AppDelegate+Notifications.swift

import Foundation
import Firebase

extension AppDelegate {
    
    func setupNotifications(application: UIApplication) {
        FIRApp.configure()
        
        let settings: UIUserNotificationSettings = UIUserNotificationSettings(forTypes: [.Alert, .Badge, .Sound], categories: nil)
        application.registerUserNotificationSettings(settings)
        application.registerForRemoteNotifications()
        
        NSNotificationCenter.defaultCenter().addObserver(
            self,
            selector: #selector(self.tokenRefreshNotification),
            name: kFIRInstanceIDTokenRefreshNotification, object: nil
        )
    }
    
    func subscribeNotificationTopics(){
        FIRMessaging.messaging().subscribeToTopic("/topics/News")
      // register token to backend
    }
    
    func tokenRefreshNotification(notification: NSNotification) {
        if let refreshedToken = FIRInstanceID.instanceID().token() {
            NSLog("InstanceID token: \(refreshedToken)")
        }
        
        // Connect to FCM since connection may have failed when attempted before having a token.
        connectToFcm()
    }
    
    func disconnectFromFcm(){
        FIRMessaging.messaging().disconnect()
        NSLog("Disconnected from FCM.")
    }
    
    func connectToFcm() {
        subscribeNotificationTopics()
        
        FIRMessaging.messaging().connectWithCompletion { (error) in
            if (error != nil) {
                NSLog("Unable to connect with FCM. \(error)")
            } else {
                NSLog("Connected to FCM.")
            }
        }
        
    }
}A

AppDelegate.swift

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
    
    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        setupNotifications(application)
        
        return true
    }
    
    func application(application: UIApplication, didRegisterUserNotificationSettings notificationSettings: UIUserNotificationSettings) {
        subscribeNotificationTopics()
    }
    
    func application(application: UIApplication, didReceiveRemoteNotification userInfo: [NSObject : AnyObject],
                     fetchCompletionHandler completionHandler: (UIBackgroundFetchResult) -> Void) {
        
        // consume notification
    }
    
    
    func applicationDidBecomeActive(application: UIApplication) {
        connectToFcm()
    }
    
    func applicationDidEnterBackground(application: UIApplication) {
        disconnectFromFcm()
    }

}