Question

我想使用FutureBuilder来构建listView，但是当我运行该应用程序时，会出现一条错误消息：构建函数返回null。

我需要的数据是一个列表，我想首先从本地数据库加载它，如果失败，那么我将发出请求并从网络获取结果。代码如下：

class AllConferencesPage extends StatefulWidget {
  @override
  AllConferencesState createState() => new AllConferencesState();
}

class AllConferencesState extends State<AllConferencesPage> {
  List<Conference> _conferences;

  Future _future;

  Future<List<Conference>> loadConferencesFromDatabase() async {

    List<Conference> conferences;

    conferences = await ConferenceProvider.instance.getConferences('1 = 1');

    await ConferenceProvider.instance.close();

    return conferences;
  }

  Future<List<Conference>> loadConferencesFromNetworkResponse(
      Response response) async {
    List<Conference> conferences = new List();
    // ...
    return conferences;
  }

  Future<List<Conference>> loadConferences() async {

    _future = loadConferencesFromDatabase();

    _future.timeout(Duration(seconds: 3),
    onTimeout: () async {
      Response response = await Dio().get(urlPrefix);
      _future = loadConferencesFromNetworkResponse(response);
    });

    return _future;
  }

  Widget _buildConferences(List<Conference> conferences) {
    return ListView.builder(
      shrinkWrap: true,
      itemCount: conferences.length,
      itemBuilder: (context, i) {
        return _buildCard(conferences[i]);
      },
      padding: const EdgeInsets.all(12.0),
    );
  }

  Widget _buildBlank() {
    return Center(
      child: CircularProgressIndicator(),
    );
  }

  @override
  void initState() {
    super.initState();

    _future = loadConferences();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text('Hello Flutter'),
      ),
      body: SafeArea(
        child: FutureBuilder<List<Conference>>(
          builder: (context, AsyncSnapshot<List<Conference>> snap) {
            if (snap.connectionState == ConnectionState.none || snap.connectionState == ConnectionState.waiting || snap.connectionState == ConnectionState.active) {
              return _buildBlank();
            } else if (snap.connectionState == ConnectionState.done) {
              if (snap.hasError) {
                return _buildBlank();
              } else if (snap.hasData) {
                if (snap.data != null) {
                  _conferences = snap.data;
                  return _buildConferences(_conferences);
                } else {
                  return _buildBlank();
                }
              }
            }
          },
          future: _future,
        ),
      ),
    );
  }
}

Answer 1

如错误消息所述，您必须在构建器函数中返回内容。

只需添加else大小写并返回Container()或Center(child: CircularProgressIndicator())

编辑：

            if (snap.connectionState == ConnectionState.none || snap.connectionState == ConnectionState.waiting || snap.connectionState == ConnectionState.active) {
              return _buildBlank();
            } else if (snap.connectionState == ConnectionState.done) {
              if (snap.hasError) {
                return _buildBlank();
              } else if (snap.hasData) { // hasData implies data is not null
                _conferences = snap.data;
                return _buildConferences(_conferences);
              } else {
                  return _buildBlank();
              }
            }

Answer 2

我认为问题是由于在if函数中使用了builder语句。运行该函数时，将执行第一个if语句，但是那时连接状态正在等待，该函数不返回任何内容。要解决此问题，请改用switch。它为我工作。我不知道我是否在解释正确，但您应该尝试一下。

代码：-


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: Text('Hello Flutter'),
      ),
      body: SafeArea(
        child: FutureBuilder<List<Conference>>(
          builder: (context, AsyncSnapshot<List<Conference>> snap) {
            switch(snap.connectionState) {
              case ConnectionState.waiting:
                return _buildBlank();
              default:
                if (snap.hasError) {
                  return _buildBlank();
                } else if (snap.hasData) {
                  if (snap.data != null) {
                    _conferences = snap.data;
                    return _buildConferences(_conferences);
                  } else {
                    return _buildBlank();
                  }
                }
            }
          },
          future: _future,
        ),
      ),
    );
  }

使用FutureBuilder在Flutter中构建列表视图时出现“生成函数返回空”错误

2 个答案: