Bevy 0.19 — Custom asset loaders

When to use this skill

• Loading a custom binary or text format (proprietary game data, sidecar configs, etc.).
• Producing an asset that depends on other assets (e.g. a level loader that pulls referenced textures).
• Migrating LoadContext::loader() or an old NestedLoader to load_builder() / NestedLoadBuilder.
• Needing async access to the underlying file (reader.read_to_end, reader.seekable()).
• Compiler error: "MyLoader: TypePath is not implemented" — 0.18 made TypePath mandatory on loaders.

Canonical pattern

use bevy::asset::io::Reader;
use bevy::asset::{Asset, AssetApp, AssetLoader, LoadContext};
use bevy::prelude::*;
use bevy::reflect::TypePath;
use serde::Deserialize;
use thiserror::Error;