iPhone App Development: The Missing Manual

Selector Projector

Because Objective-C is a dynamic language, it resolves methods at runtime, not at compile time. This setup provides a subtle but powerful mechanism: You can pass methods as parameters to other methods.

Selectors are special variables that identify which method implementation to use for an object instance. The Objective-C runtime uses internal data structures to match a message's selector to the code that gets executed.

Tip

If you've used other languages, you might want to think of a selector as a dynamic function pointer. The selector's name automatically points to the right function and adapts to whichever class it's used with.

This behavior lets you implement polymorphism in Objective-C. If you have three classes named Circle, Triangle, and Square, you can send a selector for a draw method to instances of each of these classes, resulting in different implementations for drawing the shape being executed.

Every selector is defined with a type of SEL. You can assign the selector variable with one of two methods. At compile time, you can use a compiler directive to create the value:

SEL mySelector = @selector(moreAwesomeThanEver);

At runtime, you can use a function that takes a single NSString parameter and looks up the selector:

SEL mySelector = NSSelectorFromString("moreAwesomeThanEver");

The mySelector variable now contains a shortcut for the –moreAwesomeThanEver message. And here's where it gets interesting: You can use that variable to execute the method. So instead ...