Add World::try_run_schedule

[ItsDoot]

Objective

Fix #8018

Solution

Added a new method World::try_run_schedule that returns a Result instead of panicking if the schedule doesn't exist.

---

Changelog

Added

• World::try_run_schedule: Attempt to run a schedule and return a Result rather than panicking if the schedule doesn't exist.

[alice-i-cecile]

The diff / commit history looks wrong, so this is quite hard to review. Are you able to clean that up?

[ItsDoot]

Yea sorry about that @alice-i-cecile, messed up my history trying to rebase, fixing it now. 😅

[alice-i-cecile]

Code itself LGTM. Can we use this internally anywhere? I know that state transitions wanted this: do enter / exit want it too already?

[ItsDoot]

We could certainly move towards full lazy initialization for State schedules by removing these:

bevy/crates/bevy_app/src/app.rs

Lines 359 to 366 in 4714130

	for variant in S::variants() {
	if self.get_schedule(OnEnter(variant.clone())).is_none() {
	self.add_schedule(OnEnter(variant.clone()), Schedule::new());
	}
	if self.get_schedule(OnExit(variant.clone())).is_none() {
	self.add_schedule(OnExit(variant), Schedule::new());
	}
	}

And using try_run_schedule in these places:

bevy/crates/bevy_ecs/src/schedule/state.rs

Lines 104 to 106 in 4714130

	pub fn run_enter_schedule<S: States>(world: &mut World) {
	world.run_schedule(OnEnter(world.resource::<State<S>>().0.clone()));
	}

bevy/crates/bevy_ecs/src/schedule/state.rs

Lines 120 to 128 in 4714130

	world.run_schedule(OnExit(exited.clone()));
	// If the schedule doesn't exist, we don't care.
	let _ = world.try_run_schedule(OnTransition {
	from: exited,
	to: entered.clone(),
	});
	world.run_schedule(OnEnter(entered));

@james7132 @alice-i-cecile Thoughts?

[alice-i-cecile]

I like that much better!