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.