Support publishing Flutter plugin + Dart standalone hybrid packages

[dcharkes]

I was under the impression that we could publish Flutter plugins that also run in Dart standalone (ignoring the Flutter plugin code) by a pubspec with:

name: mylib_source
description: A new Flutter FFI plugin project.
version: 0.0.1
homepage:

environment:
  sdk: ">=2.17.0 <3.0.0"
  # no flutter:

dependencies:

dev_dependencies:

flutter:
  plugin:
    platforms:
      android:
        package: com.example.mylib_source
        ffiPlugin: true
      ios:
        ffiPlugin: true
      macos:
        ffiPlugin: true
      linux:
        ffiPlugin: true
      windows:
        ffiPlugin: true

This works locally with both the Flutter and Dart standalone SDK. (For example dart pub get && dart test and flutter pub get && flutter test, where dart is not from the Flutter SDK.)

However, pub does not support publishing this:

$ dart pub publish --dry-run
// ...
Package validation found the following error:
* pubspec.yaml allows Flutter SDK version 1.9.x, which does not support the flutter.plugin.platforms key.
  Please consider increasing the Flutter SDK requirement to ^1.10.0 (environment.sdk.flutter)
  
  See https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin
Sorry, your package is missing a requirement and can't be published yet.
For more information, see: https://dart.dev/tools/pub/cmd/pub-lish.

Adding the flutter key makes dart pub get fail:

Resolving dependencies... 
Because jni requires the Flutter SDK, version solving failed.

Flutter users should run `flutter pub get` instead of `dart pub get`.

As far as I can see, the only workaround is to publish two packages.

cc @mahesh-hegde @jonasfj

edit: duplicate of #2606

[mahesh-hegde]

For me the result depends only on which SDK dart command is from.

mahesh:~/Code/jnigen/jnigen/example/pdfbox_plugin/dart_example [jni_refactor=] 
$ export PATH=$HOME/.local/dart-sdk/bin:$PATH
mahesh:~/Code/jnigen/jnigen/example/pdfbox_plugin/dart_example [jni_refactor=] 
$ dart pub get
Resolving dependencies... 
Because pdf_info depends on jni from path which requires the Flutter SDK, version solving failed.

Flutter users should run `flutter pub get` instead of `dart pub get`.
[1]
mahesh:~/Code/jnigen/jnigen/example/pdfbox_plugin/dart_example [jni_refactor=] 
$ export PATH=$HOME/.local/flutter/bin:$PATH
mahesh:~/Code/jnigen/jnigen/example/pdfbox_plugin/dart_example [jni_refactor=] 
$ dart pub get
Resolving dependencies... (2.6s)
Got dependencies!

It's confirmed also by the CI behaviour. The check in which I used setup-dart action failed. I updated it to use flutter-action and it passed.

[jonasfj]

Context: We did this years ago because older versions of the Flutter SDK didn't support flutter.plugin.platforms.

I suppose we could tweak the behavior such that the Flutter SDK constraint is expressed as a minimum Dart SDK constraint.

@sigurdm, any thoughts?

[sigurdm]

A few thoughts:

We need to have some story to tell here, but I'm not entirely sure how this kind of module should be published.

I feel it is somewhat surprising for a package that works as a dart-only package has a flutter.plugin.platforms key.

Could it be that there should be two packages, one having the dart ffi part, and another one importing the other, and wrapping it as a flutter plugin?

Even if that is technically feasible there would be downsides to that:

• branding a single package is easier
• publishing a single package is easier

[dcharkes]

@sigurdm another downside is that all your downstream packages have to do the same.

I feel it is somewhat surprising for a package that works as a dart-only package has a flutter.plugin.platforms key.

The contract is that the plugin is used to bundle the same native code for the same Dart API. But that contract is not enforced.

[dnys1]

The requirement that Flutter be installed and used simply by virtue of the environment specifier creates a whole host of issues right now. It makes it very difficult to publish hybrid packages, which is creating a lot of sprawl in the ecosystem. But worse, it means that any project which has just one transitive dependency on such a package must have Flutter installed and use that only.

This negatively impacts the ability to run CI, compile dart for the server, use in Docker, etc. where the Flutter SDK is much less portable and unnecessary because the package may not even use Flutter.

IMO, the presence of an environment specifier should be modified to mean if running with Flutter pub, then this requirement holds.

The requirement for Flutter to be installed and used should only come from an explicit dependency on a Flutter SDK package.