Dart Flutter 对 Cloud Firestore 进行排序

我正在try 对传入的列表视图进行排序.因为这个列表视图是图像，所以我向FireStore对象添加了一个数字，这样我就可以升序排序.

我似乎不能让项目排序，我确定这是我如何构建的应用程序.

我试图将.orderBy()添加到集合中，但是它说查询不是CollectionReference的类型.

这是我的页面

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

class Events extends StatefulWidget {
  @override
  _EventsState createState() => _EventsState();
}

class _EventsState extends State<Events> {
  StreamSubscription<QuerySnapshot> subscription;

  List<DocumentSnapshot> snapshot;

  CollectionReference collectionReference =
  Firestore.instance.collection("Events");


  @override

  void initState() {
    subscription = collectionReference.snapshots().listen((datasnapshot) {
      setState(() {
        snapshot = datasnapshot.documents;
      });
    });
    super.initState();
  }

//  passData(DocumentSnapshot snap) {
//    Navigator.of(context).push(
//        MaterialPageRoute(builder: (context) => EventPage(snapshot: snap,)));
//  }

  Widget build(BuildContext context) {
    return Scaffold(
         backgroundColor: Colors.white,
      body: Column(
        children: <Widget>[
          Container(
            width: MediaQuery.of(context).size.width,
            height: 200,
            decoration: BoxDecoration(
              image: DecorationImage(
                fit: BoxFit.fill,
                image: AssetImage("assets/images/events.jpg"),
              ),
            ),
          ),
          Divider(
            color: Colors.black,
          ),
          Expanded(
            child: ListView.separated(separatorBuilder: (context, index) => Divider(color: Colors.black12,
    ), itemCount: snapshot.length,
    itemBuilder: (context, index){
              return Card(
                child: Container(
                  width: MediaQuery.of(context).size.width,
                  height: 210,
                  decoration: BoxDecoration(
                    image: DecorationImage(
                      fit: BoxFit.fill,
                      image: NetworkImage(snapshot[index].data["image"]),
                    ),
                  ),
                ),

              );
    }

          ))
        ],
      ),
    );
  }
}

我真的很想在数据库中按预定的数字对这些图像进行排序.