[data_assets] Dart SDK implementation

[dcharkes]

Tracking bug for implementing a way to access Dart assets in Dart (not in Flutter) that works in both Dart standalone and Flutter, and in all backends (vm/web).

A POC: https://dart-review.googlesource.com/c/sdk/+/379841

[simolus3]

Is there an API draft / discussion issue for this somewhere? In particular, one thing I'm looking for is the ability to get a URI for a data asset on the web. Some use cases for this are:

• The sqlite3 package needs a .wasm module for SQLite on the web.
• The sqlite3_web and drift packages need a .js file to load as a web worker.

It sounds like data assets would be an excellent fit for this, especially because things like link hooks would be able to remove code from external WebAssembly modules / workers if certain methods aren't called in Dart. But if we're only able to load assets as contents, spawning web workers would be tricky or less efficient than if we had a URI.

So if would be nice to have an API to get a URI for assets on platforms where that makes sense (likely the web only). I also wonder if instead of using something like an asset class at runtime, data assets could use the syntax established for FFI, e.g. with:

@DataAsset('worker.js')
external Uri get worker; // web only, would fail to compile on other platforms

@DataAsset('sqlite3.wasm')
external Stream<Uint8List> get wasmModule;

@DataAsset('licenses.txt')
external Future<String> get text;

[dcharkes]

@simolus3 Assets which need to more than make the bytes available to Dart are not DataAssets.

The embedder/framework needs to be aware of js and wasm assets specifically:

• Either it supports or doesn't support them.
• It needs to bundle and link them somehow.
• I believe this will need to be custom for Flutter-for-web, Dart ACX, and Jaspr when bundling JS and WASM, they consume the same JSAsset and WasmAsset, but will then process them differently. Just how Dart standalone and Flutter process dylibs differently and how they process them differently for the different commands and app bundle formats.

This is tracked in:

• [native_assets_cli] build.dart / link.dart web backends and WasmCode and JsCode assets #988

@DataAsset('licenses.txt')
external Future<String> get text;

You are on the same path of thinking we are / have been! But so far we haven't found a consensus on how to achieve this yet. At the risk of you going down a rabbit hole: https://dart-review.googlesource.com/c/sdk/+/351140 That then led to https://dart-review.googlesource.com/c/sdk/+/379841. However, I'm not convinced that that is the right solution either.

The design space is vast. What do we provide for data assets? Only the bytes as Flutter does for its own data assets? But that prevents you from passing in a file path to a C API with a data asset. However, allowing file paths makes no sense again on web because there you probably only have a url. Then are the file paths and urls nullable in the API? If yes, then how do you write platform independent code against them. 😄 And if you provide file paths and urls, it prevents the embedder for shipping the file in a zip or concatenated. (Note that with @Native we do not provide access to the dylib path either. We let the embedder resolve asset ids and symbols. So the embedder can decide how to ship/link.)