What's the difference between NSInvocation and block?

Question

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?

Answer 1

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 (eg when you copy a block), blocks can require quite a bit more CPU time and memory - when compared to NSInvocation .

Answer 2

NSInvocation is an object that encapsulates a message call: the target object, the selector, the arguments and the return value. A block is an object that encapsulates a section of code and some information about the state of the program leading up to that section: specifically it records the variables on the call stack up to the creation of the block.

Both of these can clearly be used as callbacks: you can use an invocation to send a message to an object, or you can execute a block's code just like a function. What's different about them is the way you'd transport state in each case. With an invocation, you either need the target object or one of the parameters to represent the context in which the message is appearing. With a block, this context is captured automatically from the state when the block was created.

Answer 3

To put it very simply, NSInvocation is less powerful than blocks. It just encapsulates a single method call on a single object, whereas blocks can wrap many lines of arbitrary code. Even your very simple squaring block is impossible to represent using an invocation without support from an existing class that would do the squaring itself.