when i say block i mean:
^(int a) {return a*a;};
besides, block is only support by iOS4 and above.
What is the difference between these two?
An NSInvocation is a message (using a selector) to an object, with optional parameters, which can be executed later (or now), and outside the current context (mind of course what you copy vs retain or reference if you move it). NSInvocation has the benefit that you can selectively copy/refer to exactly what you need.
The block is a secret local function definition, which is able to capture portions of the current thread's context, or altogether. It's also a little easier to configure than an NSInvocation because it automatically captures, copies, and retains the thread (or scope) local context. Blocks can increase your binary size slightly, similar to functions. If taken out of the local context (e.g. when you copy a block), blocks can require quite a bit more CPU time and memory - when compared to NSInvocation.