Mutate Event/Hook

[Diddykonga]

Mutate Event/Hook

Objective

Lifecycle Hooks/Events currently cover Immutable Components changing value and changing structurally for all Components, but often times we would like it if it also covered Mutable Components changing value.

So that Lifecycle Hooks/Events cover All Components changing value or structurally.

Solution

Prior PR/Approach: #16143
Arrived at the same solution, didnt actually look at the code.

This approach attempts to give SystemParam::apply/queue the responsibility of triggering/queuing Mutate Events via Change Tick scans of their mutably-accessed Components.

Some things to note:

1. Mutate Events are grouped by individual SystemParam, which in most cases is fine and leads to normal results, although some wierdness can occur when using a collection of SystemParams that happen to overlap. ex. ParamSet with two identical queries.
2. The main driver for knowledge of access is, Access, which can contain Unbounded acessess's (ex. EntityMutExcept<...>). In these cases we are not given any definite knowledge as to what was accessed and what wasn't so we must scan every component, except the ones excluded if any.

Impled for Query, Single, Populated, ResMut, FilteredResourcesMut.

Testing

• Did you test these changes? If so, how? Barely, just ran bevy_city with and without. I'm not a good tester.
• Are there any parts that need more testing? All of it, please.

---

Showcase

fn mutate_observer(mutate: On<Mutate>) {
    let entity = mutate.entity;
    let components = &mutate.components;
    println!("{mutate:?}");
}

fn mutate_component_observer(mutate: On<Mutate, Transform>) {
    let entity = mutate.entity;
    let components = &mutate.components;
    println!("{mutate:?}");
}

#[derive(Component)]
struct MyComponent {
    // ...
}

impl MyComponent {
    fn on_mutate(mut world: DeferredWorld, hook: HookContext) {
        // ...
    }
}