Plugin-side checkup by PostStartup

Designing a plugin, the Poststartup stage in Bevy's schedule is an excellent interval to check on intial game condition with less overhead.

Simplified main schedule in Bevy upto "First"

Following examples validate some intial conditions of entity instance(s).

Example: at least one

fn at_least_one(query: Query<Entity, With<DoYouEvenExist>>) {
    if query.is_empty() {
        panic!(
            "Do you have at least one entity with `DoYouEvenExist`?"
        )
    }
}

Example: exactly one

fn the_one_and_only(query: Query<Entity, With<TheOneAndOnly>>) {
    match query.single() {
        Ok(entity) => println!("Found entity: {:?}", entity),

        Err(_) => panic!(
            "Do you have at least one entity with `TheOneAndOnly`?"
        ),
    }
}

Adding in PostStartup

impl Plugin for FooPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Startup, my_setup)
           .add_systems(PostStartup, (at_least_one, the_one_and_only))
            // Chain them if one is the prerequisite of another
            /* ... */
    }
}

Note

They are guaranteed at the beginning only. For example, an initial adjustment on camera, targetting on entity can be guaranteed by such validation. It does not guaratee any condition further from the period.

See also

Struct Main