removing_resources.md

Removing Resources

Resources can be removed after they are added to the App. We use a parameter of type Commands in systems to remove Resources.

Commands help us modifying our App even after the App is initialized. Because Commands modify the App, be sure to make it to be mutable.

Note that it is Commands with s at the end of the word.

use bevy::{
    app::{App, Startup, Update},
    ecs::system::{Commands, Res, Resource},
};

fn main() {
    App::new()
        .insert_resource(MyResource { value: 10 })
        .add_systems(Startup, remove_my_resource)
        .add_systems(Update, output_value)
        .run();
}

#[derive(Resource)]
struct MyResource {
    value: u32,
}

fn remove_my_resource(mut commands: Commands) {
    commands.remove_resource::<MyResource>();
}

fn output_value(r: Res<MyResource>) {
    // Panic!
    println!("{}", r.value);
}

The execution of the code gets panic in the output_value system, since the MyResource has been removed.

In the code above, we put remove_my_resource and output_value in different schedules (Startup and Update respectively). This gives Bevy enough time to update the internal data structure. If we put the functions in the same schedule, the update will not be reflected.

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