Interop: Objective C & Swift

[mit-mit]

Dart currently supports interop with C code via dart:ffi. We should investigate if we can provide similar interop with Objective C and Swift code.

This would be a supplement to Flutter's plugins, and would allow for calling into Objective C / Swift directly from Dart code, also from pure-Dart apps (such as CLI apps).

• Objective C interop, binary and source code: Has experimental support, see the example
• Swift interop, with source code access: Has experimental support, see the example
• Swift interop, binary/without source code access:
  TODO

If you have any feedback and suggestions about this project, please comment below.

[fotiDim]

I would be interested in trying out the Swift interop, binary/without source code access since our use case is consuming a native .xcframework written in Swift, in a Flutter plugin. We do have the source code of the .xcframework so I am not sure if it possible to use the Swift interop, with source code access method and then swap out the code with the .xcframework.

For now we are using platform channels and we are thinking of skipping Pigeon completely if FFIgen will be soon the way to go.

[a-siva]

//cc @liamappelbe

[dowobeha]

I would be interested in this functionality, for Swift specifically.

[liamappelbe]

I've looked into this a bit and I don't think it's possible to interop directly with a swift binary without some sort of intermediary layer. The only way I've seen for interop with swift is to:

1. Annotate your API with @objc (if you don't own the library, write a wrapper library and mark that with the annotation).
2. Use the swift compiler to generate an ObjC wrapper header (a 1 liner swiftc command)
3. ObjC obeys the C calling convention, so we can do interop. In Dart's case, that means using ffigen to parse the ObjC header then using FFI to load the library.

If you don't have access to the source code, just a compiled swift library, then you can still do step 1 by writing a wrapper swift library that you annotate with @objc. You'll end up with 2 libraries to link into your app: the original library and the wrapper library.

There's also a C interop annotation, @_cdecl, but it's undocumented and unofficial, and wouldn't actually simplify things.

We're also considering ways of automating steps 1 and 2, but that won't fundamentally change this workflow.

If anyone knows a cleaner way of interoping with swift, I'd love to hear about it, but afaik this is the only option atm.