Flutter: Lazy Load data from firestore

The description of your lazy loading seems to match what pagination is. Here's a simple demo using Firestore with pagination in a ListView.builder

This sample implements snippets from Firebase official doc for Firestore pagination.

There are two ways to load the data on the view in this demo.

• Refresh the entire ListView using RefreshIndicator
• Scroll down to hit the bottom of the list to load up the next documents in the ListView. ScrollController is used to determine if the user has hit the bottom part of the list.

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';

import 'DocObj.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // Initialize Firebase
  await Firebase.initializeApp();
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  var scrollController = ScrollController();

  @override
  void initState() {
    super.initState();
    getDocuments();
    scrollController.addListener(() {
      if (scrollController.position.atEdge) {
        if (scrollController.position.pixels == 0)
          print('ListView scroll at top');
        else {
          print('ListView scroll at bottom');
          getDocumentsNext(); // Load next documents
        }
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: listDocument.length != 0
            ? RefreshIndicator(
                child: ListView.builder(
                  physics: AlwaysScrollableScrollPhysics(),
                  controller: scrollController,
                  itemCount: listDocument.length,
                  itemBuilder: (context, index) {
                    return ListTile(
                      title: Text('${listDocument[index].documentName}'),
                    );
                  },
                ),
                onRefresh: getDocuments, // Refresh entire list
              )
            : CircularProgressIndicator(),
      ),
    );
  }

  List<DocObj> listDocument;
  QuerySnapshot collectionState;
  // Fetch first 15 documents
  Future<void> getDocuments() async {
    listDocument = List();
    var collection = FirebaseFirestore.instance
        .collection('sample_pagination')
        .orderBy("name")
        .limit(15);
    print('getDocuments');
    fetchDocuments(collection);
  }

  // Fetch next 5 documents starting from the last document fetched earlier
  Future<void> getDocumentsNext() async {
    // Get the last visible document
    var lastVisible = collectionState.docs[collectionState.docs.length-1];
    print('listDocument legnth: ${collectionState.size} last: $lastVisible');

    var collection = FirebaseFirestore.instance
        .collection('sample_pagination')
        .orderBy("name").startAfterDocument(lastVisible).limit(5);

    fetchDocuments(collection);
  }

  fetchDocuments(Query collection){
    collection.get().then((value) {
      collectionState = value; // store collection state to set where to start next
      value.docs.forEach((element) {
        print('getDocuments ${element.data()}');
        setState(() {
          listDocument.add(DocObj(DocObj.setDocDetails(element.data())));
        });
      });
    });
  }
}

To parse the data inside the document, you can create a model for your object.

class DocObj {
  var documentName;

  DocObj(DocObj doc) {
    this.documentName = doc.getDocName();
  }

  dynamic getDocName() => documentName;

  DocObj.setDocDetails(Map<dynamic, dynamic> doc)
      : documentName = doc['name'];
}

The sample handles this data from Firestore.

Here's how the app looks when running.

I have suffered this,and what i did is i have added scrollcontroller in listview builder and i traced if it reaches bottom of the list and if it reaches at the bottom,then update the limit of the snapshot.

I have experimented this and works well.

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';

class Sample extends StatefulWidget {
  @override
  _SampleState createState() => _SampleState();
}

class _SampleState extends State<Sample> {
  ScrollController _chatScrollController;
  int loadMoreMsgs = 25; // at first it will load only 25
  int a = 50; // 'loadMoreMsgs' will added by 'a' if we load more msgs in listview.
  
  @override
  void initState() {
    _chatScrollController = ScrollController()
      ..addListener(() {
        if (_chatScrollController.position.atEdge) {
          if (_chatScrollController.position.pixels == 0)
            print('ListView scrolled to top');
          else {
            setState(() {
              loadMoreMsgs =  loadMoreMsgs + a;
            });
            print('ListView scrolled to bottom');
          }
        }
      });
    super.initState();
  }
  
  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: FirebaseFirestore.instance.collection('CollectionName').limit(loadMoreMsgs).snapshots(),
      builder: (context, snapshot) {
        return ListView.builder(
          controller: _chatScrollController,
          itemBuilder: (context, index) {
            return Text('This is a sample');
          },
        );
      },
    );
  }
}

I have only written required codes for understanding.