iOS NSInvocation

苹果文档：

An 

NSInvocation

 is
an Objective-C message rendered static, that is, it is an action turned into an object. 

NSInvocation

 objects
are used to store and forward messages between objects and between applications, primarily by 

NSTimer

 objects
and the distributed objects system. 

An 

NSInvocation

 object
contains all the elements of an Objective-C message: a target, a selector, arguments, and the return value. Each of these elements can be set directly, and the return value is set automatically when the 

NSInvocation

 object
is dispatched.

An 

NSInvocation

 object
can be repeatedly dispatched to different targets; its arguments can be modified between dispatch for varying results; even its selector can be changed to another with the same method signature (argument and return types). This flexibility makes 

NSInvocation

 useful
for repeating messages with many arguments and variations; rather than retyping a slightly different expression for each message, you modify the 

NSInvocation

 object
as needed each time before dispatching it to a new target.

NSInvocation

 does
not support invocations of methods with either variable numbers of arguments or 

union

arguments.
You should use the 

invocationWithMethodSignature:

 class
method to create 

NSInvocation

objects;
you should not create these objects using 

alloc

 and 

init

.

This class does not retain the arguments for the contained invocation by default. If those objects might disappear between the time you create your instance of 

NSInvocation

 and
the time you use it, you should explicitly retain the objects yourself or invoke the 

retainArguments

 method
to have the invocation object retain them itself.

Getting the Method Signature

methodSignature

 Property
 

   Creating NSInvocation Objects

   
 + invocationWithMethodSignature:

   Configuring an Invocation Object

 selector

 Property

 target

 Property

 
- setArgument:atIndex:

  
- getArgument:atIndex:

argumentsRetained

 Property

  
- retainArguments 它会将传入的所有参数以及target都retain一遍。

 
- setReturnValue:

   
- getReturnValue:

Dispatching an Invocation

  
- invoke

  
- invokeWithTarget:

简单例子：