merge_observerState_observer_single_component.md

---

title: Unify `ObserverState` and `Observer` components
pull_requests: [18728]

---

ObserverState and Observer have been merged into a single component. now you can use Observer::with_dynamic_runner to build custom Observe.

let observe = unsafe {
    Observer::with_dynamic_runner(|mut world, trigger, ptr, propagate| {
        // do something
    })
    .with_event(event_a)
};
world.spawn(observe);