Question

I’m new to Cocoa/Cocoa Touch, and working through a development book. I’ve come across situations where the @selector() operator is used. I’m a bit lost on how and when the @selector() operator should be used. Can someone provide a short and sweet explanation and example of why it’s used and what benefit it gives the developer?

By the way, here is sample code taken from Apple’s iPhone development site that uses @selector()

if ([elementName isEqualToString:@'entry']) {      parsedEarthquakesCounter++;      // An entry in the RSS feed represents an earthquake, so create an instance of it.     self.currentEarthquakeObject = [[Earthquake alloc] init];     // Add the new Earthquake object to the application's array of earthquakes.     [(id)[[UIApplication sharedApplication] delegate]             performSelectorOnMainThread:@selector(addToEarthquakeList:)             withObject:self.currentEarthquakeObject waitUntilDone:YES];     return; } 

Answer 1

The selector operator provides a way to refer to a method provided by an object, somewhat similar to a function pointer in C. It is useful because it allows you to decouple the process of calling methods on an object. For example one piece of code could provide a method, and another piece of code could apply that method to a given set of objects.

Examples:

Test to see if an object implements a certain method:

[object respondsToSelector:@selector(methodName)] 

Store a method to later call on an object;

SEL method = @selector(methodName); [object performSelector:method]; 

Call a method on a different thread (useful for GUI work).

[object performSelectorOnMainThread:@selector(methodName)]