Future<List<Note>> 不是 List<Note> 类型的子类型 [重复]

Question

我正在从 sqlite 获取数据并尝试在列表视图中设置它，但出现错误：

    type 'Future<List<Note>>' is not a subtype of type 'List<Note>'

这是我的sqlite查询函数：

Future<List<Note>> readAllNotes() async {
final db = await instance.database;

final orderBy = '${NoteFields.timeAdded} ASC';

final result = await db?.query(tableNotes, orderBy: orderBy);

return result!.map((json) => Note.fromJson(json)) as List<Note>;

}

这里是调用 readAllNotes() 的地方：

class NoteListWidget extends StatefulWidget {


const NoteListWidget({
    Key? key,
  }) : super(key: key);

  @override
  State<NoteListWidget> createState() => _NoteListWidgetState();
}

class _NoteListWidgetState extends State<NoteListWidget> {
  

  @override
  Widget build(BuildContext context) {
    List<Note> note = NotesDataBase.instance.readAllNotes() as List<Note>;
return ListView.builder(
    itemCount: note.toString().characters.length,
    itemBuilder: (ctx, index) {
      return NoteTileWidget(
        body: note[index].body,
        title: note[index].title,
        timeAdded:
            ('${note[index].timeAdded.day}/${note[index].timeAdded.month}/${note[index].timeAdded.year}'),
        id: note[index].id,
      );
    });

} }

Answer 1

你必须使用FutureBuilder:

    class _NoteListWidgetState extends State<NoteListWidget> {
      @override
      Widget build(BuildContext context) {
        return FutureBuilder<List<Note>>(
          future: NotesDataBase.instance.readAllNotes(),
          builder: (context, snapshot) {
             if (snapshot.hasData){
               final List<Note> notes = snapshot.data;
               return ListView.builder(
                   itemCount: notes.length,
                   itemBuilder: (ctx, index) {
                     return NoteTileWidget(
                       body: notes[index].body,
                       title: notes[index].title,
                       timeAdded:
                       ('${notes[index].timeAdded.day}/${notes[index].timeAdded.month}/${notes[index].timeAdded.year}'),
                       id: notes[index].id,
                     );
                   });
             }else{
               return const CircularProgressIndicator();
             }
           
          }
        );
      }
    }

Answer 2

class NoteListWidget extends StatefulWidget {
  const NoteListWidget({
    Key? key,
  }) : super(key: key);

  @override
  State<NoteListWidget> createState() => _NoteListWidgetState();
}

class _NoteListWidgetState extends State<NoteListWidget> {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
        future: NotesDataBase.instance.readAllNotes(),
        builder: (context, dataSnapshot) {
          if (dataSnapshot.connectionState == ConnectionState.waiting) {
            return const Center(
              child: CircularProgressIndicator(),
            );
          } else {
            return ListView.builder(
                itemCount: (dataSnapshot.data as List).length,
                itemBuilder: (ctx, index) {
                  final note = (dataSnapshot.data as List)[index];
                  return NoteTileWidget(
                    body: note[index].body,
                    title: note[index].title,
                    timeAdded:
                        ('${note[index].timeAdded.day}/${note[index].timeAdded.month}/${note[index].timeAdded.year}'),
                    id: note[index].id,
                  );
                });
          }
        });
  }
}