如何在NotificationCenter中将Struct用作观察者

Question

In NotificationCenter Class , Why apple has created Observer of type Any?

func addObserver(Any, selector: Selector, name: NSNotification.Name?, object: Any?)

My Reasoning.

1. If observer is struct then on assigning inside as function parameter, It will be copied then how my observer can receive the notification.
2. I can't write any function which uses @objc prefix in Struct.
3. Selector is always be type of @objc.

So What is the use of Any in addObserver..... It should always be of type AnyObject.

Secondly we already known that NotificationCenter keep the weak copy of observer, And we can't use weak modifier for type Any. Then how apple is managing all this?

Any help in understanding this concept is highly appreciated.

Answer 1

No one chose to make this parameter Any . It's just what they got by default. It's automatically bridged from ObjC:

- (void)addObserver:(id)observer 
        selector:(SEL)aSelector
        name:(nullable NSNotificationName)aName
        object:(nullable id)anObject;

The default way that id is bridged is Any . It hasn't been specially refined for Swift. In practice, you can't really use structs meaningfully here. The fact that the compiler won't stop you from calling it in an unhelpful way doesn't imply that it's intended to be used that way.

Answer 2

Why type Any? - because in Objective C it is type id .

Why you can't mark your function as @obj - @obc is the keyword for Swift code which indicates what compiler should add this method to a header file for this Class , yes you can make headers only for Classes .

Selector also is the objective-c term, it just says which function to invoke, similar to msg_send

Answer 3

In NotificationCenter Class , Why apple has created Observer of type Any.

Because all Objective-C id declarations are translated into Swift as Any.

You might object that this really should be AnyObject, because only a class will work here. And indeed, that's the way id used to be translated into Swift. But nowadays you can pass anything where an id is expected, because if it's something Objective-C can't understand, it will be boxed up as a class instance (eg as a _SwiftValue ) so that it can make the round-trip into Objective-C and back again to Swift. Therefore id is translated as Any.

However, just because you can pass a struct here doesn't mean you should. It won't work, as you've discovered. Objective-C cannot introspect a Swift struct.

There are lots of situations like this, where Cocoa gives you enough room to hang yourself by passing the wrong thing. The contents of a CALayer is typed as Any, but if you pass anything other than a CGImage, nothing will happen. The layerClass if a UIView is typed as AnyClass, but you'd better pass a CALayer subclass. I could go on and on.