Stage 1: collect play handles and process automation

Collecting play handles

There are two major sources for play handles:

• User interaction: If users trigger an play action, UI classes or MIDI event processors call AudioEngine::addPlayHandle if needed

• Playing song: AudioEngine::renderNextBuffer calls Song::processNextBuffer to get play handles for the current playback position

To avoid threading issues, AudioEngine::addPlayHandle doesn't add play handles directly; it only queues play handles to process into m_newPlayHandles. Those play handles are added to the play handle list at once at this stage.

AudioEngine::removePlayHandle also work similarly when trying to remove play handles created from the mixer thread in user threads.

Processing automation

• Song::processNextBuffer calls Song::processAutomations to process automation. This function calls proper override of TrackContainer::automatedValuesAt and they call TrackContainer::automatedValuesFromTracks with proper arguments.