responsesToSelector和class_getInstanceMethod

Question

Internally, does respondsToSelector do more than finding out if class_getInstanceMethod return non-nil?

Is respondsToSelector essentially a wrapper of class_getInstanceMethod? Like so:

- (BOOL)respondsToSelector:(SEL)sel {
  return class_getInstanceMethod(self, sel) != nil;
}

Answer 1

In Apple's open-source NSObject implementation , respondsToSelector: is on line 1472 and looks like this:

- (BOOL)respondsToSelector:(SEL)sel { 
    if (!sel) return NO; 
    return class_respondsToSelector([self class], sel); 
}

class_respondsToSelector() in turn is on line 729 of objc-class.mm :

BOOL class_respondsToSelector(Class cls, SEL sel)
{
    IMP imp;

    if (!sel  ||  !cls) return NO;

    // Avoids +initialize because it historically did so.
    // We're not returning a callable IMP anyway.
    imp = lookUpMethod(cls, sel, NO/*initialize*/, YES/*cache*/, nil);
    return (imp != (IMP)_objc_msgForward_internal) ? YES : NO;
}

Answer 2

-respondsToSelector: returns a BOOL indicating, essentially, whether class_getInstanceMethod() would return a value. Does that answer your question?

-respondsToSelector: paired with one of the variants of -performSelector:* is more idiomatic in conventional Objective-C code; it's rare to see (or to need) direct calls to the runtime.

Edit : the actual implementation is likely based on calling class_respondsToSelector() , but I expect that the implementation of that runtime method is not fundamentally different than your code snippet.