Run a system when a resource changes

Use resource_changed in a run_if condition.

use bevy::prelude::*;

#[derive(Resource)]
struct Counter(u32);

fn my_system(counter: Res<Counter>) {
  println!("Counter: {}", counter.0);
}

fn main() {
  App::new().add_systems(
    Update,
    // `resource_changed` will only return true if the
    // given resource was just changed (or added)
    my_system.run_if(
      resource_changed::<Counter>
        // By default detecting changes will also trigger if the resource was
        // just added, this won't work with my example so I will addd a second
        // condition to make sure the resource wasn't just added
        .and_then(not(resource_added::<Counter>)),
    )
  );
}