AUM AUv3 init order

AUM_AUv3_init_order.md

When an AUv3 plugin is loaded from a session in AUM, this is the order of things that happens:

• If there already was a session loaded, it's saved to a hidden file (for Restore Last State).
• Wait for above state save to finish.
• The new AUM session is read from file, loading session title, notes, etc.
• Reads and sets session properties like sample rate, minimumLatency, metronome, and state for all HW and Mix busses.
• Empty channel objects are created for all channels in the session.
• Wait for this to finish.
• Loads the nodes for all channels:
  • Create the node object.
  • For AUv3 plugins, instantiate the AUAudioUnit object.
  • Enable all inputBusses of the AU, also set shouldAllocateBuffer to NO.
  • Enable all outputBusses of the AU, also set shouldAllocateBuffer to NO.
  • Set sample rate for all busses.
  • Set the AU transportStateBlock, musicalContextBlock and MIDIOutputEventBlock.
  • Call AU allocateRenderResources, failing if it returns false or if renderResourcesAllocated is not set after the call.
  • Grab the AU renderBlock and scheduleParameterBlock.
  • Add observer for kAudioComponentInstanceInvalidationNotification,
  • Get the current values of all AUParameters to store their default values.
  • Set AU fullStateForDocument with the state data stored in the session.
  • Get the list of AUParameters with kAudioUnitParameterFlag_IsWritable from AU, creating MIDI Controls for them.
  • Add observers for latency, audioUnitShortName, parameterTree and currentPreset.
  • Implement an iOS bug hackaround: get and show the view for the plugin, hidden from the user, and then close it again.
  • Setup MIDI outputs for each name in the AU MIDIOutputNames array.
  • Grab the AU scheduleMIDIEventBlock.
  • Setup MIDI inputs for each MIDI input. virtualMIDICableCount is checked to know the number of inputs, but if it says 0 we assume the AU has 1 MIDI input if isMusicDeviceOrEffect is true.
  • Repeat for the next node.
• Wait for the above to finish for all nodes and channels.
• For each channel, refresh the list of nodes on the audio thread.
• Load the MIDI matrix state (setting up MIDI routings)
• Load the MIDI Controls state (setting up all MIDI parameter bindings)
• Refresh the list of channels on the audio thread, this brings them into the actual audio render process, and will start calling their renderBlocks.
• Wait for the channel list refresh to finish, then reset the Transport Clock.

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