It's surprising that using 'transitioning' to the same state triggers OnEnter

[deifactor]

Bevy version

0.10

What you did

I was writing a system to transition to a "game over" state when the player's HP hit zero and one to show the UI on entering that state:

fn check_game_over(mut commands: Commands, query: Query<&HP, With<Player>>) {
    for hp in &query {
        if hp.0 <= 0 {
            commands.insert_resource(NextState(Some(GameState::GameOver)));
        }
    }
}

pub fn game_over_plugin(app: &mut App) {
    app.add_system(check_game_over)
        .add_system(show_game_over_screen.in_schedule(OnEnter(GameState::GameOver)));
}

What went wrong

Since NextState triggers OnEnter schedules unconditionally, this code constantly generates UI nodes. I assumed that since States: Eq, bevy would compare the next state to the current state, see that they're the same, and just do nothing (similar to DetectChangesMut::set_if_neq).

[alice-i-cecile]

I'd like to see docs for this, and a set_if_neq method on NextState IMO

[hymm]

a set_if_neq method on NextState IMO

Note that this would need to compare to the value in Res<State<T>>

[deifactor]

a set_if_neq method on NextState IMO

Note that this would need to compare to the value in Res<State<T>>

Wouldn't it be enough to just change apply_state_transition? I didn't mean that NextState should have a method, I meant that the state transition logic should check this automatically.

[alice-i-cecile]

I meant that the state transition logic should check this automatically.

Hmm. Maybe with the addition of the Transition schedules this is a safe default: users who want this behavior can add a self -> self schedule?

[deifactor]

Maybe with the addition of the Transition schedules this is a safe default: users who want this behavior can add a self -> self schedule?

What are the use cases for wanting self -> self transitions to trigger things? I'm sure there are some, I just can't think of any. But I think it's reasonable to say "this is weird enough that you have to do something different".

[alice-i-cecile]

To me this is the standard "power cycle" pattern, where you reinitialize everything to reset the state :)