As this page is outdated, please refer to Bevy's official migration guides while reading, to cover the differences: 0.14 to 0.15.

I apologize for the inconvenience. I will update the page as soon as I find the time.

Resources

Relevant official examples: ecs_guide.

Resources allow you to store a single global instance of some data type, independently of entities.

Use them for data that is truly global for your app, such as configuration / settings. Resources make it easy for you to access such data from anywhere.

To create a new resource type, simply define a Rust struct or enum, and derive the Resource trait, similar to components and events.

#[derive(Resource)]
struct GoalsReached {
    main_goal: bool,
    bonus: u32,
}

Types must be unique; there can only be at most one instance of a given type. If you might need multiple, consider using entities and components instead.

Bevy uses resources for many things. You can use these builtin resources to access various features of the engine. They work just like your own custom types.

Accessing Resources

To access the value of a resource from systems, use Res/ResMut:

fn my_system(
    // these will panic if the resources don't exist
    mut goals: ResMut<GoalsReached>,
    other: Res<MyOtherResource>,
    // use Option if a resource might not exist
    mut fancy: Option<ResMut<MyFancyResource>>,
) {
    if let Some(fancy) = &mut fancy {
        // TODO: do things with `fancy`
    }
    // TODO: do things with `goals` and `other`
}

Managing Resources

If you need to create/remove resources at runtime, you can do so using commands (Commands):

fn my_setup(mut commands: Commands, /* ... */) {
    // add (or overwrite if existing) a resource, with the given value
    commands.insert_resource(GoalsReached { main_goal: false, bonus: 100 });
    // ensure resource exists (create it with its default value if necessary)
    commands.init_resource::<MyFancyResource>();
    // remove a resource (if it exists)
    commands.remove_resource::<MyOtherResource>();
}

Alternatively, using direct World access from an exclusive system:

fn my_setup2(world: &mut World) {
    // The same methods as with Commands are also available here,
    // but we can also do fancier things:

    // Check if resource exists
    if !world.contains_resource::<MyFancyResource>() {
        // Get access to a resource, inserting a custom value if unavailable
        let _bonus = world.get_resource_or_insert_with(
            || GoalsReached { main_goal: false, bonus: 100 }
        ).bonus;
    }
}

Resources can also be set up from the app builder. Do this for resources that are meant to always exist from the start.

App::new()
    .add_plugins(DefaultPlugins)
    .insert_resource(StartingLevel(3))
    .init_resource::<MyFancyResource>()
    // ...

Resource Initialization

If you want to be able to use .init_resource to create your resource, here is how you can provide the default value.

Implement Default for simple resources:

// simple derive, to set all fields to their defaults
#[derive(Resource, Default)]
struct GameProgress {
    game_completed: bool,
    secrets_unlocked: u32,
}

#[derive(Resource)]
struct StartingLevel(usize);

// custom implementation for unusual values
impl Default for StartingLevel {
    fn default() -> Self {
        StartingLevel(1)
    }
}

// on enums, you can specify the default variant
#[derive(Resource, Default)]
enum GameMode {
    Tutorial,
    #[default]
    Singleplayer,
    Multiplayer,
}

For resources that need complex initialization, implement FromWorld:

#[derive(Resource)]
struct MyFancyResource { /* stuff */ }

impl FromWorld for MyFancyResource {
    fn from_world(world: &mut World) -> Self {
        // You have full access to anything in the ECS World from here.

        // For example, you can access (and mutate!) other things:
        {
            let mut x = world.resource_mut::<MyOtherResource>();
            x.do_mut_stuff();
        }

        // You can load assets:
        let font: Handle<Font> = world.resource::<AssetServer>().load("myfont.ttf");

        MyFancyResource { /* stuff */ }
    }
}

Beware: it can be easy to get yourself into a mess of unmaintainable code if you overuse FromWorld to do complex things.

Usage Advice

The choice of when to use entities/components vs. resources is typically about how you want to access the data: globally from anywhere (resources), or using ECS patterns (entities/components).

Even if there is only one of a certain thing in your game (such as the player in a single-player game), it can be a good fit to use an entity instead of resources, because entities are composed of multiple components, some of which can be common with other entities. This can make your game logic more flexible. For example, you could have a "health/damage system" that works with both the player and enemies.

Settings

One common usage of resources is for storing settings and configuration.

However, if it is something that cannot be changed at runtime and only used when initializing a plugin, consider putting that inside the plugin's struct, instead of a resource.

Caches

Resources are also useful if you want to store some data in a way that is easier or more efficient for you to access. For example, keeping a collection of asset handles, or using a custom datastructure for representing a game map more efficiently than using entities and components, etc.

Entities and Components, as flexible as they are, are not necessarily the best fit for all use cases. If you want to represent your data some other way, feel free to do so. Simply create a resource and put it there.

Interfacing with external libraries

If you want to integrate some external non-Bevy software into a Bevy app, it can be very convenient to create a resource to hold onto its state/data.

For example, if you wanted to use an external physics or audio engine, you could put all its data in a resource, and write some systems to call its functions. That can give you an easy way to interface with it from Bevy code.

If the external code is not thread-safe (!Send in Rust parlance), which is common for non-Rust (e.g C++ and OS-level) libraries, you should use a Non-Send Bevy resource instead. This will make sure any Bevy system that touches it will run on the main thread.