Bevy Asset Scenes Agent Skill

Bevy asset scenes

When to use

Load this when content comes from disk or a scene description should spawn entities. AssetServer loads files. bsn! describes a Scene that spawn_scene or queue_spawn_scene turns into entities.

ECS shape

• Assets: AssetServer is the resource that loads from disk. AssetServer::load returns a Handle and does not reload while a strong handle still exists. Loaded data lives in Assets<T>. AssetServer::is_loaded_with_dependencies is true when the asset and its dependencies are ready. LoadState is the status type named on that page.
• Handles: strong handles are reference counted. Dropping the last strong handle removes the asset. Weak handles do not keep it alive. Assets::add inserts a procedural asset and does not deduplicate.
• Hot reload: the file_watcher Cargo feature, and AssetPlugin's watch_for_changes_override field, are the toggles named on the asset index.
• Scenes: bevy::scene describes a Scene (entity, components, children, assets), built with the bsn! macro. World::spawn_scene and Commands::spawn_scene spawn immediately and error if asset dependencies are not loaded. World::queue_spawn_scene and Commands::queue_spawn_scene wait, then spawn in the SpawnScene schedule between Update and PostUpdate.
• Hierarchy in a scene: Children [ ... ] inside bsn!. Separate sibling entities with --. #Name assigns a Name inside that macro's scope.

Asset choice

| Need | Do this |

| --- | --- |

| Load a file | AssetServer::load returns a Handle |

| Wait until ready | is_loaded_with_dependencies before a state change |

| Spawn a scene now | spawn_scene only after dependencies are loaded |

| Spawn when ready | queue_spawn_scene (SpawnScene sits between Update and PostUpdate) |

| Children in that scene | Children inside bsn! |

| Change one entity's mesh | Swap the Handle on the component that stores it |

.bsn files on disk are still ahead of the shipping scene API. Author scenes with bsn!. Free a previous level by despawning its root, then queue the next scene. Register a custom AssetLoader with App::register_asset_loader once that loader type exists.

[dependencies]
bevy = { version = "0.20", features = ["file_watcher"] }

#[derive(Resource)]
struct PropMesh(Handle<Mesh>);

fn begin_load(mut commands: Commands, assets: Res<AssetServer>) {
    let handle: Handle<Mesh> = assets.load("models/crate.glb#Mesh0/Primitive0");
    commands.insert_resource(PropMesh(handle));
}

fn mesh_is_ready(assets: Res<AssetServer>, mesh: Option<Res<PropMesh>>) -> bool {
    match mesh {
        Some(mesh) => assets.is_loaded_with_dependencies(&mesh.0),
        None => false,
    }
}

fn store_generated_material(
    mut materials: ResMut<Assets<StandardMaterial>>,
    mut commands: Commands,
) {
    let handle = materials.add(StandardMaterial {
        base_color: Color::srgb(0.8, 0.8, 0.82),
        ..default()
    });
    commands.insert_resource(GeneratedMaterial(handle));
}

#[derive(Resource)]
struct GeneratedMaterial(Handle<StandardMaterial>);

#[derive(Component, Default, Clone)]
struct CrateProp;

#[derive(Component, Default, Clone)]
struct Lid;

#[derive(Component, Default, Clone)]
struct LevelRoot;

fn spawn_crate(mut commands: Commands) {
    commands.spawn_scene(bsn! {
        #SupplyCrate
        CrateProp
        Children [
            Lid
        ]
    });
}

fn queue_level(mut commands: Commands) {
    commands.queue_spawn_scene(bsn! {
        :"levels/yard.bsn"
        LevelRoot
    });
}

fn replace_level(mut commands: Commands, roots: Query<Entity, With<LevelRoot>>) {
    for entity in &roots {
        commands.entity(entity).despawn();
    }
    commands.queue_spawn_scene(bsn! {
        LevelRoot
    });
}

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 script that owns the whole actor |

| 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 packed-scene or resource-loader types from another engine. WHY: readiness is is_loaded_with_dependencies, and spawn is spawn_scene or queue_spawn_scene, not a node instance.
• No scene format this file did not open. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.
• No third-party scene crates. WHY: Bevy code stays idiomatic ECS and uses only documented Bevy APIs.

Reference

The library pin on bevy-master stays 0.20.0. Load references/ecs-ownership.md when a handle is a resource and a scene root is a component. Do not load it to write a custom loader body.

Official Documentation

• Assets - Official asset catalog. AssetServer, Handle, and Assets<T> are the load path. Scene, bsn!, spawn_scene, queue_spawn_scene, and SpawnScene are the spawn path.
• Examples - Example tree on the latest branch.

Related Skills

Prerequisites

• bevy-master - Who owns which piece of state.
• bevy-project-foundations - Cargo features, including asset watcher flags.
• bevy-state-machines - The asset page points at app states for load gates.

Complements

• bevy-audio-systems - Clips loaded through AssetServer.
• bevy-save-load - Player saves, separate from scene spawn.

Downstream / consumers

• bevy-camera-systems - Camera entities a scene may spawn.

Master

• bevy-master - Bevy router.