dart - Flutter:类型 'List' 不是类型 'List' 的子类型

Question

我正在使用 Flutter 开发一个简单的待办事项应用。

该应用有多个 View 和表 (SQLite)。

1. 待办事项(StatelessWidget)
2. 标签(StatefulWidget)

在 Todo ui 的情况下，它工作正常，但 Tag ui 不起作用。

第一个代码是 Todo ui。 (工作正常)。

class TodosPage extends StatelessWidget {

  final TodoRepository _todoRepository;
  final TodosBloc bloc;

  TodosPage(this._todoRepository) : this.bloc = TodosBloc(_todoRepository);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: StreamBuilder<List<Todo>>(
            stream: bloc.todos,
            builder: (context, snapshot) {
              if (snapshot.data == null) {
                return Text("Empty");
              }

              return ListView(
                children: snapshot.data.map(_buildItem).toList(),
              );
            }),
      ),
    );
  }

  Widget _buildItem(Todo todo) => Text(todo.name);

这里是 Tag ui(它不起作用)。

class TagsPage extends StatefulWidget {
  final TagRepository _tagRepository;

  TagsPage(this._tagRepository);

  @override
  State<StatefulWidget> createState() => _TagsState(_tagRepository);
}

class _TagsState extends State<TagsPage> {
  final TagRepository _tagRepository;
  final TagsBloc bloc;

  _TagsState(this._tagRepository) : this.bloc = TagsBloc(_tagRepository);

  @override
  Widget build(BuildContext context) => Scaffold(
        body: Center(
          child: StreamBuilder<List<Tag>>(
            stream: bloc.tags,
            builder: (context, snapshot) {
              if (snapshot.data == null || snapshot.data.isEmpty) {
                print("[TagsPage][JOS] build - snapshot.data is null");
              } else {
                return ListView(
                  children: snapshot.data.map(_buildTagItem).toList(),
                );
              }
            },
          ),
        ),
      );

  _buildTagItem(Tag tag) => Text(tag.name);
}

TagBloc 在这里:

class TagsBloc {
  final TagRepository _tagRepository;
  final _getTagsSubject = PublishSubject<List<Tag>>();

  Stream<List<Tag>> get tags => _getTagsSubject.stream;

  TagsBloc(this._tagRepository) {
    _getTags();
  }

  Future<Null> _getTags() async {
    await _tagRepository.getAll().then((list) {
      _getTagsSubject.add(list);
    });
  }
}

当我运行时，标签 ui 显示以下错误日志:

04-07 00:13:45.698 25181-25195/com.youknow.jos I/flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
04-07 00:13:45.708 25181-25195/com.youknow.jos I/flutter: The following assertion was thrown building StreamBuilder<List<Tag>>(dirty, dependencies:
04-07 00:13:45.708 25181-25195/com.youknow.jos I/flutter: [_LocalizationsScope-[GlobalKey#49b9d], _InheritedTheme], state: _StreamBuilderBaseState<List<Tag>,
04-07 00:13:45.708 25181-25195/com.youknow.jos I/flutter: AsyncSnapshot<List<Tag>>>#65ac3):
04-07 00:13:45.708 25181-25195/com.youknow.jos I/flutter: type 'List<dynamic>' is not a subtype of type 'List<Widget>'
04-07 00:13:45.715 25181-25195/com.youknow.jos I/flutter: Either the assertion indicates an error in the framework itself, or we should provide substantially
04-07 00:13:45.715 25181-25195/com.youknow.jos I/flutter: more information in this error message to help you determine and fix the underlying cause.
04-07 00:13:45.715 25181-25195/com.youknow.jos I/flutter: In either case, please report this assertion by filing a bug on GitHub:
04-07 00:13:45.715 25181-25195/com.youknow.jos I/flutter:   https://github.com/flutter/flutter/issues/new?template=BUG.md
04-07 00:13:45.715 25181-25195/com.youknow.jos I/flutter: When the exception was thrown, this was the stack:
04-07 00:13:45.724 25181-25195/com.youknow.jos I/flutter: #0      _TagsState.build.<anonymous closure> (package:jos/pages/tags.dart:49:62)

Todo 和Tag 没有区别。我认为唯一的区别是StatefulWidget/StatelessWidget。我不知道如何解决这个问题。

Answer 1

您还没有在 _buildTagItem 上声明返回类型.改变这个:

  _buildTagItem(Tag tag) => Text(tag.name);

为此:

  Widget _buildTagItem(Tag tag) => Text(tag.name);

因此，您的 map(...).toList()返回 List<dynamic>而不是 List<Widget> - 因此错误。