Para Swift3 / iOS10 vea este enlace:

ios10, Swift 3 y notificaciones de Firebase Push (FCM)

Estoy tratando de usar Firebase para notificaciones y lo integré exactamente como se describe en los documentos. Pero no entiendo por qué no funciona. Cuando construyo mi proyecto veo esta línea:

2016-05-25 16:09:34.987: <FIRInstanceID/WARNING> Failed to fetch default token Error Domain=com.firebase.iid Code=0 "(null)"

Este es mi AppDelegate:

 func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    FIRApp.configure()
    FIRDatabase.database().persistenceEnabled = true
     var service: DataService = DataService()
    service.start()
    registerForPushNotifications(application)
    application.registerForRemoteNotifications()
    return true
}

func registerForPushNotifications(application: UIApplication) {
    let notificationSettings = UIUserNotificationSettings(
        forTypes: [.Badge, .Sound, .Alert], categories: nil)
    application.registerUserNotificationSettings(notificationSettings)
}

func application(application: UIApplication, didRegisterUserNotificationSettings notificationSettings: UIUserNotificationSettings) {
    if notificationSettings.types != .None {
        application.registerForRemoteNotifications()
    }
}

func application(application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: NSData) {
    let tokenChars = UnsafePointer<CChar>(deviceToken.bytes)
    var tokenString = ""

    for i in 0..<deviceToken.length {
        tokenString += String(format: "%02.2hhx", arguments: [tokenChars[i]])
    }

    FIRInstanceID.instanceID().setAPNSToken(deviceToken, type: FIRInstanceIDAPNSTokenType.Unknown)
    print("Device Token:", tokenString)
}
func application(application: UIApplication, didReceiveRemoteNotification userInfo: [NSObject : AnyObject], fetchCompletionHandler completionHandler: (UIBackgroundFetchResult) -> Void)  {
    // Print message ID.
    print("Message ID: \(userInfo["gcm.message_id"]!)")

    // Print full message.
    print("%@", userInfo)
}