【Objective-C】selector in Objective-C，

【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:

view sourceprint? 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:

view sourceprint? 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:

view sourceprint? 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

view sourceprint? 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