Objective C - NSInvocation passing self as sender?

I am trying to use NSInvocation to call a method on an object and send the sender as an ar开发者_如何转开发gument. The code below calls the mthod but it seems like the object is passing to the mthod is not really self object

- (void)setTarget:(id)taret withAction:(SEL)selector
{
    NSMethodSignature *methodSignature = [target methodSignatureForSelector:action];
    _invocation = [[NSInvocation invocationWithMethodSignature:methodSignature] retain];

    _invocation.target = target;
    _invocation.selector = action;
    [_invocation setArgument:self atIndex:2];
}

- (void)callTargetWithSender
{
   [_invocation invoke];
}

See "Using NSInvocation" in Distributed Objects Programming Guide.

EDIT BASED ON NEW QUESTION *

I assume the above will be called this way:

[foo setTarget:target withAction:@selector(doSomething:)];

In that case, the final message will be:

[target doSomething:foo];

[invocation setArgument:(__bridge void *)(self) atIndex:2];

Why don't you just use

   [target performSelector:selector withObject:self];

??? When selector is @selector(foo:), this is equivalent to

   [target foo:self];

NSInvocation in your situation is an overkill, in my opinion.