Grand Central Dispatch, or GCD for short, is a low level API which introduces a new way to perform concurrent programming. For basic functionality it's a bit like NSOperationQueue, in that it allows a program's work to be divided up into individual tasks which are then submitted to work queues to run concurrently or serially. It's lower level and higher performance than NSOperationQueue, and is not part of the Cocoa frameworks.
(tags: 10.6 cocoa osx objc multithreading)
A wrapper for 10.6 associated object API
(tags: cocoa 10.6 objc categories)
Sliding doors Since we want our buttons to be über-flexible, we'll have to make the background image expand with the size of the button's text. For that, we'll use the beloved sliding doors technique; two complementing images creating the illusion of a single, stretching image.