interop_with_objc.md

Dart and Objective-C interoperability

Imagine you have the following Objective-C code:

#import <Foundation/Foundation.h>

@interface Animal : NSObject {
    NSString* name;
}

- (void)makeSound;

- (Animal*)initWithName:(NSString*) name;

+ (void)petAnAnimal:(Animal*) animal;

@end

@implementation Animal

- (void)makeSound {
}


+ (void)petAnAnimal:(Animal*) animal {
    [animal makeSound];
}


- (Animal*)initWithName:(NSString*) n {
    name = n;
    return self;
}

@end

@interface Dog : Animal {
    NSString* breed;
}

- (Dog*)initWithName:(NSString*) n andBreed:(NSString*) b;
@end

@implementation Dog

- (Dog*)initWithName:(NSString*) n andBreed:(NSString*) b {
    self = [self initWithName: n];
    breed = b;
    return self;
}


- (void)makeSound {
    NSLog(@"The Dog %@ (%@) has barked\n", name, breed);
}

@end

Can you interoperate with this from Dart? Yep, you easily can. You take ffigen and you generate bindings for it, which allow you to write code like this:

 final dog = Dog.alloc()
   .initWithName_andBreed_(
     objc.NSString('Boxer'),
     objc.NSString('pug'),
   );

 dog.makeSound();
 Animal.petAnAnimal_(dog);

This prints

2024-05-21 14:09:23.663 hello_world[26434:23013753] The Dog Boxer (pug) has barked
2024-05-21 14:09:23.663 hello_world[26434:23013753] The Dog Boxer (pug) has barked

Now the question is: can you extend Animal and make things still work?

Yes, you can - though right now you would have to do it manually. Consider for example that you want to write the following:

class Squirrel extends Animal {
  final String color;
  
  Squirrel({required String color, required String name});
  
  void makeSound() {
    print('Squirrel $name of color $color is making some squirrel sounds!');
  }
}

How would you go about it? Well, you have to do something like this:

class Squirrel extends Animal {

  static ffi.Pointer<objc.ObjCObject> _registerClass() {
    final cls = objc.allocateClassPair(class_Animal, "Squirrel",
        extraBytes: ffi.sizeOf<ffi.Pointer>());
    objc.addInstanceVariable(
      cls,
      "color",
      ffi.sizeOf<ffi.Pointer>(),
      ffi.sizeOf<ffi.Pointer>() == 4 ? 2 : 3,
      "^v",
    );
    _nameVar = objc.class_getInstanceVariable(cls, "name");
    _colorVar = objc.class_getInstanceVariable(cls, "color");

    final makeSoundMethod =
        objc.class_getInstanceMethod(class_Animal, sel_makeSound);
    final types = objc.method_getTypeEncoding(makeSoundMethod);
    objc.class_addMethod(
        cls,
        sel_makeSound,
        ffi.NativeCallable<
                ffi.Void Function(
                  ffi.Pointer<objc.ObjCObject>,
                  ffi.Pointer<objc.ObjCSelector>,
                )>.isolateLocal(_makeSoundTrampoline)
            .nativeFunction
            .cast(),
        types);
    return cls;
  }

  // Objective-C will invoke this method which will then invoke
  // [makeSound] below.
  static void _makeSoundTrampoline(
    ffi.Pointer<objc.ObjCObject> self,
    ffi.Pointer<objc.ObjCSelector> selector,
  ) {
    Squirrel._(self).makeSound();
  }

  void makeSound() {
    print(
        'Squirrel $name of color $color is making some squirrel sounds!');
  }

  factory Squirrel({required String name, required String color}) {
    final _ret = objc_msgSend(_class_Squirrel, sel_alloc);    
    final result = Squirrel._(_ret, retain: false, release: true);
    result.initWithName_(objc.NSString(name));
    result._color = color;
    return result;
  }

  ffi.Pointer<ffi.Pointer<ffi.Void>> get _colorSlot =>
      ffi.Pointer<ffi.Pointer<ffi.Void>>.fromAddress(
          pointer.address + _dartDataOffset);

  String get _color => objc.unwrapPersistentHandle(_colorSlot.value);
  set _color(_SquirrelData data) {
    _colorSlot.value = objc.createPersistentHandle(data);
  }

  objc.NSString get name =>
      objc.NSString.castFromPointer(objc.object_getIvar(pointer, _nameVar));
  String get color => _color;
}

You can then use this code like so:

  final squirrel = Squirrel(name: 'Xyz', color: 'red');
  print('Asking animal to make sound (from Dart)');
  squirrel.makeSound();

  print('Indirectly asking animal to make sound (via Objective-C)');
  Animal.petAnAnimal_(squirrel);

And this will work as expected and print:

flutter: Asking animal to make sound (from Dart)
flutter: Squirrel Xyz of color red is making some squirrel sounds!
flutter: Indirectly asking animal to make sound (via Objective-C)
flutter: Squirrel Xyz of color red is making some squirrel sounds!

Is it too manual? Yep. But most of this is boilerplate that can be just generated e.g. via macros.