Adding a component to newly spawned entities

[Sycrosity]

I've been having some trouble adding a component whenever a new entity is spawned, so that I can remove all entities with this component when switching to the MainMenu state - I first searched around for a bevy Event or similar that would allow inbuilt functionality for this, but came up short.
I attempted to fix this with two systems: one which adds the component (Loaded) to all entities Without<Loaded>, and another which OnExit of the state would remove all components With<Loaded>:

fn add_loaded_component(
    mut commands: Commands,
    query: Query<Entity, (Without<Loaded>, Without<Persist>)>,
) {
    for entity in &query {
        commands.entity(entity).insert(Loaded);
    }
}

pub fn teardown<T: Component>(
    to_despawn: Query<Entity, (With<T>, Without<Persist>)>,
    mut commands: Commands,
) {
    for entity in &to_despawn {
        commands.entity(entity).despawn_recursive();
    }
}

(Persist is so that e.g. the camera wouldn't be removed)

However, upon running a plugin using these:

impl Plugin for GalaxyWorldPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems((
            ///other stuff...///
            add_loaded_component.in_set(OnUpdate(EngineState::InGame)),
            teardown::<Loaded>.in_schedule(OnExit(EngineState::InGame)),
        ));
    }
}

I kept recieving the error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: NoEntities("bevy_ecs::query::state::QueryState<&bevy_egui::EguiContext, bevy_ecs::query::filter::With<bevy_window::window::PrimaryWindow>>")', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy-inspector-egui-0.18.3/src/quick.rs:80:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'Compute Task Pool (0)' panicked at 'A system has panicked so the executor cannot continue.: RecvError', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_ecs-0.10.1/src/schedule/executor/multi_threaded.rs:194:60
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_tasks-0.10.1/src/task_pool.rs:376:49
thread 'Compute Task Pool (1)' panicked at 'called `Result::unwrap()` on an `Err` value: RecvError', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_render-0.10.1/src/pipelined_rendering.rs:111:77
thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_tasks-0.10.1/src/task_pool.rs:376:49

and cannot work out what went wrong or how I should have approached this instead. Is there a more bevy-like way of approaching this, and if not what went wrong here?

[wilk10]

What happened is that you added the Loaded component to internal Bevy entities as well, specifically the entity with the PrimaryWindow component in this case, and then the teardown system has despawned them, but those are needed and should be managed by Bevy.

You'll need a solution that marks what should be despawned more granularly. You could add the Loaded component at the time of spawning your own entities, or you could add Loaded to custom bundles of your entities. Or similarly, you could add ToBeLoaded at the time of spawning, and then the added_loaded_component system adds Loaded only to the entities With<ToBeLoaded>. Just avoid using a blanket solution because then you despawn everything with teardown. Hope it helps!

[Sycrosity]

Ah yes that was it - I had added a persist label to the camera entity but forgot about the window... I resolved this by in my startup system adding Persist to all entities, so that my system would work - I'm aware this is probably not the best way to do it but to avoid adding a component manually to every entity I want torn down, this seems to work. I'll mark this as answered - thanks for your help and your detailed explanation!