Bevy App Resources Agent Skill

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.