title: How to detect absent network connection when setting Firestore document
body: <p>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</p>
  
  <pre class="lang-swift prettyprint-override"><code>let newMsgRef = database.document(“/users/\(userId)/messages/\(docId)“)
  newMsgRef.setData(payload) { err in
     if let error = err {
         // handle error
     } else {
        // handle OK
     }
  }
  </code></pre>
  
  <p>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.</p>
  
  <p>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.  </p>
  
  <p>We thought that disabling offline persistence (which is on by default) would make it trigger the failure callback immediately, but unexpectedly - it does not.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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.</p>
  
  <p>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. </p>