Is there a simple way to load/get more Firebase documents as a user scrolls?

you can use limit and startAfter method of firebase to fetch items(data).

• limit used to limit the number of documents retrieved from the firebase(check the firebase doc here)

• startAfter or startAt methods to define the start point for a query. (Check the firebase doc here)

       ` class Users extends StatefulWidget {
         Users({Key? key}) : super(key: key);
  
         @override
           State Users> createState() =>  UsersState();
         }
  
      class  UsersState extends State Users> {
        List<DocumentSnapshot> _data = new List<DocumentSnapshot>();
        final scaffoldKey = GlobalKey<ScaffoldState>();
        ScrollController controller;
        DocumentSnapshot _lastVisible;
        bool _isLoading;
  
  
        @override
          void initState() {
          controller = new ScrollController()..addListener(_scrollListener);
          super.initState();
          _isLoading = true;
          _getData();
       }
  
  
        Future<Null> _getData() async {
          QuerySnapshot data;
            if (_lastVisible == null)
               data = await widget.firestore
                     .collection('users') //collection name
                     .orderBy('created_at', descending: true)
                      // to retrieve the latest data first
                      .limit(3)
                      .getDocuments();
             else
               data = await widget.firestore
              .collection('users')//collection name
              .orderBy('created_at', descending: true)
              // to retrieve the latest data first
              .startAfter([_lastVisible['created_at']])
              .limit(3)
              .getDocuments();
  
             if (data != null && data.documents.length > 0) {
               _lastVisible = data.documents[data.documents.length - 1];
             if (mounted) {
                setState(() {
                  _isLoading = false;
                  _data.addAll(data.documents);
                });
              }
            } else {
                setState(() => _isLoading = false);
                scaffoldKey.currentState?.showSnackBar(
                SnackBar(
                  content: Text('No Data'),
             ),
            );
           }
              return null;
             }
  
        @override
          Widget build(BuildContext context) {
            return Scaffold(
              key: scaffoldKey,
               appBar: new AppBar(),
               body: RefreshIndicator(
               child: ListView.builder(
                  controller: controller,
                  itemCount: _data.length + 1,
                  itemBuilder: (_, int index) {
                   if (index < _data.length) {
                    final DocumentSnapshot document = _data[index];
                     return new Container(
                     height: 200.0,
                     child: new Text(document['value']),// get specfic 
                       documentSnapshot, assign the name value(key) here
                     );
                   }
               return Center(
                child: new Opacity(
                 opacity: _isLoading ? 1.0 : 0.0,
                 child: new SizedBox(
                  width: 32.0,
                  height: 32.0,
                    child: new CircularProgressIndicator()),
                 ),
               );
              },
            ),
          onRefresh: ()async{
          _data.clear();
          _lastVisible=null;
           await _getData();
         },
        ),
      );
    }
  
      @override
      void dispose() {
       controller.removeListener(_scrollListener);
       super.dispose();
     }
  
      void _scrollListener() {
       if (!_isLoading) {
        if (controller.position.pixels == controller.position.maxScrollExtent) {
         setState(() => _isLoading = true);
         _getData();
         }
       }
     } 
    }
  

`