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    <p>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:</p> <pre><code>-(void)methodWithNoParameters; SEL noParameterSelector = @selector(methodWithNoParameters); -(void)methodWithOneParameter:(id)parameter; SEL oneParameterSelector = @selector(methodWithOneParameter:); // notice the colon here -(void)methodWIthTwoParameters:(id)parameterOne and:(id)parameterTwo; SEL twoParameterSelector = @selector(methodWithTwoParameters:and:); // notice the parameter names are omitted </code></pre> <p>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:</p> <pre><code>-(void)someMethod:(NSTimer*)timer; </code></pre> <p>So when you schedule the timer you would use @selector to specify which method on your object will actually be responsible for the callback:</p> <pre><code>@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; } </code></pre> <p>In this case you are specifying that the object obj be messaged with myTimerCallback every 30 seconds. </p>
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    1. COcoco -- can you provide the code that NSTimer is executing to call the selector that it has received?
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