Proper way to use selectors in Swift

1. Well, it is called evolution

2. When there are some arguments in the method, you should declare the selector as:

   let selector = #selector(YourClass.selector(_:))
   

   You can type only #selector(selector(_:)) if the selector is in the same class of the caller. _: means that accept one parameter. So, if it accept more parameters, you should do something like: (_:, _:) and so on.

3. I found out that the @objc is needed only when the function is declared as private or the object doesn't inherit from NSObject

1. why can't I no longer pass simply the function name String to the action?

Using strings for selectors has been deprecated, and you should now write #selector(methodName)instead of "methodName". If the methodName() method doesn't exist, you'll get a compile error – another whole class of bugs eliminated at compile time. This was not possible with strings.

2. how is the proper way to implement this following the Swift Way? Using the Selector class?

You did it the right way:

button.addTarget(self, action: #selector(ClassName.methodName(_:)), forControlEvents: UIControlEvents.TouchUpInside)

3. why do we need to pass the @objc keyword and how it affects the function?

In Swift the normal approach is to bind method's calls and method's bodies at compile time (like C and C++ do). Objective C does it at run time. So in Objective C you can do some things that are not possible in Swift - for example it is possible to exchange method's implementation at run time (it is called method swizzling). Cocoa was designed to work with Objective C approach and this is why you have to inform the compiler that your Swift method should be compiled in Objective-C-like style. If your class inherits NSObject it will be compiled ObjC-like style even without @objc keyword.

1: Currently you can, but it will create a deprecated warning. In Swift 3 this will be an error, so you should fix it soon. This is done because just using a String can not be checked by the compiler if the function really exists and if it is a valid Objective C function which can be resolved dynamically during runtime.

2: Do it in this way:

button.addTarget(self, action: #selector(MyViewControllerClass.buttonPressed(_:)), forControlEvents: UIControlEvents.TouchUpInside)

3: Usually you not have to use the @objc attribute. I assume your class ViewController is (for any reason) not derived from UIViewController. If it derives from UIViewController is inherits also the needed ObjC behavior for calling selectors on functions.