Running systems with both a state and a stage limitation causes systems to sometimes just be ignored.

[haihala]

Bevy version

0.6 off of crates.io

Operating system & version

Windows 10.

What you did

1. Wanted to make system execution order (more) deterministic.
2. Did this by assigning systems to stages with app.add_system_to_stage
3. Got a white screen and the program froze on load with the warning WARN bevy_ecs::schedule::graph_utils: Chain(bevy_ecs::schedule::state::State<time::game_flow::GameState>::on_enter::{{closure}}, bevy_ecs::schedule::state::should_run_adapter<time::game_flow::GameState>) wants to be after unknown label: DriverLabel(TypeId { t: 3089577881103655877 })
4. Added a driver system set (don't understand how this works at all) with app.add_system_set_to_stage(CoreState::First, State::<my custom state enum>::get_driver())
5. That worked for CoreState::First
6. Want to do the same for CoreState::Last with state run criteria, repeat 3-4 for the new stage
7. Doesn't crash, but the run criteria never passes. Removing the run criteria the system runs every tick (want it to run on entry to a state)
8. Switched order of ...::get_driver() commands,
9. Systems in CoreStage::Last work, systems in CoreStage::First with run_criteria don't

What you expected to happen

Expected all systems to run

What actually happened

Systems get executed based on the order of the added stages. Only impacts systems with further run criteria and doesn't impact ones that always run in a stage.

Additional information

Snippet from the plugin that adds the state:

        .add_state(GameState::PreRound)
        .add_system_set_to_stage(CoreStage::Last, State::<GameState>::get_driver())
        .add_system_set_to_stage(CoreStage::First, State::<GameState>::get_driver())

Usage:

        .add_system_to_stage(
            CoreStage::First,
            update_clock
                .with_run_criteria(FixedTimestep::steps_per_second(constants::FPS as f64))
                .label(TimeSystemLabel::UpdateClock),
        )
        ...
        .add_system_to_stage(
            CoreStage::Last,
            text::update_timer
                .with_run_criteria(State::on_update(GameState::Combat))
                .label(UISystemLabel::Timer),
        )

[haihala]

Can't say I still understand how it works, but I added the state to some more stages and changed from app.add_state to app.add_state_to_stage and now State::on_update seems to work, while State::on_enter doesn't. I can work with this, although I'd really wish for an explanation.

[alice-i-cecile]

Thank you for the thorough writeup!

States that work across multiple stages are very much a fragile hack, so I'm unsurprised that this broke in surprising ways. I think that the reason the on-enter doesn't work properly is because the state already decides that entry is complete at the end of the first stage that it has been entered in.

This will almost certainly just be fixed / obsoleted by the stageless redesign: bevyengine/rfcs#45.

[haihala]

Awesome thanks