Question about Added<T> filter

[MuongKimhong]

When i use Added<T> filter in a Query, it returns 2 types of result:

1. When the entity is created
2. When the entity is added to another entity as a child

// spawning
let child = commands.spawn(MyComponent).id();
commands.spawn(OtherComponent).add_child(child);

fn system(query: Query<&MyComponent, Added<MyComponent>>) {
    for component in query.iter() {
        println!("something");
    }
}

I got "something" printed out 2 times.

Is there native way to get only when an entity is created?

[jfouche]

You should have something wrong in your code as the behavior of the Added filter is what you expect.

If I cargo test -- --nocapture the following, I got what I expect : only 1 "Added".

[derive(Component)]
struct MyComponent;

#[test]
fn test_added() {
    let mut app = App::new();
    app.add_systems(Update, |q: Query<Entity, Added<MyComponent>>| {
        let mut i = 0;
        for e in &q {
            i += 1;
            eprintln!("Added {e}");
        }
        assert_eq!(1, i);
    });

    let root = app.world_mut().spawn(()).id();
    let child = app.world_mut().spawn(MyComponent).id();
    app.world_mut().entity_mut(root).add_child(child);
    app.update();
}

Output :

Added 1v1
test test::test_added ... ok

Do your OtherComponent requires MyComponent ? Or maybe your system is called muliple times.

I suggest you to check what entity is added to ensure it's not the same by changing your system to

fn system(query: Query<Entity, Added<MyComponent>>) {
    for e in query.iter() {
        println!("Added<MyComponent>: {e}");
    }
}

[MuongKimhong]

I did deep check again and you're right, the component being used with another entity. Thanks you!