Explanation of Cocoa @selector usage

Question

I\'m new to Cocoa/Cocoa Touch, and working through a development book. I\'ve come across situations where the @selector() operator is used. I\'m a bit lost on how and when the

Answer 1

In addition to what's been said, you can also wrap up the @selector in an NSInvocation for later use. You can set the arguments to the NSInvocation a long time after it's created, and activate it when you need the message to be fired. This gives you a lot of power.

For an introduction to the concept, Scott Stevenson has a great post entitled "Dynamic Objective-C with NSInvocation".

Answer 2

The selector operator provides a way to refer to a method provided by an object, somewhat similar to a function pointer in C. It is useful because it allows you to decouple the process of calling methods on an object. For example one piece of code could provide a method, and another piece of code could apply that method to a given set of objects.

Examples:

Test to see if an object implements a certain method:

[object respondsToSelector:@selector(methodName)]

Store a method to later call on an object;

SEL method = @selector(methodName);
[object performSelector:method];

Call a method on a different thread (useful for GUI work).

[object performSelectorOnMainThread:@selector(methodName)]

Answer 3

@selector() is used each time you need to pass the name of a method as an argument to another method, a function or as a variable value. Passing directly the name doesn't work in objective-C.

Answer 4

One reference to look at:

http://en.wikipedia.org/wiki/Multiple_dispatch

Answer 5

You can use a selector to invoke a method on an object—this provides the basis for the implementation of the target-action design pattern in Cocoa.

[myObject performSelector:@selector(runMYmethod:) withObject:parameters];

is equivalent to:

[myObject runMYmethod:parameters];

Answer 6

One practical example is validateMenuItem method where menu items are identified with their target actions.

Simplified example:

- (BOOL)validateMenuItem:(NSMenuItem *)item {
    if ([item action] == @selector(selectFiles:) && otherCondition) {
        return YES;
    } else {
        return NO;
    }
}