How to detect absent network connection when setting Firestore document

Question

We are building a real-time chat app using Firestore. We need to handle a situation when Internet connection is absent. Basic message sending code looks like this

let newMsgRef = database.document(“/users/\(userId)/messages/\(docId)“)
newMsgRef.setData(payload) { err in
   if let error = err {
       // handle error
   } else {
      // handle OK
   }
}

When device is connected, everything is working OK. When device is not connected, the callback is not called, and we don't get the error status.

When device goes back online, the record appears in the database and callback triggers, however this solution is not acceptable for us, because in the meantime application could have been terminated and then we will never get the callback and be able to set the status of the message as sent.

We thought that disabling offline persistence (which is on by default) would make it trigger the failure callback immediately, but unexpectedly - it does not.

We also tried to add a timeout after which the send operation would be considered failed, but there is no way to cancel message delivery when the device is back online, as Firestore uses its queue, and that causes more confusion because message is delivered on receiver’s side, while I can’t handle that on sender’s side.

If we could decrease the timeout - it could be a good solution - we would quickly get a success/failure state, but Firebase doesn’t provide such a setting.

A built-in offline cache could be another option, I could treat all writes as successful and rely on Firestore sync mechanism, but if the application was terminated during the offline, message is not delivered.

Ultimately we need a consistent feedback mechanism which would trigger a callback, or provide a way to monitor the message in the queue etc. - so we know for sure that the message has or has not been sent, and when that happened.

Answer 1

The completion callbacks for Firestore are only called when the data has been written (or rejected) on the server. There is no callback for when there is no network connection, as this is considered a normal condition for the Firestore SDK.

Your best option is to detect whether there is a network connection in another way, and then update your UI accordingly. Some relevant search results:

• Check for internet connection with Swift
• How to check for an active Internet connection on iOS or macOS?
• Check for internet connection availability in Swift

As an alternatively, you can check use Firestore's built-in metadata to determine whether messages have been delivered. As shown in the documentation on events for local changes:

Retrieved documents have a metadata.hasPendingWrites property that indicates whether the document has local changes that haven't been written to the backend yet. You can use this property to determine the source of events received by your snapshot listener:

db.collection("cities").document("SF")
    .addSnapshotListener { documentSnapshot, error in
        guard let document = documentSnapshot else {
            print("Error fetching document: \(error!)")
            return
        }
        let source = document.metadata.hasPendingWrites ? "Local" : "Server"
        print("\(source) data: \(document.data() ?? [:])")
    }

With this you can also show the message correctly in the UI