Hard lock using State across stages

[siler]

Bevy version

main: 284889c

Operating system & version

Windows 10 20H2

What you did

use bevy::{
    prelude::*,
    input::{
        keyboard::KeyboardInput,
        ElementState
    }
};

fn main() {
    App::build()
        .add_plugins(DefaultPlugins)
        .add_state(GameState::Start)
        .add_system_set(SystemSet::on_update(GameState::Start).with_system(transition_check.system()))
        .add_system_set_to_stage(CoreStage::PreUpdate, SystemSet::on_update(GameState::Freeze).with_system(do_something.system()))
        .run();
}

#[derive(Clone, Eq, PartialEq)]
pub enum GameState {
    Start,
    Freeze
}

fn transition_check(mut state: ResMut<State<GameState>>, mut keyboard_event_reader: EventReader<KeyboardInput>) {
    for event in keyboard_event_reader.iter() {
        if event.key_code == Some(KeyCode::Return) && event.state == ElementState::Pressed {
            state.set_next(GameState::Freeze).unwrap();
        }
    }
}

fn do_something() {}

What you expected to happen

When the return key is pressed the state transitions to GameState::Freeze and the do_something system begins firing.

What actually happened

When the return key is pressed the application hard locks and enters "not responding".

Additional information

If I try to use a state that I added with add_state(..) as a run criteria for a system I add to any stage other than update (via add_system_set_to_stage(..), the application freezes as soon as the state transition occurs.

[rparrett]

Able to reproduce on macos. It seems that the do_nothing system executes over and over (in the same frame?), but and no other systems do after the transition.

[mockersf]

this freeze for me:

use bevy::prelude::*;

fn main() {
    App::build()
        .add_plugins(DefaultPlugins)
        .add_state(GameState::Freeze)
        .add_system_set_to_stage(
            CoreStage::PreUpdate,
            SystemSet::on_update(GameState::Freeze).with_system(do_something.system()),
        )
        .run();
}

#[derive(Clone, Eq, PartialEq)]
pub enum GameState {
    Freeze,
}

fn do_something() {}

it does not freeze if the system set is added to stage CoreStage::Update

[mockersf]

this is because the system set make_driver is needed to handle state correctly but is added only to stage CoreStage::Update

bevy/crates/bevy_app/src/app_builder.rs

Lines 180 to 183 in 284889c

	pub fn add_state<T: Component + Clone + Eq>(&mut self, initial: T) -> &mut Self {
	self.insert_resource(State::new(initial))
	.add_system_set(State::<T>::make_driver())
	}

bevy/crates/bevy_app/src/app_builder.rs

Lines 139 to 141 in 284889c

	pub fn add_system_set(&mut self, system_set: SystemSet) -> &mut Self {
	self.add_system_set_to_stage(CoreStage::Update, system_set)
	}

[rparrett]

Indeed, this patch to the code from the original issue seems to make things work properly

         .add_state(GameState::Start)
+        .add_state_to_stage(CoreStage::PreUpdate, GameState::Start)

[cart]

Removing this from the 0.5 milestone in favor of #1746

[djeedai]

Can confirm both bug and workaround from @rparrett (thanks!).

[james7132]

Checking in, is this still relevant after stageless? States now work across base sets and the run conditions have significantly simplified the mental model for states.