What does the performSelector method do?

Question

What does performSelector do? What is the difference between creating a new NSThread and the performSelector method?

How it works and where should we use it?

Spragens

Answer 1

All of these perform the same task, that is make the doStuff method on anObject execute synchronously on the current thread:

// 1
[anObject doStuff];

// 2
[anObject performSelector:@selector(doStuff)];

// 3
objc_msgSend(anObject, @selector(doStuff));

// 4
IMP imp = [anObject methodForSelector:@selector(doStuff)];
imp(anObject, @selector(doStuff));

1. Is how you normally should go about to do stuff.
2. Is for dynamically dispatching a message. Use if the selector is unknown, or provided by a client, for example if you implement an target-action pattern. or if the class of anObject is unknown, usually used by first asking if the object has the method with -[NSObject respondsToSelector:].
3. Is what no 1. is actually compiled down to. Usually never any real need to do this.
4. Cached the actual IMP (implementation) for a method, and then call it directly. Can sometimes be faster than 1. if used in a tight loop. Just remember; premature optimization is evil.

What you need to grasp is that in Objective-C methods are more important than classes/interfaces. Usually you do not query an object if it belongs to a particular class, or conforms to any protocol, that is for the compiler to complain about. At run-time you instead query for specific methods.

In short: It does not matter what you are, just what you can do.

As a convenience NSObject also have several siblings to performSelector that are asynchronios. Most notably:

• performSelector:withObject:afterDelay: - To execute the method on the current thread after a delay.
• performSelectorInBackground:withObject: - To execute the method on a new background thread.
• performSelectorOnMainThread:withObject:waitUntilDone: - To execute the method on the main thread.
• performSelector:onThread:withObject:waitUntilDone: - To execute the method on any thread.

The asynchronous performers all depend on a NSRunLoop to function. This is not something you need to worry about unless you spawn a thread yourself. If you do then you need to also run the new threads run loop. Just skip that for now.

Issiah

You can use the direct method call when you don't know the class of an object -- id foo; if ([foo respondsToSelector:@selector(bar)]) [foo bar]; When you do need performSelector: is if you don't know the selector to call at compile-time (e.g. it's sent in from a different object, or it's user-selectable, etc). – Piranesi

(3) is sometimes useful because (2) only works with methods whose arguments are all objects and whose return type is an object or void, and which only has 0-2 arguments. For anything else you can use (3) – Wyon

That comment about methods are more important than classes/interfaces is insightful, and explains a key difference between Objective-C and other OOP environments. Thanks. – Ditheism

Answer 2

performSelector executes a selector. In other words, it calls a method.

It is very different from running a new thread.

I think it would be best for you to read up on selectors.

Diadiabase