Swift example

This example shows how to use FFIgen to interact with Swift libraries.

Swift APIs can be made compatible with Objective-C, using the @objc annotation. Then you can use the swiftc tool to build a dylib for the library using -emit-library, and generate an Objective-C wrapper header using -emit-objc-header-path filename.h:

swiftc -c swift_api.swift                           \
    -module-name swift_module                       \
    -emit-objc-header-path third_party/swift_api.h  \
    -emit-library -o libswiftapi.dylib

This should generate libswiftapi.dylib and swift_api.h. For more information about Objective-C / Swift interoperability, see the Apple documentation.

Once you have an Objective-C wrapper header, FFIgen can parse it like any other header:

dart run tool/ffigen.dart

This will generate swift_api_bindings.dart.

Finally, you can run the example using this command:

dart run example.dart

Config notes

The FFIgen configuration is defined in tool/ffigen.dart. FFIgen only sees the Objective-C wrapper header, swift_api.h. So you need to enable Objective-C support and set the entry-point to the header.

final generator = FfiGenerator(
  output: Output(
    dart: DartOutput(path: packageRoot.resolve('swift_api_bindings.dart')),
  ),
  objectiveC: const ObjectiveC(),
  input: Input(entryPoints: [packageRoot.resolve('third_party/swift_api.h')]),
  visitors: [
    Visitor(
      objCInterface: (node) {
        if (node.name == 'SwiftClass') {
          node.isIncluded = true;
          node.module = 'swift_module';
        }
      },
    ),
  ],
);

There are two important things to note about this example:

1. Swift classes become Objective-C interfaces, so include them using an objCInterface visitor.
2. When swiftc compiles the library, it gives the Objective-C interface a module prefix. Internally, our SwiftClass is actually registered as swift_module.SwiftClass. So you need to tell FFIgen about this prefix using node.module. The module is whatever you passed to swiftc in the -module-name flag.