How to avoid duplicate event processing across states

[rcorre]

Is there a way for one system to consume events, so another system won't see them? I'm running into an issue where an event triggers a state change, and both the old an new states process the events.
This is similar to #1700, which suggests Input::reset, but with events instead.

I'd assumed clear would be what I wanted, but it doesn't seem to work:

use bevy::prelude::*;

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

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .init_state::<GameState>()
        .add_event::<Pause>()
        .add_systems(PreUpdate, process_input)
        .add_systems(Update, pause.run_if(in_state(GameState::Play)))
        .add_systems(Update, unpause.run_if(in_state(GameState::Pause)))
        .run();
}

#[derive(Event, Default)]
struct Pause;

fn process_input(key: Res<ButtonInput<KeyCode>>, mut events: EventWriter<Pause>) {
    if key.just_pressed(KeyCode::Escape) {
        info!("Pause pressed");
        events.send_default();
    }
}

fn pause(mut events: EventReader<Pause>, mut state: ResMut<NextState<GameState>>) {
    for _ in events.read() {
        info!("Pausing");
        state.set(GameState::Pause);
    }
    events.clear();
}

fn unpause(mut events: EventReader<Pause>, mut state: ResMut<NextState<GameState>>) {
    for _ in events.read() {
        info!("Unpausing");
        state.set(GameState::Play);
    }
    events.clear();
}

When I press escape, I see "Pausing" and "Unpausing".

[alice-i-cecile]

Use ResMut<Events<T>>::drain, which will remove the events from the central storage, rather than just one of the readers.