应为“List

Question

我试图在FutureBuilder中构建一个Project列表的下拉按钮，但是我收到了错误：

需要类型为List的值

我理解错误和它发生的地方，但我不明白为什么它不返回List

违规代码：

@override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _futureProjects,
      builder: (BuildContext context, AsyncSnapshot snapshot) {
        if (snapshot.hasData) {
          return Column(children: [   
            Text('Project:'),
            new DropdownButton<Project>(
              value: _selectedProject,
              items: snapshot.data
                  .map((value) => new DropdownMenuItem<String>(
                        value: value.name,
                        child: Text(value.name),
                      ))
                  .toList(),
            ),
        } else {
          // ...

_futureProjects通过超文本传输协议填充：

Future<List<Project>> getJiraProjectsList() async {
    final response = await http.get(url + 'jira/projects');

    if (response.statusCode == 200) {
      var jsonData = jsonDecode(response.body);
      List<Project> projects = [];

      for(var data in jsonData){
        Project proj = Project.fromJson(data);
        projects.add(proj);
      }

      return projects;
    } else {
      throw Exception('Failed to load Projects List');
    }
  }

如果能帮我解决这个问题，我将不胜感激




Answer 1

你可以使用AsyncSnapshot

FutureBuilder(
    future: _futureProjects,
    builder: (BuildContext context, AsyncSnapshot<List<Project>> snapshot) {
      if (snapshot.hasData) {
        return Column(children: [
          Text('Project:'),
          DropdownButton<Project>(
            value: snapshot.data[0],
            onChanged: (value) {},
            items: snapshot.data
                .map((project) => DropdownMenuItem<Project>(
                      value: project,
                      child: Text(project.name),
                    ))
                .toList(),
          ),
        ]);
      } else {
        return Text('... waiting');
      }
    })