在 null 上调用了 getter 'length'。接收方：null 尝试调用：长度 FutureBuilder 和列表

Question

我在 Flutter 应用程序运行时不断收到上述错误。基本上我想要实现的是从 api 获取数据并使用 FutureBuilder 以 SliverList 的形式显示它。

在我将列表的代码从 FutureBuilder 更改为 FutureBuilder 以使用 EntertainerEvent 类之前，它工作得非常好，该类具有我需要从 json 文件中显示的所有字段。 我该如何解决这个问题，因为当我使用自定义类时，构造函数或应用程序本身似乎没有获取数据。

这是 EntertainerEvent 类的代码：

    class EntertainerEvent {
      final int eventId;
      final int entertainerId;
      final int eventTypeId;
      final int categoryId;
      final String eventName;
      final String description;
      final String imagePoster;
      final String location;
      final DateTime startDate;
      final DateTime endDate;
      final double entreeFee;
    
      const EntertainerEvent({required this.eventId, required this.entertainerId, required this.eventTypeId,
        required this.categoryId, required this.eventName, required this.description, required this.imagePoster,
        required this.location, required this.startDate, required this.endDate, required this.entreeFee});
    
      factory EntertainerEvent.fromJson(Map<String, dynamic> event) {
        return EntertainerEvent(
          eventId: event['EventID'],
          entertainerId: event['EntertainerID'],
          eventTypeId: event['EventTypeID'],
          categoryId: event['CategoryID'],
          eventName: event['EventName'],
          description: event['Description'],
          imagePoster: event['ImagePoster'],
          location: event['Location'],
          startDate: event['StartDate'],
          endDate: event['EndDate'],
          entreeFee: event['EntryFee'],
        );
      }
    }

下面是从api获取数据的代码：

Future<List<EntertainerEvent>> fetchEvents() async {
    var result = await http.get(Uri.parse(apiUrl));

    if (result.statusCode == 200) {
      var content = result.body;
      var arr = json.decode(content) as List;

      return arr.map((eve) => new EntertainerEvent.fromJson(eve)).toList();
    } else {
      print('Not loaded');
      throw Exception('Unable to fetch data from the Rest API');
    }
  }

  late Future<List<EntertainerEvent>> _fetchEvents;

  @override
  void initState() {
    _fetchEvents = fetchEvents();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<List<EntertainerEvent>>(
        future: _fetchEvents,
        builder: (BuildContext context, AsyncSnapshot snapshot) {
          var childCount = 0;
          if (snapshot.connectionState != ConnectionState.done) {
            childCount = 1;
          } else {
            childCount = snapshot.data.length;
          }
          return SliverList(
            delegate: SliverChildBuilderDelegate((context, index) {
              if (snapshot.hasData) {
                List<EntertainerEvent> someData = snapshot.data;
                print('data here');
               
                //Do some stuff

               }
              },  childCount: childCount),
           );
       });
  }

我不知道我到底错过了什么，因为如果我使用动态类型而不是自定义类 EntertainerEvent，则此代码有效。

提前谢谢大家！

Answer 1

用hasData包裹它：

if(snapshot.hasData){
return SliverList(
            delegate: SliverChildBuilderDelegate((context, index) {
          
                List<EntertainerEvent> someData = snapshot.data;
                print('data here');
               
                //Do some stuff

             
              },  childCount: childCount),
           );}
    return CircularProgressIndicator();