Introduce CheckChangeTicks event that is triggered by World::check_change_ticks

[urben1680]

Objective

In the past I had custom data structures containing Ticks. I learned that these need to be regularly checked to clamp them. But there was no way to hook into that logic so I abandoned storing ticks since then.

Another motivation to open this up some more is to be more able to do a correct implementation of System::check_ticks.

Solution

Add CheckChangeTicks and trigger it in World::check_change_ticks.
Make Tick::check_tick public.

This event makes it possible to store ticks in components or resources and have them checked.

I also made Schedules::check_change_ticks public so users can store schedules in custom resources/components for whatever reasons.

Testing

The logic boils down to a single World::trigger call and I don't think this needs more tests.

Alternatives

Making this obsolete like with #15683.

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Showcase

From the added docs:

use bevy_ecs::prelude::*;
use bevy_ecs::component::CheckChangeTicks;

#[derive(Resource)]
struct CustomSchedule(Schedule);

let mut world = World::new();
world.add_observer(|tick: Trigger<CheckChangeTicks>, mut schedule: ResMut<CustomSchedule>| {
    schedule.0.check_change_ticks(tick.get());
});

[urben1680]

An idea I had as well was to make all check_change_ticks methods to take CheckChangeTicks instead of Tick for a stricter context and to make it self-explaining where this value is taken from.

This would however touch numerous files, a good few of them because it changes the System trait method too. What do others think?

[chescock]

Huh, I would have expected this to take &self, but I guess this is actually cheaper since it's Copy.

[urben1680]

Right, never use &self for tiny Copy types.