Mark Dalrymple continues his long running, deep dive series into Objective-C:
Behold, the Objective-C Runtime API. Here are the tools to let you peer into the metadata that the Objective-C compiler keeps around. Here are the tools that let you manipulate the underlying data structures. Here be the gateway to information and control. Here also be some sharp corners.
This is a highly recommended series if you are interested to know more about how things work under the covers of the Objective-C.