Support for Dart Extensions

[eseidelGoogle]

https://www.dartlang.org/articles/native-extensions-for-standalone-dart-vm/

We've had at least one request for this. I suspect it's implemented entirely in the dart CLI.

My understanding is it's just a bit of code to map dart-ext: urls to making a dlopen call and passing the symbols off to the VM. @johnmccutchan

I'm mostly seeking to document how we'd do this in this bug. It's not clear if this is generically useful.

[eseidelGoogle]

Here is the relevant code in the dart CLI:
https://github.com/dart-lang/sdk/blob/1e5eab3a143f0736d925f546952006fb31127e9a/runtime/bin/dartutils.cc#L445

[johnmccutchan]

Assuming we keep this in the embedder, what happens when an import 'dart-ext:' occurs is the following:

1. Load the .so/.dll
2. Invoke the extensionName_Init() method.

In existing extensions, in the _Init method mostly just wires up the native resolver for the Dart code.

I wonder if we could formulate extensions differently and support them directly in the VM. I'll investigate this.

[johnmccutchan]

Related- https://codereview.chromium.org/1773643002/

[Hixie]

I've moved this to the 2.0 milestone since it doesn't seem like a blocker for a quality first release.

[eseidelGoogle]

This is not needed by any near-term customer. The only bug I've seen which might use this is opengl support: #179 iceboxing.

[nacardin]

I'm currency working on an app that needs to use digital signatures, specifically EdDSA. Since there are C implementations which are stable and known to be reliable, I think it would be safer to use FFI, rather than trying to port the algorithm to dart.

Especially with WebAssembly becoming widely supported, Flutter would be at a disadvantage if it didn't allow FFI to use native libraries.

[sethladd]

You may be able to use our plugins system to communicate between the Flutter Dart code and platform-specific libraries: https://flutter.io/platform-plugins/

[eseidelGoogle]

#7053 is probably what you want. You can integrate with C code today with Flutter, you just have to bounce through Java/Obj-C first.

[eseidelGoogle]

This need will hopefully be covered by FFI work: dart-lang/sdk#34452 and related tooling work in: #7053

[ping996]

does the flutter support dart native extension? according to use import 'dart-ext:library_name';
https://dart.dev/server/c-interop-native-extensions

[github-actions]

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