How to make a query that only runs when the given component has been mutated

[Dumpinground]

From Bevy 0.1 introduction I learn that we could make a query only runs when the given component has been mutated with Mutated, but in the latest version (0.16) Added and Changed are left, Mutated is gone. I have searched all released news and migration guides but not found an explanation or any alternatives. Is there an example that let a query only runs when the given component mutated ?

[MrGVSV]

This is what Changed is for. I suppose if you really want to excluded Added you could probably do (Changed<MyComponent>, Without<Added<MyComponent>>).

If that does work (I’m pretty sure it should but I've never personally used it), you could wrap it into your own QueryFilter:

#[derive(QueryFilter)]
struct Mutated<T: Component> {
  changed: Changed<T>,
  not_added: Without<Added<T>>,
}

[Dumpinground]

I had tried with Without<Added<Component>> and it is illegal in rust. I think @rparrett 's answer is feasible. Anyway, thanks for answering.

[MrGVSV]

What do you mean? What error are you getting? It shouldn't be illegal in Rust.

Maybe you’re not using tuples to separate query data from query filters? For example, Query<&Foo, Changed<Foo>>, Without<Added<Foo>>> should actually be Query<(&Foo), (Changed<Foo>>, Without<Added<Foo>>)>.

[MrGVSV]

Oh wait I see. Without only works on components but I’m suggesting giving it a filter. 🤦‍♂️

Never mind me lol

[rparrett]

I think what's generally done now is something like

fn system(things: Query<Ref<Thing>, Changed<Thing>>) {
    for thing in things.iter().filter(|r| !r.is_added()) {
        // ...
    }
}

Relevant issue: #15070