【Objective-C】selector in Objective-C

selector在Objective-C裡面，通常被拿來當作callback function使用。以下是網路上看來的，先記起來，以免日後要找時忘記。

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:

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1.-(void)methodWithNoArguments;

2.SEL noArgumentSelector = @selector(methodWithNoArguments);

3. 

4.-(void)methodWithOneArgument:(id)argument;

5.SEL oneArgumentSelector = @selector(methodWithOneArgument:); // notice the colon here

6. 

7.-(void)methodWIthTwoArguments:(id)argumentOne and:(id)argumentTwo;

8.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. For instance, when you create a timer, the callback method is specifically defined as:

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1.-(void)someMethod:(NSTimer*)timer;

So when you schedule the timer you would use @selector to specify which method on your object will actually be responsible for the callback:

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01.@implementation MyObject

02. 

03.-(void)myTimerCallback:(NSTimer*)timer

04.{

05.// do some computations

06.if( timerShouldEnd ) {

07.[timer invalidate];

08.}

09.}

10. 

11.@end

12. 

13.// ...

14. 

15.int main(int argc, const char **argv)

16.{

17.// do setup stuff

18.MyObject* obj = [[MyObject alloc] init];

19.SEL mySelector = @selector(myTimerCallback:);

20.[NSTimer scheduleTimerWithTimeInterval:30.0 target:obj selector:mySelector userInfo:nil repeats:YES];

21.// do some tear-down

22.return 0;

23.}

In this case you are specifying that the object obj be messaged with myTimerCallback every 30 seconds.

假如一個function在該做的事完成後要執行selector callback，可以類似下面這樣寫

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1.-(void) someMethod:(id)handler selector:(SEL)selector {

2.// some codes here...

3. 

4.// execute callback function

5.if( handler != nil && selector != nil && [handler respondsToSelector:selector] ) {

6.[handler performSelector:selector];

7.}

8.}

Reference:
http://stackoverflow.com/questions/297680/how-do-sel-and-selector-work-in-iphone-sdk