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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
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
SEL aSelector = @selector(methodName);
At runtime, you use the
NSSelectorFromStringfunction, 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);
(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.)