Function resource_exists 

pub fn resource_exists<T>(res: Option<Res<'_, T>>) -> bool
where
    T: Resource,

A SystemCondition-satisfying system that returns true if the resource exists.

To skip a system with a Res or ResMut parameter if the resource does not exist, you may instead wrap the parameter in If, like If<Res<T>> or If<ResMut<T>>.

Example

app.add_systems(
    // `resource_exists` will only return true if the given resource exists in the world
    my_system.run_if(resource_exists::<Counter>),
);

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

// `Counter` hasn't been added so `my_system` won't run
app.run(&mut world);
world.init_resource::<Counter>();

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