[Bug?] State is not updated when manually updating App

[not-elm]

Bevy version

v0.14.0-rc.2

What you did

My library has a similar to the following test code, but it failed after I updated the bevy version from v0.13.2 to v0.14.0-rc2.

use bevy::prelude::*;

fn main() {
    let mut app = App::new();
    app.add_plugins(MinimalPlugins);
    app.init_resource::<StateChanged>();
    app.init_state::<GameState>();
    app.add_systems(Update, wait_state);
    app.update();
    assert!(!app.world().resource::<StateChanged>().0);
    app.insert_state(GameState::InGame);
    app.update();
    // panic!! 
    assert!(app.world().resource::<StateChanged>().0);
}

#[derive(Resource, Default, Debug)]
struct StateChanged(bool);

#[derive(Eq, PartialEq, Debug, Default, Copy, Clone, Hash, States)]
enum GameState {
    #[default]
    Title,
    InGame,
}

fn wait_state(
    mut changed: ResMut<StateChanged>,
    state: Res<State<GameState>>,
) {
    if matches!(state.get(), GameState::InGame) {
        changed.0 = true;
    }
}

What went wrong

Console output:

thread 'main' panicked at src\main.rs:19:5:
assertion failed: app.world().resource::<StateChanged>().0

[alice-i-cecile]

FYI @lee-orr @benfrankel @MiniaczQ

[lee-orr]

@alice-i-cecile - I believe the source of this is App::new() doesn't set up the main schedule, so StateTransition isn't added in.

Do you think a good solution here is documenting the need to set up that schedule when you aren't building on the main schedule, or ensuring it's run by default even if no meta-schedule has been added?

[alice-i-cecile]

I agree with your diagnosis of the problem. IMO we should warn about this during .init_state / insert_state. Users won't read the docs for this sort of weirdness, but they will check for warnings.

[MiniaczQ]

I believe the source of this is App::new() doesn't set up the main schedule, so StateTransition isn't added in.

Not quite, StateTransition is no longer part of the default main schedule, it's added through StatesPlugin

Can the state installation methods check for plugin existence? That'd be the best scenario, we shouldn't implicitly install plugins.

[alice-i-cecile]

is_plugin_added is the method you want :)

[MiniaczQ]

Great, I'll have a fix in 10 min

[MiniaczQ]

For the author, you want to add StatesPlugin manually when using MinimalPlugins