AssetBundle is a container that provides asynchronous access to application resources (e.g., images, strings, fonts). Resources are associated with a string-based key and can be retrieved as bytes (via AssetBundle.load), a string (via AssetBundle.loadString), or structured data (via AssetBundle.loadStructuredData). A variety of subclasses support different methods for obtaining assets (e.g., PlatformAssetBundle, NetworkAssetBundle). Some bundles also support caching; if so, keys can be evicted from the bundle’s cache (via AssetBundle.evict).
CachingAssetBundle caches strings and structured data throughout the application’s lifetime (unless explicitly evicted). Binary data is not cached since the higher level methods are built atop AssetBundle.load, and the final representation is more efficient to store.
Every application is associated with a rootBundle. This AssetBundle contains the resources that were packaged when the application was built (i.e., as specified by pubspec.yaml). Though this bundle can be queried directly, DefaultAssetBundle provides a layer of indirection so that different bundles can be substituted (e.g., for testing or localization).
NetworkAssetBundle loads resources over the network. It does not implement caching; presumably, this is provided by the network layer. It provides a thin wrapper around dart’s HttpClient.
PlatformAssetBundle is a CachingAssetBundle subclass that fetches resources from a platform-specific application directory via platform messaging (specifically, Engine::HandleAssetPlatformMessage).