About @selector

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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 Dog, Athlete, andComputerSimulation (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);      // 没有冒号表示methodName没有参数

注意：在声明selector的时候

SEL aSelector = @selector(methodName:);// 冒号表示methodName带有一个参数

SEL aSelector = @selector(methodName:data:);// 表示methodName带有两个参数

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.)

如何检测一个类中有没有某个selector

-respondsToSelector:+instancesRespondToSelector:

例子：

@interface Parent : - (void)onUpdate;@end@interface Child : Parent @end

则

Child * childObj = [[Child alloc] init];BOOL hasSelector = [childObj respondsToSelector : @selector(onUpdate)];CCLOG(@"child has selector = %d", hasSelector);        BOOL hasInstanceSelector = [Child instancesRespondToSelector : @selector(onUpdate)];CCLOG(@"child has instance selector = %d", hasInstanceSelector);        BOOL hasParentInstanceSelector = [Parent instancesRespondToSelector : @selector(onUpdate)];CCLOG(@"parent has parent instance selector = %d", hasParentInstanceSelector);结果为child has selector = 1child has instance selector = 1parent has parent instance selector = 1


	
					
					   About @selector
	  	   selector
	  	   selector
	  	   @selector
	  	   selector
	  	   @selector
	  	   Selector
	  	   @selector
	  	   @selector
	  	   Selector
	  	   selector
	  	   @selector
	  	   selector
	  	   selector
	  	   Selector
	  	   @selector
	  	   SELECTOR
	  	   selector
	     		  
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