Resources

Entities and Components are great for representing complex, query-able groups of data. But most Apps will also require "globally unique" data of some kind. In Bevy ECS, we represent globally unique data using Resources.

Here are some examples of data that could be encoded as Resources:

• Elapsed Time
• Asset Collections (sounds, textures, meshes)
• Renderers

Tracking Time with Resources

Lets solve our App's "hello spam" problem by only printing "hello" once every two seconds. We'll do this by using the Time resource, which is automatically added to our App via add_plugins(DefaultPlugins).

For simplicity, remove the hello_world system from your App. This way we only need to adapt the greet_people system.

Resources are accessed in much the same way that we access components. You can access the Time resource in your system like this:

fn greet_people(time: Res<Time>, query: Query<&Name, With<Person>>) {
    for name in query.iter() {
        println!("hello {}!", name.0);
    }
}

Res and ResMut pointers provide read and write access (respectively) to resources.

The delta_seconds field on Time gives us the time that has passed since the last update. But in order to run our system once every two seconds, we must track the amount of time that has passed over a series of updates. To make this easier, Bevy provides the Timer type. Lets create a new Resource for our system to track elapsed time with a Timer:

struct GreetTimer(Timer);

fn greet_people(
    time: Res<Time>, mut timer: ResMut<GreetTimer>, query: Query<&Name, With<Person>>) {
    // update our timer with the time elapsed since the last update
    // if the timer hasn't finished yet, we return
    if !timer.0.tick(time.delta_seconds()).just_finished() {
        return;
    }

    for name in query.iter() {
        println!("hello {}!", name.0);
    }
}

Now all thats left is adding a GreetTimer Resource to our HelloPlugin:

impl Plugin for HelloPlugin {
    fn build(&self, app: &mut AppBuilder) {
        // the reason we call from_seconds with the true flag is to make the timer repeat itself
        app.add_resource(GreetTimer(Timer::from_seconds(2.0, true)))
            .add_startup_system(add_people.system())
            .add_system(greet_people.system());
    }
}

Now cargo run the App. It should now greet people at a reasonable rate.