Bevy Data Resources Agent Skill

Bevy data resources

When to use

Load this when you are deciding where a piece of data lives: on many entities, or once for the whole app. The Resources page and the ECS page are the source for the names below.

ECS shape

• Components are for data you query across entities (Component, Query). Use them when many things carry the same kind of fields.
• Resources are globally unique data (Resource). The Resources page names elapsed time, asset collections, and renderers as examples. Access them with Res<T> (read) and ResMut<T> (write).
• Insert. insert_resource places a resource on the App. The page's sample derives Resource on GreetTimer(Timer) and inserts Timer::from_seconds(2.0, TimerMode::Repeating).
• Time. Time is inserted by add_plugins(DefaultPlugins). time.delta() is the elapsed time since the last update. Timer::tick and just_finished are the page's timer methods.
• Schedule: The sample reads Res<Time> and ResMut<GreetTimer> inside an Update system. Startup versus update is on the ECS page.

A resource can hold asset handles loaded through AssetServer. The handles are data; the loader stays on the asset server.

Where data lives

| Need | Bevy idiom |

| --- | --- |

| Data on one gameplay thing | A Component |

| One copy for the whole app | A Resource inserted with insert_resource |

| Shared clock | Time from DefaultPlugins, or your own Resource |

| Authored data file | serde plus ron::from_str into a Resource, or Handle<Mesh> from AssetServer |

Scope

• Handles, RON tables, and plugin insert_resource calls are in the examples below.
• Per-entity data files and save blobs are out of scope. See bevy-save-load.

[package]
name = "data-resources"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"
serde = { version = "1", features = ["derive"] }
ron = "0.10"

One greet timer resource, ticked from Time on Update.

#[derive(Resource)]
struct GreetTimer(Timer);

fn greet(time: Res<Time>, mut timer: ResMut<GreetTimer>) {
    timer.0.tick(time.delta());
    if timer.0.just_finished() {
        let _ready = true;
    }
}

pub struct GreetPlugin;

impl Plugin for GreetPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(GreetTimer(Timer::from_seconds(2.0, TimerMode::Repeating)))
            .add_systems(Update, greet);
    }
}

fn main() {
    App::new().add_plugins(DefaultPlugins).add_plugins(GreetPlugin).run();
}

Per-entity speed stays a component. The shared multiplier stays a resource.

#[derive(Component)]
struct BaseSpeed(f32);

#[derive(Component)]
struct Speed(f32);

#[derive(Resource)]
struct SpeedScale(f32);

fn scale_speed(scale: Res<SpeedScale>, mut bodies: Query<(&BaseSpeed, &mut Speed)>) {
    for (base, mut speed) in &mut bodies {
        speed.0 = base.0 * scale.0;
    }
}

pub struct SpeedPlugin;

impl Plugin for SpeedPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(SpeedScale(1.0))
            .add_systems(Startup, |mut commands: Commands| {
                commands.spawn((BaseSpeed(4.0), Speed(4.0)));
            })
            .add_systems(Update, scale_speed);
    }
}

Load a scene handle into a resource, and parse a RON table into another.

use serde::Deserialize;

#[derive(Resource)]
struct LevelAssets {
    mesh: Handle<Mesh>,
}

#[derive(Deserialize, Resource)]
struct Tuning {
    speed: f32,
}

fn load_level(mut commands: Commands, assets: Res<AssetServer>) {
    commands.insert_resource(LevelAssets {
        mesh: assets.load("models/gate.glb#Mesh0/Primitive0"),
    });
}

fn tuning_from_ron(raw: &str) -> Result<Tuning, ron::error::SpannedError> {
    ron::from_str(raw)
}

Expert Knowledge Delta

Target: Bevy 0.20+.

Core ECS types: Component, Resource, Query, Commands, Startup, Update, Fixed / FixedMain (bevy::time), .chain(), Plugin, DefaultPlugins, MinimalPlugins. Messages and observers are an ownership slot. See bevy-events-messaging for their APIs. Systems run in parallel unless .chain() orders the ones that conflict.

Who owns state

| Piece | Owner | Who may mutate | Schedule |

| --- | --- | --- | --- |

| Data on one entity | Component | One system via Query | Update or FixedMain |

| One value for the whole app | Resource | One system | Startup to insert, then the schedule that reads it |

| Spawn | Commands | The spawning system | Startup for the initial world; later schedules for runtime spawn |

| Order that must not race | .chain() | The chained systems only | Same schedule |

| Plugin boundary | Plugin | build registers systems and resources | Before the app runs |

Borrow rule: two systems that both mutably query the same component must be ordered (.chain()) or split so only one writes. Unrelated systems stay parallel. That is the ECS page rule, not a guess at a system-set API.

Decision

| If you need | Do this | Do not |

| --- | --- | --- |

| Per-entity data | Component + Query | One struct that stores every entity's fields |

| App-wide config or clock | Resource | Copy the value onto every entity |

| Stable simulation step | FixedMain and the Fixed clock | Tie simulation to raw frame Update |

| Startup spawn | Commands in Startup | Spawn from a render system |

| Cross-system notice that is not shared state | Message or observer slot in bevy-events-messaging | A new event type invented in this file |

| An API outside the documented set | Read the linked Bevy pages | A made-up crate or API |

Load references/ecs-ownership.md for the ownership table.

NEVER

• Do not pin /0.20.0/ in docs URLs. WHY: links stay on latest; the 0.20 target is stated once in prose.
• Do not mutably query the same component from two unordered systems. WHY: the schedule runs systems in parallel unless .chain() orders them.
• Do not invent a physics, navigation, networking, or XR crate here. WHY: this skill only names documented Bevy crates and modules.

• No packed scene or node-script stand-ins for Resource. WHY: app-wide data is a Resource inserted on the App or Commands.
• No invented asset or serialization APIs. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No third-party crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Official links stay on the unversioned learn pages.

Official Documentation

• Resources - Resource, Res, ResMut, insert_resource, Time, Timer, TimerMode.
• Quick Start: ECS - Component and Query for per-entity data.
• Apps - Resources as the single copy of global data in the World.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Component versus resource ownership.
• bevy-ecs-composition - How components are spawned and queried.

Complements

• bevy-app-resources - Schedules and plugins around those resources.

Downstream / consumers

• Inventory, stats, and save skills cover items, attributes, and persistence.

Master

• bevy-master - Bevy router.