Question

I have a code sample that gets a SEL from the current object,

SEL callback = @selector(mymethod:parameter2); 

And I have a method like

 -(void)mymethod:(id)v1 parameter2;(NSString*)v2 { } 

Now I need to move mymethod to another object, say myDelegate.

I have tried:

SEL callback = @selector(myDelegate, mymethod:parameter2); 

but it won’t compile.

Answer 1

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)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 

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:

-(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:

@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; } 

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