How to use a Future in a StreamBuilder in Flutter

Question

I'm trying to build my first mobile application with flutter and firebase.

• Error: The argument type 'Future Function()' can't be assigned to the parameter type 'String'

   class ShowUserData extends StatefulWidget {
     @override
     _ShowUserDataState createState() => _ShowUserDataState();
   }
  
   class _ShowUserDataState extends State<ShowUserData> {
  
     final email = () {
       final userEmail =
           FirebaseAuth.instance.currentUser().then((user) => user.email);
       print('userEmail + $userEmail');
       return userEmail;
     };
  
     @override
     Widget build(BuildContext context) {
       return StreamBuilder(
           // stream: user.getUserData().asStream(),
           stream: Firestore.instance.collection('users').document(email).snapshots(),
           builder: (context, snapshot) {
             if (!snapshot.hasData) {
               return Text("Loading");
             }
  
             print('snapshot + $snapshot');
             var userDocument = snapshot.data;
             print('userDocument + $userDocument');
             return Text(userDocument['firstName']);
           });
     }
   }
  

Answer 1

For the first part of your code, consider something like this:

   Future<String> getUserEmail() async {
     final userEmail =
         FirebaseUser user = await FirebaseAuth.instance.currentUser();
     String userEmail = user.email;
     print('userEmail + $userEmail');
     return userEmail;
   };

That should solve that error (hence answering your question). Although the rest of the code will still fail. You should read up on futures and async programming.

---

Now my suggested solution for the whole thing.
Since you have 2 different async sources that create your stream, you might want to consider creating your own Stream! (Yes, streams are powerful like that).

In sequence, you’re wanting to

• await on the future: .currentUser()
• use the result of that to create your firestore snapshot stream

Try something like this

class ShowUserData extends StatefulWidget {
   @override
   _ShowUserDataState createState() => _ShowUserDataState();
 }

 class _ShowUserDataState extends State<ShowUserData> {

   Stream currentUserStream() async* {
     FirebaseUser user = await FirebaseAuth.instance.currentUser();
     String userEmail = user.email;
     yield* Firestore.instance.collection('users').document(userEmail).snapshots();     
   };

   @override
   Widget build(BuildContext context) {
     return StreamBuilder(
         // stream: user.getUserData().asStream(),
         stream: currentUserStream(),
         builder: (context, snapshot) {
           if (!snapshot.hasData) {
             return Text("Loading");
           }

           print('snapshot + $snapshot');
           var userDocument = snapshot.data;
           print('userDocument + $userDocument');
           return Text(userDocument['firstName']);
         });
   }
 }