Bevy app resources

When to use

Load this when data or logic must exist for the whole app: one clock, one ruleset, startup spawn, or an ordered pair of systems. Pair with bevy-data-resources for the component-versus-resource split.

ECS shape

• App. The Apps page: an App contains a World and the outer loop. It initializes resources (one copy), adds systems to a Schedule, and imports other App changes through plugins.
• Resources. Resource, Res, ResMut, and insert_resource (Resources page). Time arrives from add_plugins(DefaultPlugins).
• Plugins. The Resources page implements Plugin and inserts the resource inside build. DefaultPlugins is the plugin set that adds Time. Name only the plugins this page documents.
• Schedules. add_systems(Startup, ...) runs once before other systems. add_systems(Update, ...) is the repeating schedule in the ECS page. .chain() orders systems that must see each other's writes. bevy::ecs::message names FixedUpdate only as the schedule after which signal_message_update_system runs. Fixed-timestep clock types stay in bevy-character-movement.
• States. App-wide modes are bevy::state, not a global node. See bevy-state-machines.

App choices

Need	Bevy idiom
One copy for the whole app	A Resource inserted on the App
Logic that ticks every frame	A system on Update (or a chained pair)
Boot setup	A Startup system or insert_resource in a Plugin
One-shot notice	A message or event. See bevy-events-messaging.

Scope

• Insert app-wide data with insert_resource inside Plugin::build, or from a Startup system.
• Order two systems that both mutably use the same resource with .chain().
• Save files are a Resource you serialize yourself. The running copy stays in the World.

Examples

[package]
name = "app_resources"
version = "0.1.0"
edition = "2021"

[dependencies]
bevy = "0.20"

One rules resource, one startup spawn, and a pair of systems that must not race on Score.

#[derive(Resource)]
struct Ruleset {
    gravity: f32,
}

#[derive(Resource, Default)]
struct Score(u32);

#[derive(Component)]
struct Falling;

fn spawn_session(mut commands: Commands) {
    commands.spawn((Name::new("session"), Transform::default(), Falling));
}

fn apply_gravity(
    rules: Res<Ruleset>,
    time: Res<Time>,
    mut bodies: Query<&mut Transform, With<Falling>>,
) {
    let step = rules.gravity * time.delta_secs();
    for mut transform in &mut bodies {
        transform.translation.y += step;
    }
}

fn award(mut score: ResMut<Score>) {
    score.0 = score.0.saturating_add(1);
}

fn publish(score: Res<Score>, mut bodies: Query<&mut Transform, With<Falling>>) {
    let lift = score.0 as f32;
    for mut transform in &mut bodies {
        transform.translation.x = lift;
    }
}

pub struct RulesPlugin;

impl Plugin for RulesPlugin {
    fn build(&self, app: &mut App) {
        app.insert_resource(Ruleset { gravity: -9.8 })
            .init_resource::<Score>()
            .add_systems(Startup, spawn_session)
            .add_systems(Update, (apply_gravity, award, publish).chain());
    }
}

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

award and publish both touch Score or Transform. apply_gravity also writes Transform, so the three systems stay in one chain.

A second plugin only reads the rules resource.

#[derive(Resource)]
struct Ruleset {
    gravity: f32,
}

fn log_rules(rules: Res<Ruleset>) {
    let _gravity = rules.gravity;
}

pub struct HudPlugin;

impl Plugin for HudPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(Update, log_rules);
    }
}

Fixed-step simulation reads Time<Fixed> and stays on FixedUpdate.

#[derive(Component)]
struct Falling;

fn fixed_step(time: Res<Time<Fixed>>, mut bodies: Query<&mut Transform, With<Falling>>) {
    let step = time.delta_secs();
    for mut transform in &mut bodies {
        transform.translation.z += step;
    }
}

pub struct FixedRulesPlugin;

impl Plugin for FixedRulesPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(FixedUpdate, fixed_step);
    }
}

fixed_step owns Falling in this module. The startup example uses its own Falling marker.

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	A value stored off the entity
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	Check the official docs first	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 process-global static for app data. WHY: a Resource on the App is the single owner, and systems borrow it through Res or ResMut.

• No invented plugin lists beyond DefaultPlugins and the Plugin trait the Resources page uses. 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. Permalinks stay unversioned.

Official Documentation

• Apps - App, World, resources, Schedule, plugins.
• Resources - Resource, insert_resource, Plugin::build, DefaultPlugins, Time.
• Quick Start: ECS - Startup, Update, add_systems, .chain().
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Hub and schedules.
• bevy-project-foundations - Crate and App::new().

Complements

• bevy-data-resources - When data is a component instead of a resource.
• bevy-state-machines - App-wide States for mode changes.

Downstream / consumers

• Save, audio, and scene skills cover persistence, playback, and scene spawn.

Master

• bevy-master - Bevy router.