Question

I’m writing a wrapper around a C library in Objective-C. The library allows me to register callback functions when certain events occur.

The register_callback_handler() function takes a function pointer as one of the parameters.

My question to you gurus of programming is this: How can I represent an Objective-C method call / selector as a function pointer?

• Would NSInvocation be something useful in this situation or too high level?
• Would I be better off just writing a C function that has the method call written inside it, and then pass the pointer to that function?

Any help would be great, thanks.

Answer 1

Does register_callback_handler() also take a (void*) context argument? Most callback APIs do.

If it does, then you could use NSInvocation quite easily. Or you could allocate a little struct that contains a reference to the object and selector and then cobble up your own call.

If it only takes a function pointer, then you are potentially hosed. You need something somewhere that uniquely identifies the context, even for pure C coding.

Given that your callback handler does have a context pointer, you are all set:

typedef struct {
    id target;
    SEL selector;
    // you could put more stuff here if you wanted
    id someContextualSensitiveThing;
} TrampolineData;

void trampoline(void *freedata) {
    TrampolineData *trampData = freedata;
    [trampData->target performSelector: trampData->selector withObject: trampData-> someContextualSensitiveThing];
}

...
TrampolineData *td = malloc(sizeof(TrampolineData));
... fill in the struct here ...
register_callback_handler(..., trampoline, td);

That is the general idea, anyway. If you need to deal with non-object typed arguments and/or callbacks, it gets a little bit trickier, but not that much. The easiest way is to call objc_msgSend() directly after typecasting it to a function pointer of the right type so the compiler generates the right call site (keeping in mind that you might need to use objc_msgSend_stret() for structure return types).