call AudioProcessor::sendParamChangeMessageToListeners(), the same way it does for VST. That should not be too difficult to accomplish.
I’ll’ start with this over the next weekend. If anyone else has already done this, please let me know.