Protocol extension on an ObjC protocol

Question

I have an Objective-C protocol which is used by mostly objective-C objects and one or two Swift objects.

I would like to extend the protocol in Swift and add 2 functions. One to register for a notification and another to handle the notification.

If I add these

func registerForPresetLoadedNotification() {     NSNotificationCenter.defaultCenter().addObserver(self as AnyObject,                                                      selector: #selector(presetLoaded(_:)),                                                      name: kPresetLoadedNotificationName,                                                      object: nil) }  func presetLoaded(notification: NSNotification) {      } 

I get an error on the #selector which says:

Argument of '#selector' refers to a method that is not exposed to Objective-C

If I then mark presetLoaded as @objc I get an error which says:

@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes

I also cannot mark the protocol extension as @objc

When I create the Objective-C protocol as a Swift protocol I get the same error.

Is there a way to achieve this that will work for Objective-C and Swift classes that use the protocol?

Answer 1

Indeed, you can't really mark a function of a protocol extension as @objc (or dynamic, which is equivalent by the way). Only methods of a class are allowed to be dispatched by Objective-C runtime.

In your particular case, if you really want to make it through protocol extension, I can propose the following solution (assuming your original protocol is named ObjcProtocol).

Let's make a wrapper for our notification handler:

final class InternalNotificationHandler {     private let source: ObjcProtocol      init(source: ObjcProtocol) {         // We require source object in case we need access some properties etc.         self.source = source     }      @objc func presetLoaded(notification: NSNotification) {         // Your notification logic here     } } 

Now we need extend our ObjcProtocol to introduce required logic

import Foundation import ObjectiveC  internal var NotificationAssociatedObjectHandle: UInt8 = 0  extension ObjcProtocol {     // This stored variable represent a "singleton" concept     // But since protocol extension can only have stored properties we save it via Objective-C runtime     private var notificationHandler: InternalNotificationHandler {         // Try to an get associated instance of our handler         guard let associatedObj = objc_getAssociatedObject(self, &NotificationAssociatedObjectHandle)             as? InternalNotificationHandler else {             // If we do not have any associated create and store it             let newAssociatedObj = InternalNotificationHandler(source: self)             objc_setAssociatedObject(self,                                      &NotificationAssociatedObjectHandle,                                      newAssociatedObj,                                      objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)             return newAssociatedObj         }          return associatedObj     }      func registerForPresetLoadedNotification() {         NSNotificationCenter.defaultCenter().addObserver(self,                                                          selector: #selector(notificationHandler.presetLoaded(_:)),                                                          name: kPresetLoadedNotificationName,                                                          object: self)     }      func unregisterForPresetLoadedNotification() {         // Clear notification observer and associated objects         NSNotificationCenter.defaultCenter().removeObserver(self,                                                             name: kPresetLoadedNotificationName,                                                             object: self)         objc_removeAssociatedObjects(self)     } } 

I know this might look not so elegant, so I'd really consider changing a core approach.

One note: You do might want to restrict your protocol extension

extension ObjcProtocol where Self: SomeProtocolOrClass