How to call original implementation when overwriting a method with a category?

Question

I try to figure out how things really work. So I thought when I would overwrite certain methods using categories, I would get interesting NSLogs.

@implementation UIView(Learning)
- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event {
    NSLog(@"-hitTest:withEvent: event=%@", event);
    return [self hitTest:point withEvent:event];
}
@end

super and self don't work here. Is there a way to call the original implementation of -hitTest:withEvent:? What I want is an NSLog every time -hitTest:withEvent: is called on an UIView.

It's just for personal learning purposes. I want to see the event delivery in action.

Answer 1

You can do it, but not using a category. A category replaces a method. (Warning, car analogy) If you have a car, and you destroy that car and replace it with a new car, can you still use the old car? No, because it is gone and does not exist anymore. The same with categories.

What you could do is use the Objective-C runtime to add the method under a different name at runtime (say, "bogusHitTest:withEvent:"), then swap the implementations of hitTest:withEvent: and bogusHitTest:withEvent:. That way when the code calls hitTest:withEvent:, it's going to execute the code that was originally written for bogusHitTest:withEvent:. You can then have that code invoke bogusHitTest:withEvent:, which will execute the original implementation.

So the bogus method would look like:

- (UIView *) bogusHitTest:(CGPoint)point withEvent:(UIEvent *)event {
  NSLog(@"executing: %@", NSStringFromSelector(_cmd));
  return [self bogusHitTest:point withEvent:event];
}

The code to swap the methods would be something along the lines of:

Method bogusHitTest = class_getInstanceMethod([UIView class], @selector(bogusHitTest:withEvent:));
Method hitTest = class_getInstanceMethod([UIView class], @selector(hitTest:withEvent:));
method_exchangeImplementations(bogusHitTest, hitTest);

Answer 2

What you want to do is called method swizzling : http://www.cocoadev.com/index.pl?MethodSwizzling

Answer 3

Unfortunately, no, there is no way to call the original implementation of a method that you override. Once you implement it in the category you have eliminated the original method.

Sending the same message to super should work in your method; it will call the method on the superclass as normal (if there is one).

Sending the same message to self will create an infinite loop, as I'm sure you've discovered.