live_paginating.swift

    var lastDoc: QueryDocumentSnapshot?
    var listeners: [ListenerRegistration] = []
    var query: Query = db.collection("channels").document("the doc id for this channel").collection("messages").limit(to: 25)

    override func viewDidLoad() {
        super.viewDidLoad()
        addListener() // adds listener for first 25 messages
    }

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        // fetch nexst 25 items after scrolling to some point...
        addListener()
    }

    func addListener() {
        if let lastDoc = lastDoc {
            query.start(afterDocument: lastDoc)
        }
      
        let messageListener = query
            .addSnapshotListener(includeMetadataChanges: true, listener: { (querySnapshot, error) in
                guard let snapshot = querySnapshot else {
                    print(error)
                    return
                }
                                                                          
                self.lastDoc = snapshot.documents.last
                                                                          
                for diff in snapshot.documentChanges {                    
                    // update data source here
                }

                self.tableView.reloadData()
        })
      
      listeners.append(messageListener)
    }