What is the best practice to use selector in Notification

Question

In Swift 3, to register a notification, I can do the following ways:

NotificationCenter.default.addObserver(self, selector: #selector(ViewController.n1(notification:)), name: Notification.Name("123"), object: nil)
func n1(notification: Notification){
    print("123")
}


// #selector is more brief
NotificationCenter.default.addObserver(self, selector: #selector(n2), name: Notification.Name("456"), object: nil)
func n2(notification: Notification){
    print("456")
}

However, in Xcode 9.0 beta 2 (Swift 4.0), when I register a notification this way, the object method should have a prefix @objc, why? What is the best practice to use Notification?

Argument of '#selector' refers to instance method 'n1(notification:)' that is not exposed to Objective-C

//Add '@objc' to expose this instance method to Objective-C
@objc func n1(notification: Notification){
    print("123")
}

@objc func n2(notification: Notification){
    print("456")
}

Pochi · Accepted Answer · 2017-06-30T02:47:53.737

You are not wrong.

As a matter of fact this is how apple explains you should be using selectors in Swift 4:

In Objective-C, a selector is a type that refers to the name of an Objective-C method. In Swift, Objective-C selectors are represented by the Selector structure, and can be constructed using the #selector expression. To create a selector for a method that can be called from Objective-C, pass the name of the method, such as #selector(MyViewController.tappedButton(sender:)). To construct a selector for a property’s Objective-C getter or setter method, pass the property name prefixed by the getter: or setter: label, such as #selector(getter: MyViewController.myButton).

Documentation link here.

And to answer your question about why, well selectors are actually a way to send messages between cocoa classes, not a swift feature. So they are actually based on Objective-C thus why you need to keep compatibility between them.

Selectors:

A selector is the name used to select a method to execute for an object, or the unique identifier that replaces the name when the source code is compiled. A selector by itself doesn’t do anything. It simply identifies a method. The only thing that makes the selector method name different from a plain string is that the compiler makes sure that selectors are unique. What makes a selector useful is that (in conjunction with the runtime) it acts like a dynamic function pointer that, for a given name, automatically points to the implementation of a method appropriate for whichever class it’s used with.

You can read more about selectors here.

But basically, they are just part of the "messaging" interface used by cocoa.

What is the best practice to use selector in Notification

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