[dartdev] Add `dart prebuild` command to build native assets

[Zekfad]

How can I precompile assets for release packaging and debug development?
You can pub get the packages, but there are no command for post-processing (compiling) got packages.

Also it would be great to expose some sort of config that will allow to map some particular Natives to precompiled dynamic libraries.

[dcharkes]

Hi @Zekfad,

You can pub get the packages, but there are no command for post-processing (compiling) got packages.

With build.dart you can do whatever you want in the script. You can compile your packages (and RELEASE or DEBUG is defined).
Alternatively, if you want to precompile your packages, you can precompile your dynamic libraries both in release and debug mode, host them somewhere on the web and download them in build.dart.

Also it would be great to expose some sort of config that will allow to map some particular Natives to precompiled dynamic libraries.

This is exactly what build.dart outputs: a mapping from asset id (the asset: in @Native) to the path of the dynamic library (per os_arch).

You can give support for build.dart a spin in Dart by passing the experimental flag --enable-experiment=native-assets. In Flutter support hasn't landed yet flutter/flutter#129757.

[Zekfad]

Hi @dcharkes , thank you for your answer!

What if I have multiple native dependencies, should I manually call build for each of them from pub cache?

Apart from this I understand meaning behind build.dart to have precise control over build phase, but isn't yaml config easier for simple solutions (e.g. just compile c-code) than having to copy-paste build script and keep it updated for API changes?

[dcharkes]

What if I have multiple native dependencies, should I manually call build for each of them from pub cache?

If you have dart package that have native assets, they should be automatically built by their build.dart. The Dart SDK will automatically invoke the build.dart for all your dependencies.

If your package has multiple dynamic libraries, you should build/download all of these in your build.dart. The Dart SDK will automatically invoke your build.dart when running dart run / dart test when passing --enable-experiment=native-assets on the dev branch.

You should not be manually calling any build.dart scripts, the Dart SDK should do this for you. Maybe I misunderstood the question, was this what you were asking?

but isn't yaml config easier for simple solutions

We considered this. However, you'll need a different line in your config for every _, and Dart and Flutter support 21 combinations, so you'll likely want to script that. (And then default the OS to the relevant file extensions etc. .so for Linux, .dylib for MacOS...) Moreover, it means you have to push your dynamic libraries to pub.dev with dart pub publish, and this only works up to a certain size as pub has a size restriction for package versions.

keep it updated for API changes?

The API changes in the Dart API of the helper package are versioned by the package, so you're not forced to update.

The API changes in the communication between Dart SDK and the packages with native assets would always have issues if we ever want to change that protocol. Having a package actually allows you to just update the package in your dependencies instead of having to manually fix up some yaml file.

(In general, we'd like to minimize breaking API changes! However, currently this package is experimental, so no promises.)

[Zekfad]

Thanks for clarifying the config part.

Regarding automatic build.dart call: I was talking about scenario where I don't want to run tests or program itself. That's what I meant by precompilation.
One solution I see to have empty test, to force Dart SDK into calling related builds?

[dcharkes]

Ah, that's what you meant with precompiling. I thought you were talking about having the C code compiled when shipping a package as a package author rather than on a build.dart invocation as a package user.

You are correct, there is currently no way to force pre-compilation. In the Dart SDK it could make sense to precompile, because we almost always are running in JIT mode (meaning we need dynamic libraries, not static libraries) and Dart does not have a concept of debug/release mode (so the build mode is always the same). However, in Flutter, we have multiple target OSes/architectures, and debug and release build modes. So it's hard to know for what to pre-compile in that case.

The workaround, having an empty test in your package, works.

cc @mit-mit Our first user asking for being able to run the native builds before doing a dart run or dart test invocation. Maybe it would be good to introduce something similar to dart pub get to be able to build the native assets. Possibly dart build native-assets.

[mit-mit]

So the use case here is targeted at the developer creating the package containing native assets who wants to adhoc test that the build works?

[dcharkes]

That (1), and (2) the "airplane mode" approach as a user, where you get all your dependencies with dart pub get and your native dependencies (which are possibly downloaded as well) with dart build native-assets. And then you enter the airplane and hack on your own Dart code that uses some downloaded native assets as a dependency.

[mit-mit]

Got it. How about dart prebuild as a hidden subcommand (i.e. hidden in dart help, only shown in dart help -v)? That is aligned with flutter precache.

[dcharkes]

Got it. How about dart prebuild as a hidden subcommand (i.e. hidden in dart help, only shown in dart help -v)? That is aligned with flutter precache.

While the feature is experimental it should be dart --enable-experiment=native-assets prebuild, and then later dart prebuild.

[ykmnkmi]

Please add same logic/feature for JS packages.

For example, definition:

// native_add_library.d.ts

export function add(a: number, b: number): number;

// native_add_library.js

import { add as _add } from "library";

export function add(a, b) {
  return _add(a, b);
}

generated:

// native_add_library.dart

JS()
library;

import 'package:js/js.dart';

@JS()
external int add(int a, int b);