[swiftgen] Generating bindings for super simple Swift examples

[orestesgaolin]

In my attempts to understand swiftgen a bit more I'm trying to play with simple Swift concepts and expose bindings to them in Dart.

Given this simple Swift file

import Foundation
// enum '@objc' enum must declare an integer raw type
@objc enum CompassPoint: Int {
    case north
    case south
    case east
    case west
}

// struct
public struct Person {
    var name: String
    var age: Int
}

// class
@objc public class Dog : NSObject {
    public var name: String
    public var breed: String
    
    public init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

// function
public func greet(person: Person) -> String {
    return "Hello, \(person.name)!"
}

I'm trying to use current generate_code.dart script to create bindings to Dog, Person, greet() or CompassPoint. I've learned some details from the error messages (e.g. '@objc' enum must declare an integer raw typ) but I'm still not able to generate all the bindings:

• class Dog is accessible but cannot be initialized, properties are not generated
• greet is not available
• Person is not available
• CompassPoint is not available

Source code:

• basic_swift.h comes from manual swiftc -c basic_swift.swift -module-name basic_swift -emit-objc-header-path basic_swift.h -emit-library -o libbasicswift.dylib
• entire sample basic_swift.zip

I verified the Swift code can be run

Is there any way to make this code work with Dart?

[liamappelbe]

You're using ObjCCompatibleSwiftFileInput, so it will only generate bindings for APIs that are already annotated with @objc.

The full pipeline looks like this:

If you use ObjCCompatibleSwiftFileInput, you're starting from the "Swift API with @objc" node. This lets you skip the swift2objc step, but only works on APIs that already have @objc annotations. This can be handy as a time saver, but also because swift2objc is the most experimental link in the chain. In particular, swift2objc doesn't support enums yet (#1364). That's the next feature on the to-do list.

[liamappelbe]

Sorry, forgot to answer your actual question

Is there any way to make this code work with Dart?

Since some of these APIs don't have @objc, you want SwiftFileInput. You won't be able to generate bindings for CompassPoint atm, since swift2objc doesn't support enums yet, but in your example that enum doesn't seem to be used anyway.

Other than that, we'll have to resolve that build issue you've been having (#2637), since it sounds like it's also occurring here.

[liamappelbe]

• CompassPoint is not available

This one confused me for a bit because it is annotated with @objc, but I think this one is missing because it's not declared as public. If you add public, you should get bindings for this enum. Though if you switch from ObjCCompatibleSwiftFileInput to SwiftFileInput you won't get bindings for it anyway, due to #1364.

[orestesgaolin]

I managed to get somewhere with following config but still getting an exception when running.

• enum is not getting generated either with or without @objc
• PersonWrapper is missing the properties if they are not public (expected)
• greet is exposed via GlobalsWrapper

Generated /Users/dominik/Projects/_personal/plugins_presentation/native/pkgs/objective_c/test/main.c.exe
BasicSwiftWrapper/basic_swift_wrapper.swift:13: Fatal error: Use of unimplemented initializer 'init()' for class 'BasicSwiftWrapper.DogWrapper'

===== CRASH =====
si_signo=Trace/BPT trap: 5(5), si_code=TRAP_BRKPT(1), si_addr=0x1070dd9a0
version=3.9.2 (stable) (Wed Aug 27 03:49:40 2025 -0700) on "macos_arm64"
pid=45083, thread=15111, isolate_group=main(0x135809c00), isolate=main(0x135008790)
os=macos, arch=arm64, comp=no, sim=no
isolate_instructions=10466a720, vm_instructions=10466a720
fp=16c13a390, sp=16c13a380, pc=1070dd9a0
  pc 0x00000001070dd9a0 fp 0x000000016c13a390 $s17BasicSwiftWrapper03DogC0CACycfc+0x60
  pc 0x00000001070dd9b8 fp 0x000000016c13a3b0 $s17BasicSwiftWrapper03DogC0CACycfcTo+0x14
  pc 0x0000000107187fc8 fp 0x000000016c13a3d8 Unknown symbol
  pc 0x000000010f0fdf2c fp 0x000000016c13a418 Unknown symbol
  pc 0x000000010f0fd844 fp 0x000000016c13a468 Unknown symbol
  pc 0x000000010f0fd698 fp 0x000000016c13a490 Unknown symbol
  pc 0x000000010f0aa268 fp 0x000000016c13a4e8 Unknown symbol
  pc 0x000000010f0a9f30 fp 0x000000016c13a518 Unknown symbol
  pc 0x000000010f0a8a0c fp 0x000000016c13a580 Unknown symbol
  pc 0x000000010f0a8da8 fp 0x000000016c13a5c8 Unknown symbol
  pc 0x000000010f0a8a0c fp 0x000000016c13a630 Unknown symbol
  pc 0x000000010f0a7304 fp 0x000000016c13a688 Unknown symbol
  pc 0x0000000107183a64 fp 0x000000016c13a760 Unknown symbol
  pc 0x0000000104826438 fp 0x000000016c13a7d0 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x1ee438
  pc 0x0000000104827f60 fp 0x000000016c13a810 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x1eff60
  pc 0x00000001048482a8 fp 0x000000016c13adc0 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x2102a8
  pc 0x000000010486c17c fp 0x000000016c13ae30 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x23417c
  pc 0x000000010486c584 fp 0x000000016c13ae90 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x234584
  pc 0x000000010494cec0 fp 0x000000016c13af20 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x314ec0
  pc 0x000000010494d10c fp 0x000000016c13af50 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x31510c
  pc 0x00000001048fdea0 fp 0x000000016c13afc0 /Users/dominik/fvm/versions/stable/bin/cache/dart-sdk/bin/dart+0x2c5ea0
  pc 0x000000018b323c0c fp 0x000000016c13afe0 _pthread_start+0x88
-- End of DumpStackTrace
  pc 0x0000000000000000 fp 0x000000016c13a3d8 sp 0x0000000000000000 Cannot find code object
  pc 0x000000010f0fdf2c fp 0x000000016c13a418 sp 0x000000016c13a3e8 [Optimized] init:_objc_msgSend_151sglz@23115747.#ffiClosure3
  pc 0x000000010f0fd844 fp 0x000000016c13a468 sp 0x000000016c13a428 [Unoptimized] DogWrapper.new$
  pc 0x000000010f0fd698 fp 0x000000016c13a490 sp 0x000000016c13a478 [Unoptimized] new DogWrapper.
  pc 0x000000010f0aa268 fp 0x000000016c13a4e8 sp 0x000000016c13a4a0 [Unoptimized] main
  pc 0x000000010f0a9f30 fp 0x000000016c13a518 sp 0x000000016c13a4f8 [Unoptimized] main
  pc 0x000000010f0a8a0c fp 0x000000016c13a580 sp 0x000000016c13a528 [Unoptimized] _Closure@0150898.dyn:call
  pc 0x000000010f0a8da8 fp 0x000000016c13a5c8 sp 0x000000016c13a590 [Unoptimized] _delayEntrypointInvocation@1026248.<anonymous closure>
  pc 0x000000010f0a8a0c fp 0x000000016c13a630 sp 0x000000016c13a5d8 [Unoptimized] _Closure@0150898.dyn:call
  pc 0x000000010f0a7304 fp 0x000000016c13a688 sp 0x000000016c13a640 [Unoptimized] _RawReceivePort@1026248._handleMessage@1026248
  pc 0x0000000107183a64 fp 0x000000016c13a760 sp 0x000000016c13a698 [Stub] InvokeDartCode

basic_swift_example.dart

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:ffi';
import 'dart:io';

import 'package:objective_c/objective_c.dart';

// TODO(https://github.com/dart-lang/native/issues/1068): Remove this.
import '../../objective_c/test/setup.dart' as objc_setup;

import 'basic_swift_bindings.dart';

const _wrapperDylib = 'basic_swift_wrapper.dylib';

void main(List<String> args) async {
  if (args.isEmpty) {
    print('Usage: dart basic_swift_example.dart message');
    return;
  }

  objc_setup.main([]);
  DynamicLibrary.open(Platform.script.resolve(_wrapperDylib).toFilePath());

  final message = args.join(' ').toNSString();

  final dog = DogWrapper();
  dog.name = 'Fido'.toNSString();
  print('Dog name: ${dog.name.toDartString()}');

  final owner = PersonWrapper();
  // owner.name = 'Alice'.toNSString();

  GlobalsWrapper.greetWrapperWithPerson(owner);
}

Source basic_swift.swift

import Foundation

enum CompassPoint: Int {
    case north = 0
    case south = 1
    case east = 2
    case west = 3
}

// struct
public struct Person {
    public var name: String
    public var age: Int
}

// class
public class Dog  {
    public var name: String
    public var breed: String
    
    public init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

// function
public func greet(person: Person) -> String {
    return "Hello, \(person.name)!"
}

Generator:

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:io';

import 'package:ffigen/ffigen.dart' as fg;
import 'package:logging/logging.dart';
import 'package:pub_semver/pub_semver.dart';
import 'package:swiftgen/swiftgen.dart';

Future<void> main() async {
  final logger = Logger('swiftgen');
  logger.onRecord.listen((record) {
    stderr.writeln('${record.level.name}: ${record.message}');
  });

  await SwiftGenerator(
    target: Target(
      triple: 'arm64-apple-macosx14.0',
      sdk: Uri.directory(
        '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk',
      ),
    ),
    inputs: [
      // ObjCCompatibleSwiftFileInput(files: [Uri.file('basic_swift.swift')]),
      SwiftFileInput(files: [Uri.file('basic_swift.swift')]),
    ],
    // include: (d) => d.name == 'BasicSwiftWrapper',
    output: Output(
      swiftWrapperFile: SwiftWrapperFile(
        path: Uri.file('basic_swift_wrapper.swift'),
        preamble: '''
// Generated code. Do not modify.
''',
      ),
      module: 'BasicSwiftWrapper',
      dartFile: Uri.file('basic_swift_bindings.dart'),
      objectiveCFile: Uri.file('basic_swift_wrapper.m'),
      preamble: '''
// ignore_for_file: always_specify_types
// ignore_for_file: camel_case_types
// ignore_for_file: non_constant_identifier_names
// ignore_for_file: unnecessary_non_null_assertion
// ignore_for_file: unused_element
// ignore_for_file: unused_field
// coverage:ignore-file
''',
    ),
    ffigen: FfiGeneratorOptions(
      objectiveC: fg.ObjectiveC(
        externalVersions: fg.ExternalVersions(
          ios: fg.Versions(min: Version(12, 0, 0)),
          macos: fg.Versions(min: Version(10, 14, 0)),
        ),

        interfaces: fg.Interfaces(
          include: (decl) {
            if (decl.originalName.startsWith('NS')) {
              return false;
            }
            print(decl.originalName);

            return [
              'Dog',
              'Person',
              'DogBreed',
              'DogOwner',
              'CompassPoint',
              'GlobalsWrapper',
              'PersonWrapper',
              'DogWrapper',
            ].contains(decl.originalName);
          },
        ),
      ),
    ),
  ).generate(logger: logger, tempDirectory: Uri.directory('temp'));

  final result = Process.runSync('swiftc', [
    '-emit-library',
    '-o',
    'basic_swift_wrapper.dylib',
    '-module-name',
    'BasicSwiftWrapper',
    'basic_swift_wrapper.swift',
    '-framework',
    'Foundation',
    'basic_swift.swift',
  ]);
  if (result.exitCode != 0) {
    print('Failed to build the swift wrapper library');
    print(result.stdout);
    print(result.stderr);
  }
}

[liamappelbe]

I managed to get somewhere with following config but still getting an exception when running.

I think we should focus on fixing that exception for now. Now that I can look at the stack trace, it looks a bit different to the other one. It's not a Dart exception that fails to load the symbol, it's some sort of Swift exception I haven't seen before.

Can you send the latest source code, including the temp directory? I want to take a look at the dylib and the swift wrapper code, and see if I can repro the runtime exception.

[orestesgaolin]

Sure, here's the entire codebase including logs and temp folder. I'm using current tip of the main branch with Dart 3.9.2.

basic_swift_example.zip

[liamappelbe]

There 3 different issues here.

BasicSwiftWrapper/basic_swift_wrapper.swift:13: Fatal error: Use of unimplemented initializer 'init()' for class 'BasicSwiftWrapper.DogWrapper'

That specific error makes sense now that I'm looking at the bindings. DogWrapper doesn't have a zero argument constructor. So one issue to be fixed is that ffigen needs to parse this special annotation that's generated in the ObjC header, if possible, and omit the zero arg constructor:

- (nonnull instancetype)init SWIFT_UNAVAILABLE;
+ (nonnull instancetype)new SWIFT_UNAVAILABLE_MSG("-init is unavailable");

Filed #2665 to track this.

So the example shouldn't be calling DogWrapper(), because that ctor doesn't exist. Instead, the example should be calling the ctor that takes two strings, public init(name: String, breed: String). But there's a second issue that prevents that. The corresponding constructor on DogWrapper isn't declared as public: @objc init(name: String, breed: String) {. That's a bug in swift2objc, but it's a trivial fix: #2667

After fixing that, the DogWrapper can be rewritten like this, and works correctly:

final dog = DogWrapper.alloc().initWithName(
  'Fido'.toNSString(),
  breed: 'Shiba'.toNSString(),
);
print('Dog name: ${dog.name.toDartString()}');

Now I get the same runtime error for the PersonWrapper initializer:

Dog name: Fido
BasicSwiftWrapper/basic_swift_wrapper.swift:44: Fatal error: Use of unimplemented initializer 'init()' for class 'BasicSwiftWrapper.PersonWrapper'

Person is a struct, so it has an implicit constructor, so when swift2objc generates a wrapper class for this struct, it should generate a wrapper for the implicit constructor. Filed a separate bug about this: #2668. You can work around this issue by explicitly declaring a constructor for the struct:

public struct Person {
    public var name: String
    public var age: Int

    public init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

Rewriting the PersonWrapper initializer, the whole example now works:

final dog = DogWrapper.alloc().initWithName(
  'Fido'.toNSString(),
  breed: 'Shiba'.toNSString(),
);
print('Dog name: ${dog.name.toDartString()}');

final owner = PersonWrapper.alloc().initWithName(
  'Alice'.toNSString(),
  age: 30,
);

print(GlobalsWrapper.greetWrapperWithPerson(owner).toDartString());

Output:

Dog name: Fido
Hello, Alice!

[orestesgaolin]

This is so good! Thanks for such a clear explanation. Waiting for the fixes to land in repo and will probably provide you with more of similar little examples :)

[liamappelbe]

With the second issue fixed, you should be unblocked. #2665 isn't blocking, and you can work around #2668

[orestesgaolin]

Thanks I can confirm now it works

example.dart

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:ffi';
import 'dart:io';

import 'package:objective_c/objective_c.dart';

// TODO(https://github.com/dart-lang/native/issues/1068): Remove this.
import '../../objective_c/test/setup.dart' as objc_setup;

import 'basic_swift_bindings.g.dart';

const _wrapperDylib = 'basic_swift_wrapper.dylib';

void main(List<String> args) async {
  if (args.isEmpty) {
    print('Usage: dart basic_swift_example.dart message');
    return;
  }

  objc_setup.main([]);
  DynamicLibrary.open(Platform.script.resolve(_wrapperDylib).toFilePath());

  final message = args.join(' ').toNSString();
  print('Message: ${message.toDartString()}');

  final dog = DogWrapper.alloc().initWithName(
    'Fido'.toNSString(),
    breed: 'Shiba'.toNSString(),
  );
  print('Dog name: ${dog.name.toDartString()}');

  final owner = PersonWrapper.alloc().initWithName(
    'Alice'.toNSString(),
    age: 30,
  );
  print('Owner name: ${owner.name.toDartString()}');

  print(GlobalsWrapper.greetWrapperWithPerson(owner).toDartString());
}

generate_code.dart

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:io';

import 'package:ffigen/ffigen.dart' as fg;
import 'package:logging/logging.dart';
import 'package:pub_semver/pub_semver.dart';
import 'package:swiftgen/swiftgen.dart';

Future<void> main() async {
  final logger = Logger('swiftgen');
  logger.onRecord.listen((record) {
    stderr.writeln('${record.level.name}: ${record.message}');
  });

  await SwiftGenerator(
    target: Target(
      triple: 'arm64-apple-macosx14.0',
      sdk: Uri.directory(
        '/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk',
      ),
    ),
    inputs: [
      // ObjCCompatibleSwiftFileInput(files: [Uri.file('basic_swift.swift')]),
      SwiftFileInput(files: [Uri.file('basic_swift.swift')]),
    ],
    // include: (d) => d.name == 'BasicSwiftWrapper',
    output: Output(
      swiftWrapperFile: SwiftWrapperFile(
        path: Uri.file('basic_swift_wrapper.swift'),
        preamble: '''
// Generated code. Do not modify.
''',
      ),
      module: 'BasicSwiftWrapper',
      dartFile: Uri.file('basic_swift_bindings.g.dart'),
      objectiveCFile: Uri.file('basic_swift_wrapper.m'),
      preamble: '''
// ignore_for_file: always_specify_types
// ignore_for_file: camel_case_types
// ignore_for_file: non_constant_identifier_names
// ignore_for_file: unnecessary_non_null_assertion
// ignore_for_file: unused_element
// ignore_for_file: unused_field
// coverage:ignore-file
''',
    ),
    ffigen: FfiGeneratorOptions(
      objectiveC: fg.ObjectiveC(
        externalVersions: fg.ExternalVersions(
          ios: fg.Versions(min: Version(12, 0, 0)),
          macos: fg.Versions(min: Version(10, 14, 0)),
        ),

        interfaces: fg.Interfaces(
          include: (decl) {
            if (decl.originalName.startsWith('NS')) {
              return false;
            }
            print(decl.originalName);

            return [
              'Dog',
              'Person',
              'DogBreed',
              'DogOwner',
              'CompassPoint',
              'GlobalsWrapper',
              'PersonWrapper',
              'DogWrapper',
            ].contains(decl.originalName);
          },
        ),
      ),
    ),
  ).generate(logger: logger, tempDirectory: Uri.directory('temp'));

  final result = Process.runSync('swiftc', [
    '-emit-library',
    '-o',
    'basic_swift_wrapper.dylib',
    '-module-name',
    'BasicSwiftWrapper',
    'basic_swift_wrapper.swift',
    '-framework',
    'Foundation',
    'basic_swift.swift',
  ]);
  if (result.exitCode != 0) {
    print('Failed to build the swift wrapper library');
    print(result.stdout);
    print(result.stderr);
  }
}

basic_swift.swift

import Foundation

enum CompassPoint: Int {
    case north = 0
    case south = 1
    case east = 2
    case west = 3
}

// struct
public struct Person {
    public var name: String
    public var age: Int

    public init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

// class
public class Dog  {
    public var name: String
    public var breed: String
    
    public init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

// function
public func greet(person: Person) -> String {
    return "Hello, \(person.name)!"
}