Swift and Objective-C
Although you will write your classes in Swift, the classes in the Cocoa frameworks are written in Objective-C. Swift was designed to work seamlessly with Objective-C classes. While you can write Cocoa apps in pure Swift, without a line of Objective-C, it is important to have a basic understanding of how Objective-C works.
Objective-C methods (which are only available on classes, not structures) are not called like functions or like Swift methods. Instead of calling a method on an object, Objective-C sends the object a message.
A message consists of a receiver, selector, and any parameters. The selector is the name of the method you want executed. The receiver is the object that you want to execute that ...
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