Support generating bindings for Objective-C

[john-h-k]

Clang supports Objective-C and having the ability to generate code for it would be useful for many Apple-centric interop scenarios.

Work items

• [ ] Support @protocols [current]
• [ ] Support standalone methods
• [ ] Support @interfaces and @implementations

Support @protocols

Protocols are analogous to abstract classes/interfaces in C++/C#. They are a series of required methods and properties (getters and/or setters as in C#), and then optional methods and properties. Implementing types (called @interfaces) must implement required attributes, but not optional attributes, and a special method (respondsToSelector) is used to determine whether they are implemented. Due to this, publicly exposing all selectors is quite important.

Protocol methods are called via the magic-method objc_msgSend(*), which you cast to the prototype of the method you wish to call, but prepend 2 implicit parameters (self of type void*, and _cmd of type SEL). self is the this pointer for instance methods, or the class pointer (retrieved via object_getClass("className")) for class (static) methods. _cmd is the "selector" for the method, which is basically an opaque pointer handle that represents a method name (Obj-C does not support method overloading, but as parameter names are considered part of the method name, 2 methods with different parameter names are differently named, and this is reflected by the selector) and is used to identify the method you wish to call. Selectors are retrieved by calling sel_registerName("selector"), where the selector has the format methodName (for no params), or methodName:param1:param2:param3 (you can remove param names but it can lead to ambiguities).

* there are actually a couple of different forms of objc_msgSend for different types of return value, but the principle is the same

Roughly how it would work for C# is:

@protocol MyObjcType <NSObject>

@required
- (void)hello;
- (int)add_two:(int)left, right:(int)right;

@end

struct MyObjType
{
    public static class Selectors
    {
        private static readonly SEL sel_hello = sel_registerName("hello");
        private static readonly SEL sel_add_two_left_right = sel_registerName("add_two:right:");
    }

    public void hello() => ((delegate* unmanaged <void*, SEL, void>)(delegate* unmanaged <void>)&objc_msgSend)(Unsafe.AsPointer(ref this), Selectors.sel_hello);
    public int add_two(int left, int right) => ((delegate* unmanaged <void*, SEL, int, int, int>)(delegate* unmanaged <void>)&objc_msgSend)(Unsafe.AsPointer(ref this), Selectors.sel_add_two_right, value);
}

Changes to clangsharp

Requires adding

CXString clangsharp_cursor_getSelector(/* expected: ObjCMethodDecl */ CXCursor C);
CXString clangsharp_cursor_getGetterName(/* expected: ObjCPropertyDecl */ CXCursor C);
CXString clangsharp_cursor_getSetterName(/* expected: ObjCPropertyDecl */ CXCursor C);

[IsaacMarovitz]

It's been several years since this issue was created, is there a possibility of this getting picked up? The lack of Objective-C support is blocking other projects like Silk.NET from binding to Apple APIs.

[IsaacMarovitz]

~~Perhaps a job for @Developer-Ecosystem-Engineering~~ Ig not lol