Question

Objective-C keeps all its methods in a huge hashtable – so shouldn’t it possible to patch this table and replace an existing method with my own patched method (which then calls the original)?

I need a way to hook up the NSWindow KeyUp method in a window which i can’t subclass cause it’s already created.

I need some code or at least some keywords i can use for further searching.

Answer 1

Of course it is possible. In fact, you don’t even need to look into the hash table — there’s standard API for this.

For example:

typedef void (*NSWindow_keyUp__IMP)(NSWindow* self, SEL _cmd, NSEvent* evt);
static NSWindow_keyUp__IMP original_NSWindow_keyUp_;

void replaced_NSWindow_keyUp_(NSWindow* self, SEL _cmd, NSEvent* evt) {
  NSLog(@"Entering keyUp:. self = %@, event = %@", self, evt);
  original_NSWindow_keyUp_(self, _cmd, evt);
  NSLog(@"Leaving keyUp:. self = %@, event = %@", self, evt);
}

...

Method m = class_getInstanceMethod([NSWindow class], @selector(keyUp:));
original_NSWindow_keyUp_ = method_setImplementation(m, replaced_NSWindow_keyUp_);