what does type 'Future<dynamic>' is not a subtype of type 'Stream<dynamic>?' means?

Question

Can someone please let me know where im doing wrong and what changes have to be done in the code in order to overcome this error of 'Future' is not a subtype of type 'Stream?'.Earlier the error was like Future' is not a subtype of type 'Widget' for which i wrapped the wwidget undet the StreamBuilder.Doing so resulted in this error.

The below is the code which results in this error

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(),
          );
        }),
  ),
);

}

Solution

Your displayPercentage function is marked as async which makes whatever the function returns wrapped into a Future automaticallu.

You are returning a Widget which makes the function's return type Future<Widget>. But since you have not mentioned it specifically while defining the function, dart is assuming it to Future<dynamic>.

Now, coming to your StreamBuilder, the parameter stream is supposed to be assigned a Stream. But you are assigning it your function which is actually a Future<dynamic>.

This is why it is giving the error saying that, it expects a Stream<dynamic> but you are giving it a Future<dynamic> and since Future is not a sub class that extends from Stream, the error says Future<dynamic> is not a subtype of type Stream<dynamic>.

To fix this, change your StreamBuilder into a FutureBuilder like this,

@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, 
          );
        },
      ),
    ),
  );
}

Comments

[Learn]

great! That worked. But var val = await doc.data()['TotalClassesAttended']; in the displayPercentage() function changes dynamically , those realtime changes will reflect by using FutureBuilder? If not what is the alternative please let me know

[Nisanth Reddy]

In that case, you would still need to use StreamBuilder but you should get the snapshot instead of data. I suggest once you go thorugh the docs for getting realtime updates. If it is confusing tell me, Ill update the answer.

[Learn]

i have gone through the doc of StreamBuilder, but I'm a bit confused how can i use this in my case,in my code as im calling a displayPercentage() function. I'm unable to call the same function in "stream" of StreamBuilder.

[Nisanth Reddy]

Give me a few minutes, I will create a chat room and tell you there.

[Nisanth Reddy]

chat.stackoverflow.com/rooms/232816/discussion

[Learn]

I must have atleast 20 reputation on Stack Overflow to be able to join chatroom?

[Nisanth Reddy]

I have upvoted your question, you must be more than 20 now

[Learn]

Let us continue this discussion in chat.