Ari Grant describes in detail the debugging command pinvocation (print invocation) availabe in Facebook’s Chisel library:
“The command is pinvocation (for print invocation) which is most useful for debugging Objective-C methods in Apple’s code (or anything where you don’t have access to the source and thus symbols). pinvocation finds the values for self and _cmd, packages them up into an NSInvocation and then prints it all out for you, including the arguments.”
This posts gets pretty deep into the details. A good read if you want to take your debugging to a new level.