How to run a system X duration after a state change

[ItsDoot]

I've seen this come up every now and then as a question in the Discord, so it's probably worth documenting in the cheatbook.

The following is a solution I've posted to discord:

// We'll use a bevy::time::Timer to wait X time.
// Prefer a bevy Timer over a generic Timer for more accurate readings
#[derive(Resource)]
struct MyStateTimer(Timer);

// When we enter a specific state, start the timer.
fn start_timer(timer: ResMut<MyStateTimer>) {
    timer.0 = Timer::new(YOUR_DURATION_HERE, TimerMode::Once);
}

// We initialize the app with a non-running timer
app.add_resource(MyStateTimer(Timer::new(Duration::from_secs(0), TimerMode::Once)));
// Don't forget to register the timer starting system for when the state is entered.
app.add_systems(OnEnter(MyState), start_timer);

// Now here's where we do our delayed logic
fn my_system(timer: ResMut<MyStateTimer>, /* ... */) {
    // We only run logic if the timer has completed, and only when it has *just* completed
    if (timer.just_finished()) {
        // run your logic here
    }
}

// Don't forget to register it!
app.add_systems(my_system.run_if(in_state(MyState)));