AbletonOSC's property listener uses Live's add_<prop>_listener API and immediately sends the current value on subscribe

The start_listen method in AbletonOSCHandler registers a callback via Live’s add<property_name>_listener() method, then immediately calls the callback to push the current value to the client. The callback sends an OSC message to /live/<class_identifier>/get/ with the track/clip index prepended. Listener functions are stored in a dict keyed by (prop, params_tuple) to allow per-object subscriptions (e.g. volume on track 0 and track 1 are independent subscriptions). On stop_listen, the corresponding remove_listener() is called. This pattern is the standard Ableton Live remote script observer pattern.

Examples

Python handler pattern: add_volume_listener(callback) → callback fires on any volume change → sends /live/track/get/volume (track_id, new_value) to client. On reload, _clear_listeners() iterates all keys and removes all observers before re-registering fresh ones.

Assessment

Why does _start_listen immediately send the current value when called? What data structure does AbletonOSC use to prevent duplicate listeners for the same property on the same object?