How to subscribe to changes for a public database in CloudKit?

Here are a few other things you can check:

= 1 =

Make sure the CloudKit container defined in your code is the same one you are accessing in the CloudKit dashboard. Sometimes we overlook what we selected in Xcode as the CloudKit container when we create and test multiple containers.

= 2 =

Check the Subscriptions tab in the CloudKit dashboard and make sure your Person subscription is being created when you launch your app. If you see it, try deleting it in the CK Dashboard and then run your app again and make sure it shows up again.

= 3 =

Check the logs in the CK Dashboard. They will show a log entry of type push whenever a push notification is sent. If it's logging it when you update/add a record in the CK Dashboard, then you know the issue lies with your device.

= 4 =

Remember that push notifications don't work in the iOS simulator. You need an actual device (or a Mac if you are making a macOS app).

= 5 =

Through extensive testing, I've found notifications are more reliable if you always set the alertBody even if it's blank. Like this:

let info = CKSubscription.NotificationInfo()
info.shouldSendContentAvailable = true
info.alertBody = "" //This needs to be set or pushes don't always get sent
subscription.notificationInfo = info

= 6 =

For an iOS app, my app delegate handles notifications like this:

class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterDelegate {

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

    //Ask Permission for Notifications
    UNUserNotificationCenter.current().delegate = self
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .badge, .sound], completionHandler: { authorized, error in
      DispatchQueue.main.async {
        if authorized {
          UIApplication.shared.registerForRemoteNotifications()
        }
      }
    })

    return true
  }

  //MARK: Background & Push Notifications
  func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any]{
    let dict = userInfo as! [String: NSObject]
    let notification = CKNotification(fromRemoteNotificationDictionary: dict)

    if let sub = notification.subscriptionID{
      print("iOS Notification: \(sub)")
    }
  }

  //After we get permission, register the user push notifications
  func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
    //Add your CloudKit subscriptions here...
  }

}

Getting permission for notifications isn't required if you are only doing background pushes, but for anything the user sees in the form of a popup notification, you must get permission. If your app isn't asking for that permission, try deleting it off your device and building again in Xcode.

Good luck! : )

I am using RxCloudKit library, here's an a code snippet of how it handles query notifications -

public func applicationDidReceiveRemoteNotification(userInfo: [AnyHashable: Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
  let dict = userInfo as! [String: NSObject]
  let notification = CKNotification(fromRemoteNotificationDictionary: dict)
        
  switch notification.notificationType {
  case CKNotificationType.query:
    let queryNotification = notification as! CKQueryNotification
    self.delegate.query(notification: queryNotification, fetchCompletionHandler: completionHandler)
...

This method is called from func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void)

Before you can receive notifications, you will need to do the following -

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.
    application.registerForRemoteNotifications()        
...

UPDATE: Info.plist should contain the following -

<key>UIBackgroundModes</key>
<array>
  <string>fetch</string>
  <string>remote-notification</string>
</array>

Update: As Reinhard mentioned in his comment: you can in fact still subscribe to changes from the public database and manually import the changes into Core Data. Still I am unsure whether it is a good idea to rely on subscriptions in the public database if Apple specifically mentioned these differences

Original answer:

I don't think the accepted answer fully answers the question here. Short answer would be that the CloudKit public database does not support subscriptions like the private database. Instead only a polling mechanism can be used. NSPersistentCloudKitContainer handles this automatically, but only updates very rarely.

This talk from WWDC2020 explains this in detail and I recommend watching it because there are other important details mentioned where public database differs from private database: https://developer.apple.com/wwdc20/10650

In the talk they mentioned thet a pull is initiated on each app start and after about 30 mins of application usage.