populateUsers.swift

// Create users and update users counter
  private func populateUsers(addedCount: Int) {
    let db = Firestore.firestore()
    db.runTransaction({ (transaction, errorPointer) -> Any? in
      // Update user counter to added users amount
      let usersCounterRef = db.collection("counters").document("users")
      let usersCounterDoc: DocumentSnapshot
      
      do {
        try usersCounterDoc = transaction.getDocument(usersCounterRef)
      }
      catch let fetchError as NSError {
        errorPointer?.pointee = fetchError
        return nil
      }
      
      guard let oldCount = usersCounterDoc.data()?["count"] as? Int else {
        let error = NSError(
          domain: "AppErrorDomain",
          code: -1,
          userInfo: [
            NSLocalizedDescriptionKey: "Unable to retrieve population from snapshot \(usersCounterDoc)"
          ]
        )
        errorPointer?.pointee = error
        return nil
      }
      
      // Note: this could be done without a transaction
      // by updating the population using FieldValue.increment()
      transaction.updateData(["count": oldCount + addedCount], forDocument: usersCounterRef)
      
      // Create users
      for _ in 0..<addedCount {
        let userRef = db.collection("users").document()
        transaction.setData(User.fake().dictionary, forDocument: userRef)
      }

      return nil
    })
    { (object, error) in
      if let error = error {
        print("Transaction failed: \(error)")
      } else {
        print("Transaction successfully committed!")
      }
    }
  }