什么是“Future<dynamic>”类型不是“Stream<dynamic>”类型的子类型？方法？

Question

谁能告诉我我哪里做错了，为了克服 'Future' is not a subtype of type 'Stream?' 的错误，必须在代码中进行哪些更改？ 。之前的错误就像 Future' 不是“Widget”类型的子类型

以下是导致此错误的代码

class _StudentDashboardState extends State<StudentDashboard> {
  userdetails userdetail;
  String res;

  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;

  final CollectionReference student_details =
      FirebaseFirestore.instance.collection('students');
  final CollectionReference tutor_details =
      FirebaseFirestore.instance.collection("tutors");

  String uid = FirebaseAuth.instance.currentUser.uid;

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

  displayPercentage() async {
    var percentage = 0.0;
    var totalClassesTook;
    var totalClassesAttended;
    try {
      totalClassesTook = await tutor_details.doc(uid).get().then((doc) async {
        var val = await doc.data()['TotalClassesTook'];
        return val;
      });

      totalClassesTook = await totalClassesTook == null ? 0 : totalClassesTook;

      totalClassesAttended =
          await student_details.doc(uid).get().then((doc) async {
        var val = await doc.data()['TotalClassesAttended'];
        return val;
      });
      totalClassesAttended =
          await totalClassesAttended == null ? 0 : totalClassesAttended;
      percentage =
          await ((totalClassesAttended / totalClassesTook) * 100.0) / 100.0;
    } catch (e) {
      percentage = 0.0;
    }
    return Center(
      child: ListView(children: <Widget>[
        Padding(
          padding: EdgeInsets.symmetric(vertical: 150.0),
          child: new CircularPercentIndicator(
            radius: 120.0,
            lineWidth: 13.0,
            animation: true,
            percent: percentage,
            center: new Text(
              "$percentage",
              style: new TextStyle(fontWeight: FontWeight.bold, fontSize: 20.0),
            ),
            footer: Padding(
              padding: EdgeInsets.symmetric(horizontal: 10.0, vertical: 10.0),
              child: new Text(
                "Attendance Percentage",
                style:
                    new TextStyle(fontWeight: FontWeight.bold, fontSize: 17.0),
              ),
            ),
            circularStrokeCap: CircularStrokeCap.round,
            progressColor: Colors.purple,
          ),
        ),
      ]),
    );
  }
      @override
  Widget build(BuildContext context) {
     return Container(
      child: SafeArea(
        child: StreamBuilder(
            stream: displayPercentage(),
            builder: (context, snapshot) {
              return Scaffold(
                appBar: AppBar(
                  backgroundColor: Colors.lightGreen,
                  title: Text('Student Welcome'),
                  actions: <Widget>[
                    TextButton.icon(
                        onPressed: () async {
                          await _auth.signOut();
                          Navigator.of(context).pushNamed('/homepage');
                   
                    },
                    icon: Icon(Icons.person),
                    label: Text('Logout'))
              ],
            ),
            body: 
                displayPercentage(),
          );
        }),
  ),
);

}

Answer 1

您的displayPercentage 函数被标记为async，这使得函数返回的wrapped 变为Future 自动allu。

您正在返回一个Widget，它使函数的返回类型为Future<Widget>。但是由于您在定义函数时没有具体提及它，所以dart 假设它为Future<dynamic>。

现在，来到您的StreamBuilder，参数stream 应该被分配一个Stream。但是你给它分配了你的函数，它实际上是一个Future<dynamic>。

这就是为什么它给出错误说，它需要一个Stream<dynamic>，但你给它一个Future<dynamic>，因为Future不是从Stream扩展的sub class，错误说Future<dynamic> is not a subtype of type Stream<dynamic>.

要解决此问题，请将您的 StreamBuilder 更改为 FutureBuilder，如下所示，

@override
Widget build(BuildContext context) {
  return Container(
    child: SafeArea(
      child: FutureBuilder(
        future: displayPercentage(),
        builder: (context, snapshot) {
          
          // since futures dont immediately give a value, if data is null the show loader
          if (snapshot.data == null) return CircularProgressIndicator();

          // Since data is not null, we can return Scaffold
          return Scaffold(
            appBar: AppBar(
              backgroundColor: Colors.lightGreen,
              title: Text('Student Welcome'),
              actions: <Widget>[
                TextButton.icon(
                    onPressed: () async {
                      // await _auth.signOut();
                      Navigator.of(context).pushNamed('/homepage');
                    },
                    icon: Icon(Icons.person),
                    label: Text('Logout'))
              ],
            ),
            // Since your Future holds the Center widget that you are sending, use it like this
            body: snapshot.data, 
          );
        },
      ),
    ),
  );
}