如何检测真正的 iOS/APNS 推送令牌何时注册到 Firebase 云消息传递 (FCM)？

Question

Firebase's didReceiveRegistrationToken as seen below is getting called even if the user hasn't granted push notification permissions. I need to make sure that APNS push tokens are being registered for analytics, as well as for saving it on my server, but this function is getting called even when the user hasn't granted push permission. ♂️

/**
 * Requirement for Firebase push notifications.
 * See documentation here: https://firebase.google.com/docs/cloud-messaging/ios/client
 */
extension AppDelegate: MessagingDelegate {
    // Note: This callback is fired at each app startup and whenever a new token is generated.
    func messaging(_ messaging: Messaging, didReceiveRegistrationToken fcmToken: String) {
        print("Firebase registration token: \(fcmToken)")
        let dataDict: [String: String] = ["token": fcmToken]
        NotificationCenter.default.post(name: Notification.Name("FCMToken"), object: nil, userInfo: dataDict)
        print("Getting called even if the user hasn't opted in for push notifications!")
    }
}

Answer 1

TL;DR : FCM token is not a reliable way to detect if APNS token is registered. Disable method swizzling in firebase and listen to APNS registration manually .

1. FCM Tokens: An FCM token is also known as a device instance ID; it identifies that specific app on that specific device. In other words, FCM token doesn't equal an APNS token. Here is one explanation: APNs, FCM or GCM token .

2. FCM Token Auto Generation: Surprisingly, even without iOS user granting permission, Firebase generates an FCM token on app launch. I guess it wants to find a way to identify the app & device pair, so as soon as you launch, you're gonna have an FCM token. If you want, you can disable auto-generation and wait for the user to opt-in.

By default, the FCM SDK generates a registration token for the client app instance on app launch. If you want to get an explicit opt-in before using Instance ID, you can prevent generation at configure time by disabling FCM. To do this, add a metadata value to your Info.plist (not your GoogleService-Info.plist):

Here lies one of the problems! Even if the user has disabled auto-generated of FCM tokens, one will be generated when the user is prompted to enable push notifications not when the user has accepted push notification permission. This seems quiet odd and I ended up reporting this to Firebase.

3. APNS Token Swizzling : So by default Firebase takes over AppDelegate's didRegisterForRemoteNotificationsWithDeviceToken (called "swizzling"). You can disable this and override the delegate method yourself. This will then give you access to both APNS token as well as FCM token.

func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    print("Successfully registered for notifications!")
    // Required to make FCM work since we disabled swizzling!
    Messaging.messaging().apnsToken = deviceToken
    // String version of APNS so we can use save it/use it later/do analytics.
    let apnsToken = deviceToken.map { String(format: "%02.2hhx", $0) }.joined()
}

Answer 2

Getting a callback for a device ID token and getting a callback for an actual pushed message are two different things. The device ID token callback just gives you a token that identifies the device for a later push notification. It isn't actually a pushed notification itself. It's just an ID.

That said, if you disagree with the behavior of the client SDK from a security perspective, you're free to contact Firebase support directly with feedback.