Function Pointers in Objective C

Question

Quick question. Does anyone know how to get the function pointer of an objective c method? I can declare a C++ method as a function pointer, but this is a callback method so that C++ method would need to be part of the class SO THAT IT CAN ACCESS THE INSTANCE FIELDS. I don't know how to make a C++ method part of an objective c class. Any suggestions?

Answer 1

Typically, you need two pieces of information to call back into Objective-C; the method to be invoked and the object to invoke it upon. Neither just a selector or just the IMP -- the instanceMethodForSelector: result -- will be enough information.

Most callback APIs provide a context pointer that is treated as an opaque value that is passed through to the callback. This is the key to your conundrum.

I.e. if you have a callback function that is declared as:

typedef void (*CallBackFuncType)(int something, char *else, void *context);

And some API that consumes a pointer of said callback function type:

void APIThatWillCallBack(int f1, int f2, CallBackFuncType callback, void *context);

Then you would implement your callback something like this:

void MyCallbackDude(int a, char *b, void *context) {
    [((MyCallbackObjectClass*)context) myMethodThatTakesSomething: a else: b];
}

And then you would call the API something akin to this:

MyCallbackObjectClass *callbackContext = [MyCallbackObjectClass new];
APIThatWillCallBack(17, 42, MyCallbackDude, (void*)callbackContext);

If you need to switch between different selectors, I would recommend creating a little glue class that sits between the callback and the Objective-C API. The instance of the glue class could contain the configuration necessary or logic necessary to switch between selectors based on the incoming callback data.