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# Objective Improve performance scalability when adding new event types to a Bevy app. Currently, just using Bevy in the default configuration, all apps spend upwards of 100+us in the `First` schedule, every app tick, evaluating if it should update events or not, even if events are not being used for that particular frame, and this scales with the number of Events registered in the app. ## Solution As `Events::update` is guaranteed `O(1)` by just checking if a resource's value, swapping two Vecs, and then clearing one of them, the actual cost of running `event_update_system` is *very* cheap. The overhead of doing system dependency injection, task scheduling ,and the multithreaded executor outweighs the cost of running the system by a large margin. Create an `EventRegistry` resource that keeps a number of function pointers that update each event. Replace the per-event type `event_update_system` with a singular exclusive system uses the `EventRegistry` to update all events instead. Update `SubApp::add_event` to use `EventRegistry` instead. ## Performance This speeds reduces the cost of the `First` schedule in both many_foxes and many_cubes by over 80%. Note this is with system spans on. The majority of this is now context-switching costs from launching `time_system`, which should be mostly eliminated with #12869.  The actual `event_update_system` is usually *very* short, using only a few microseconds on average.  --- ## Changelog TODO ## Migration Guide TODO --------- Co-authored-by: Josh Matthews <josh@joshmatthews.net> Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com> |
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