An Experiment in Bloggery

Method Swizzling is one common technique of people writing hacks, such as Safari Plugins. Unfortunately, it’s always suffered from a flaw, wherein swizzling inherited methods affects all classes which inherit that method (including the base class), rather than the intended subclass. This problem is discussed on the CocoaDev Method Swizzling page.

As part of writing YubNubSearch, I decided to solve this problem.

First I looked into dynamic subclass generation + posing. Unfortunately, this has a big problem. In this technique, calling the original implementation would naturally be done through a [super foo] call. Unfortunately, when the compiler sees super, it hardcodes a reference to the superclass at which to start the search. This means you cannot write this code in, say, a category on NSObject, then pull up the IMP into a dynamically-generated subclass and have it work. So that throws out that idea.

The other idea I had, which I eventually went with, was to copy inherited methods into the subclass that you wish to swizzle, before swizzling. It turned out to be fairly easy, and still has the same semantics as the old, flawed technique for calling the original implementation.

You can download my implementation here.

This entry was posted on Saturday, December 30th, 2006 at 8:36 pm and is filed under Cocoa, Programming. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.