Cannot convert value of type '(String!, NSError!) -> ()' to expected argument type 'GGLInstanceIDTokenHandler!'

Question

I have this problem with Google Cloud Messaging ( GCM ) after refactoring to Swift 3. Can anyone help?

I got this error: Cannot convert value of type '(String!, NSError!) -> ()' to expected argument type 'GGLInstanceIDTokenHandler!'

Here:

func application( _ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data ) {

// ...

GGLInstanceID.sharedInstance().tokenWithAuthorizedEntity(gcmSenderID, scope: kGGLInstanceIDScopeGCM, options: registrationOptions, handler: registrationHandler) 
}

And same error here:

func onTokenRefresh() {
    GGLInstanceID.sharedInstance().token(withAuthorizedEntity: gcmSenderID, scope: kGGLInstanceIDScopeGCM, options: registrationOptions, handler: registrationHandler)
}

This is the registrationHandler:

func registrationHandler(_ registrationToken: String!, error: NSError!) {
    // ...
}

Answer 1

Change registrationHandler to

func registrationHandler(_ registrationToken: String?, error: Error?) {
    // …
}

Answer 2

David's answer doesn't work with Swift 3, unfortunately. However, this does:

func registrationHandler(registrationToken: Optional<String>, error: Optional<Error>) {
    // ...
}