flutter - 在 null Flutter 错误上调用该方法

Question

我在其他 StackOverflow 帖子中看到很多人遇到此错误，但是当我努力为自己的代码实现其他问题的答案时。正在使用的代码来自 GitHub 存储库https://github.com/sbis04/events_demo。这就是我调用该方法的方式：

class _ThirdRoute extends State<ThirdRoute> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Calendar"),
      ),
      body: Column(
        children: <Widget>[
          DashboardScreen(),
        ],
      ),
    );
  }
}

这是另一个 .dart 文件中的方法：

class DashboardScreen extends StatefulWidget {
  @override
  _DashboardScreenState createState() => _DashboardScreenState();
}

class _DashboardScreenState extends State<DashboardScreen> {
  Storage storage = Storage();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        elevation: 0,
        backgroundColor: Colors.white,
        iconTheme: IconThemeData(
          color: Colors.grey, //change your color here
        ),
        title: Text(
          'Event Details',
          style: TextStyle(
            fontSize: 22,
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          Navigator.of(context).push(
            MaterialPageRoute(
              builder: (context) => CreateScreen(),
            ),
          );
        },
      ),
      body: Padding(
        padding: const EdgeInsets.only(
          left: 16.0,
          right: 16.0,
        ),
        child: Container(
          child: StreamBuilder(
            stream: storage.retrieveEvents(),
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                if (snapshot.data.documents.length > 0) {
                  return ListView.builder(
                    physics: BouncingScrollPhysics(),
                    itemCount: snapshot.data.documents.length,
                    itemBuilder: (context, index) {
                      Map<String, dynamic> eventInfo = snapshot.data.documents[index].data();

                      EventInfo event = EventInfo.fromMap(eventInfo);

                      DateTime startTime = DateTime.fromMillisecondsSinceEpoch(event.startTimeInEpoch);
                      DateTime endTime = DateTime.fromMillisecondsSinceEpoch(event.endTimeInEpoch);

                      String startTimeString = DateFormat.jm().format(startTime);
                      String endTimeString = DateFormat.jm().format(endTime);
                      String dateString = DateFormat.yMMMMd().format(startTime);

                      return Padding(
                        padding: EdgeInsets.only(bottom: 16.0),
                        child: InkWell(
                          onTap: () {
                            Navigator.of(context).push(
                              MaterialPageRoute(
                                builder: (context) => EditScreen(event: event),
                              ),
                            );
                          },
                          child: Column(
                            mainAxisAlignment: MainAxisAlignment.start,
                            crossAxisAlignment: CrossAxisAlignment.start,
                            mainAxisSize: MainAxisSize.min,
                            children: [
                              Container(
                                padding: EdgeInsets.only(
                                  bottom: 16.0,
                                  top: 16.0,
                                  left: 16.0,
                                  right: 16.0,
                                ),
=                                child: Column(
                                  crossAxisAlignment: CrossAxisAlignment.start,
                                  children: [
                                    SizedBox(height: 10),
                                    Text(
                                      event.description,
                                      maxLines: 2,
                                      overflow: TextOverflow.ellipsis,
                                    ),
                                    SizedBox(height: 10),
                                    Padding(
                                      padding: const EdgeInsets.only(top: 8.0, bottom: 8.0),
                                      child: Text(
                                        event.link,
                                      ),
                                    ),
                                    SizedBox(height: 10),
                                    Row(
                                      crossAxisAlignment: CrossAxisAlignment.center,
                                      children: [
                                        Container(
                                          height: 50,
                                          width: 5,
                                        ),
                                        SizedBox(width: 10),
                                        Column(
                                          crossAxisAlignment: CrossAxisAlignment.start,
                                          children: [
                                            Text(
                                              dateString,
                                            ),
                                            Text(
                                              '$startTimeString - $endTimeString',
                                            ),
                                          ],
                                        )
                                      ],
                                    ),
                                  ],
                                ),
                              ),
                            ],
                          ),
                        ),
                      );
                    },
                  );
                } else {
                  return Center(
                    child: Text(
                      'No Events',
                    ),
                  );
                }
              }
              return Center(
                child: CircularProgressIndicator(
                  strokeWidth: 2,
                ),
              );
            },
          ),
        ),
      ),
    );
  }
}

如果有人能给我一些关于如何解决错误的建议：

在 null 上调用了方法 'collection'。接收方：空。尝试调用：集合（“事件”）。相关的导致错误的小部件是 DashboardScreen

编辑这是我在评论中要求的 storage.dart 文件：

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

import 'calendar_client.dart';

final CollectionReference mainCollection = FirebaseFirestore.instance.collection('event');
final DocumentReference documentReference = mainCollection.doc('test');

class Storage {
  Future<void> storeEventData(EventInfo eventInfo) async {
    DocumentReference documentReferencer = documentReference.collection('events').doc(eventInfo.id);

    Map<String, dynamic> data = eventInfo.toJson();

    print('DATA:\n$data');

    await documentReferencer.set(data).whenComplete(() {
      print("Event added to the database, id: {${eventInfo.id}}");
    }).catchError((e) => print(e));
  }

  Future<void> updateEventData(EventInfo eventInfo) async {
    DocumentReference documentReferencer = documentReference.collection('events').doc(eventInfo.id);

    Map<String, dynamic> data = eventInfo.toJson();

    print('DATA:\n$data');

    await documentReferencer.update(data).whenComplete(() {
      print("Event updated in the database, id: {${eventInfo.id}}");
    }).catchError((e) => print(e));
  }

  Future<void> deleteEvent({@required String id}) async {
    DocumentReference documentReferencer = documentReference.collection('events').doc(id);

    await documentReferencer.delete().catchError((e) => print(e));

    print('Event deleted, id: $id');
  }

  Stream<QuerySnapshot> retrieveEvents() {
    Stream<QuerySnapshot> myClasses = documentReference.collection('events').orderBy('start').snapshots();

    return myClasses;
  }
}

标签： flutter