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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-(void)methodWithNoArguments;
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SEL noArgumentSelector = @selector(methodWithNoArguments);
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-(void)methodWithOneArgument:(id)argument;
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SEL oneArgumentSelector = @selector(methodWithOneArgument:); // notice the colon here
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-(void)methodWIthTwoArguments:(id)argumentOne and:(id)argumentTwo;
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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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-(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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@implementation MyObject
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-(void)myTimerCallback:(NSTimer*)timer
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{
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// do some computations
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if( timerShouldEnd ) {
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[timer invalidate];
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}
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}
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@end
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// ...
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int main(int argc, const char **argv)
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{
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// do setup stuff
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MyObject* obj = [[MyObject alloc] init];
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SEL mySelector = @selector(myTimerCallback:);
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[NSTimer scheduleTimerWithTimeInterval:30.0 target:obj selector:mySelector userInfo:nil repeats:YES];
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// do some tear-down
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return 0;
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}
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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-(void) someMethod:(id)handler selector:(SEL)selector {
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// some codes here...
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// execute callback function
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if( handler != nil && selector != nil && [handler respondsToSelector:selector] ) {
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[handler performSelector:selector];
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}
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}
Reference:
http://stackoverflow.com/questions/297680/how-do-sel-and-selector-work-in-iphone-sdk