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