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The Adventurer ceases her sprint and rests her hand on a nearby tree trunk. She stands still but oddly her hand moves, like the tree is breathing. She sharpens her senses and notices that the breathing rhythm is strangely similar to the melody in her head. In fact, it's completely in sync.
While most of your audio integration will rely on sending calls from Unity to Wwise, like posting Events or setting Game Syncs, you can also return information from Wwise to Unity by the use of callbacks, hereby completing the circle of interplay between the soundscape and the game visuals and mechanisms.
As you might deduce from the name, callbacks allow you to get a call back from an Event you have posted. It is a way for the game to be notified of certain events, like when a sound has started or stopped playing, or the moment a cymbal hit occurs in the orchestral music ensemble.
As callbacks allow you to get a call back to Unity from an Event, you will naturally have to post the Event in the first place to get callbacks. Additionally, Wwise should be aware that it needs to send a callback from a specific Event, and so to set up callbacks you need to extend the way you post the Event.