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.