Objective C SEl 和@selector是怎么工作的||How do SEL and @selector work in iphone sdk?

SEL is a type that represents a selector in Objective-C. The @selector() keyword returns a SEL that you describe. It's not a function pointer and you can't pass it any objects or references of any kind. For each variable in the selector (method), you have to represent that in the call to @selector. For example:

-(void)methodWithNoArguments;
SEL noArgumentSelector =@selector(methodWithNoArguments);
-(void)methodWithOneArgument:(id)argument;
SEL oneArgumentSelector =@selector(methodWithOneArgument:);// notice the colon here注意引号
-(void)methodWIthTwoArguments:(id)argumentOne and:(id)argumentTwo;
SEL twoArgumentSelector =@selector(methodWithTwoArguments:and:);// notice the argument names are omitted

Selectors are generally passed to delegate methods and to callbacks to specify which method should be called on a specific object during a callback. 

一个计时器的例子：@implementation MyObject

-(void)myTimerCallback:(NSTimer*)timer
{
    // do some computations
    if( timerShouldEnd ) {
      [timer invalidate];
    }
}

@end

// ...

int main(int argc, const char **argv)
{
    // do setup stuff
    MyObject* obj = [[MyObject alloc] init];
    SEL mySelector = @selector(myTimerCallback:);

    [NSTimer scheduledTimerWithTimeInterval:30.0 target:obj selector:mySelector userInfo:nil repeats:YES];
    // do some tear-down
    return 0;
}

+ (NSTimer *)scheduledTimerWithTimeInterval:(NSTimeInterval)seconds target:(id)target selector:(SEL)aSelector userInfo:(id)userInfo repeats:(BOOL)repeat
这个是NSTimer里的方法；
这句代码的意思就是每隔30s会调用selector 代表的方法（回调方法）myTimerCallback；

你可以理解 @selector()就是取类方法的编号,他的行为基本可以等同C语言的中函数指针，但不是指针,只不过C语言中，可以把函数名直接赋给一个函数指针，而Objective-C的类不能直接应用函数指针，这样只能做一个@selector语法来取.