Facing issues in Chat Pagination

Question

I am running the code to load the chats in swift iOS chat application. The problem in chat application is at "query in else{}" in function paginateData() while running the app, the first "query in if{}" runs smoothly as expected, but in other one does not run. I am using firestore database for implementing chat. My goal is to paginate the chats in the chat room. If any more information is required, please let me know.

CODE

func paginateData() {
    fetchingMore = true
    var query: Query!

    if messages.isEmpty {
        query = Firestore.firestore().collection("messageRoom").document(messageRoomIdFinal).collection("messages").order(by: "timestamp", descending: false).limit(to: 20)
        print("First 20 Messages loaded")
    } else {
        query = Firestore.firestore().collection("messageRoom").document(messageRoomIdFinal).collection("messages").order(by: "timestamp", descending: false).start(afterDocument: lastDocumentSnapshot).limit(to: 7)
        print("Next 7 Messages loaded")
    }

    query.addSnapshotListener { (snapshot, err) in
        if let err = err {
            print("\(err.localizedDescription)")
        } else if snapshot!.isEmpty {
            self.fetchingMore = false
            return
        } else {
            snapshot!.documentChanges.forEach { diff in
                if (diff.type == .added) {
                    let snap = diff.document
                    let aMessage = Message(withSnap: snap)
                    self.messages.append(aMessage)
                    DispatchQueue.main.async {
                        self.collectionView.reloadData()
                        let indexPath = IndexPath(item: self.messages.count - 1, section: 0)
                        self.collectionView.scrollToItem(at: indexPath, at: .bottom, animated: true)
                    }
                }

                if (diff.type == .modified) {
                    let docId = diff.document.documentID
                    DispatchQueue.main.async {
                        self.collectionView.reloadData()
                        let indexPath = IndexPath(item: self.messages.count - 1, section: 0)
                        self.collectionView.scrollToItem(at: indexPath, at: .bottom, animated: true)
                    }
                    //update the message with this documentID in the array
                }

                if (diff.type == .removed) {
                    let docId = diff.document.documentID
                    //remove the message with this documentID from the array
                }

                self.lastDocumentSnapshot = snapshot!.documents.last
            }
        }
    }
}

Answer 1

Unrelated to the question but UI calls within Firebase closures are run on the main thread so you can remove the DispatchQueue.

I don't think your code is very far off. I re-wrote it to paginate loading users 3 at a time by age and the below code works correctly.

Take a look and compare with your code. Each time this is called, it loads in the next three users.

var lastDocumentSnapshot: DocumentSnapshot?

func observeUsersWithPagination() {
    var query: Query!

    let usersCollectionRef = self.db.collection("users")

    if let nextStartingSnap = self.lastDocumentSnapshot {
        query = usersCollectionRef.order(by: "age", descending: false).start(afterDocument: nextStartingSnap).limit(to: 3)
    } else {
        query = usersCollectionRef.order(by: "age", descending: false).limit(to: 3)
    }

    query.addSnapshotListener { querySnapshot, error in
        guard let snapshot = querySnapshot else {
            print("Error fetching snapshots: \(error!)")
            return
        }

        self.lastDocumentSnapshot = snapshot.documents.last

        snapshot.documentChanges.forEach { diff in
            let userName = diff.document.get("name") as? String ?? "No Name"
            let age = diff.document.get("age") as? Int ?? 0
            if (diff.type == .added) {
                print("Added user: \(userName)", age)
            }
            if (diff.type == .modified) {
                print("Modified user: \(userName)", age)
            }
            if (diff.type == .removed) {
                print("Removed user: \(userName)", age)
            }
        }
    }
}

It's not clear if documentChanges.forEach is really needed.