add GameCenterHelper to login to GameCenter and recognize authenticated user

GoldWars/GoldWars/GameCenterHelper.swift

@@ -0,0 +1,47 @@
+//
+//  GameCenterHelper.swift
+//  GoldWars
+//
+//  Created by Jakob Haag on 02.05.20.
+//  Copyright © 2020 SP2. All rights reserved.
+//
+
+import GameKit
+
+final class GameCenterHelper: NSObject {
+    typealias CompletionBlock = (Error?) -> Void
+    
+    // 1
+    static let helper = GameCenterHelper()
+
+    // 2
+    var viewController: UIViewController?
+    
+    static var isAuthenticated: Bool {
+      return GKLocalPlayer.local.isAuthenticated
+    }
+    
+    override init() {
+        super.init()
+      
+        GKLocalPlayer.local.authenticateHandler = { gcAuthVC, error in
+            NotificationCenter.default
+            .post(name: .authenticationChanged, object: GKLocalPlayer.local.isAuthenticated)
+
+            if GKLocalPlayer.local.isAuthenticated {
+                print("Authenticated to Game Center!")
+            } else if let vc = gcAuthVC {
+                self.viewController?.present(vc, animated: true)
+            }
+            else {
+                print("Error authentication to GameCenter: " +
+                    "\(error?.localizedDescription ?? "none")")
+            }
+        }
+    }
+}
+
+extension Notification.Name {
+  static let presentGame = Notification.Name(rawValue: "presentGame")
+  static let authenticationChanged = Notification.Name(rawValue: "authenticationChanged")
+}