AddSnapShotListener is repeating the document reading in one instance

Question

When I am using addSnapshotListener for realtime updates, the documents are repeated which should not be the case, but when using getDocuments() the documents are repeated once only, I need to use addSnaphotListener but not want to duplicate the document reading, please assist where I am wrong in using snapshot listener.

I am using Firestore database in Swift iOS. Below is the code I am using

Code with addSnapShotListener():

func getComments() {

      //print(postId + "received")
        let commentsRef = Firestore.firestore().collection("posts").document(postId).collection("comments")

        commentsRef.addSnapshotListener { (snapshot, error) in

            if let error = error {

                print(error.localizedDescription)

            } else {

                if let snapshot = snapshot {

                    for document in snapshot.documents {

                      // self.length = snapshot.count


                        let data = document.data()
                        let username = data["comment_author_username"] as? String ?? ""
                        let comment = data["comment_author_comment"] as? String ?? ""
                        let spinnerC = data["comment_author_spinnerC"] as? String ?? ""
                        let fullname = data["comment_author_fullname"] as? String ?? ""
                        let email = data["comment_author_email"] as? String ?? ""
                        let commentUserImageUrl = data["comment_user_image"] as? String ?? ""
                        let commentuser_id = data["comment_author_id"] as? String ??  ""
                        self.checkl1value = data["l1"] as? Bool


                        let newComment = Comment(_documentId: document.documentID, _commentAuthorUsername: username, _commentAuthorFullName: fullname, _commentAuthorComment: comment, _commentUserImage: commentUserImageUrl, _commentAuthorSpinnerC: spinnerC, _commentAuthorId:commentuser_id, _checkl1value: self.checkl1value)

                        self.comments.append(newComment)
                    //   print(self.length!)

                    }
                   self.tableView.reloadData()
                }
            }
        }
    }

Code With getDocuments():

func getComments() {

      //print(postId + "received")
        let commentsRef = Firestore.firestore().collection("posts").document(postId).collection("comments")

        commentsRef.getDocuments { (snapshot, error) in

            if let error = error {

                print(error.localizedDescription)

            } else {

                if let snapshot = snapshot {

                    for document in snapshot.documents {

                      // self.length = snapshot.count


                        let data = document.data()
                        let username = data["comment_author_username"] as? String ?? ""
                        let comment = data["comment_author_comment"] as? String ?? ""
                        let spinnerC = data["comment_author_spinnerC"] as? String ?? ""
                        let fullname = data["comment_author_fullname"] as? String ?? ""
                        let email = data["comment_author_email"] as? String ?? ""
                        let commentUserImageUrl = data["comment_user_image"] as? String ?? ""
                        let commentuser_id = data["comment_author_id"] as? String ??  ""
                        self.checkl1value = data["l1"] as? Bool


                        let newComment = Comment(_documentId: document.documentID, _commentAuthorUsername: username, _commentAuthorFullName: fullname, _commentAuthorComment: comment, _commentUserImage: commentUserImageUrl, _commentAuthorSpinnerC: spinnerC, _commentAuthorId:commentuser_id, _checkl1value: self.checkl1value)

                        self.comments.append(newComment)
                    //   print(self.length!)

                    }
                   self.tableView.reloadData()
                }
            }
        }
    }

Answer 1

You're probably looking to only handle the changes between the snapshots. To do that you'll want to loop over instead of, as shown in the documentation on viewing changes between snapshots:

db.collection("cities").whereField("state", isEqualTo: "CA")
    .addSnapshotListener { querySnapshot, error in
        guard let snapshot = querySnapshot else {
            print("Error fetching snapshots: \(error!)")
            return
        }
        snapshot.documentChanges.forEach { diff in
            if (diff.type == .added) {
                print("New city: \(diff.document.data())")
            }
            if (diff.type == .modified) {
                print("Modified city: \(diff.document.data())")
            }
            if (diff.type == .removed) {
                print("Removed city: \(diff.document.data())")
            }
        }
    }

Initially your listener will get called with diff.type == .added for each existing document, and then when there are changes it'll get called with the right mix of types.