A selector is the name used to select a method to execute for an object, or the unique identifier that replaces the name when the source code is compiled. A selector by itself doesn’t do anything. It simply identifies a method. The only thing that makes the selector method name different from a plain string is that the compiler makes sure that selectors are unique. What makes a selector useful is that (in conjunction with the runtime) it acts like a dynamic function pointer that, for a given name, automatically points to the implementation of a method appropriate for whichever class it’s used with. Suppose you had a selector for the method run, and classes Dog, Athlete, and ComputerSimulation (each of which implemented a method run). The selector could be used with an instance of each of the classes to invoke its run method—even though the implementation might be different for each.

Getting a Selector

Compiled selectors are of type SEL. There are two common ways to get a selector:

  • At compile time, you use the compiler directive @selector.

    SEL aSelector = @selector(methodName);
  • At runtime, you use the NSSelectorFromString function, where the string is the name of the method:

    SEL aSelector = NSSelectorFromString(@"methodName");

    You use a selector created from a string when you want your code to send a message whose name you may not know until runtime.

Using a Selector

You can invoke a method using a selector with performSelector: and other similar methods.

SEL aSelector = @selector(run);
[aDog performSelector:aSelector];
[anAthlete performSelector:aSelector];
[aComputerSimulation performSelector:aSelector];

(You use this technique in special situations, such as when you implement an object that uses the target-action design pattern. Normally, you simply invoke the method directly.)

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