Function bevy::ecs::schedule::common_conditions::state_changed

pub fn state_changed<S>() -> impl FnMut(Res<'_, State<S>>) + Clonewhere
    S: States,

Generates a Condition-satisfying closure that returns true if the state machine changed state.

To do things on transitions to/from specific states, use their respective OnEnter/OnExit schedules. Use this run condition if you want to detect any change, regardless of the value.

Panics

The condition will panic if the resource does not exist.

Example

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

world.init_resource::<State<GameState>>();

app.add_systems(
    // `state_changed` will only return true if the
    // given states value has just been updated or
    // the state has just been added
    my_system.run_if(state_changed::<GameState>()),
);

fn my_system(mut counter: ResMut<Counter>) {
    counter.0 += 1;
}

// `GameState` has just been added so `my_system` will run
app.run(&mut world);
assert_eq!(world.resource::<Counter>().0, 1);

// `GameState` has not been updated so `my_system` will not run
app.run(&mut world);
assert_eq!(world.resource::<Counter>().0, 1);

*world.resource_mut::<State<GameState>>() = State::new(GameState::Paused);

// Now that `GameState` has been updated `my_system` will run
app.run(&mut world);
assert_eq!(world.resource::<Counter>().0, 2);