Question 176545 in Flutter Question posted in
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How 2 return String frm Future<String> in Dart 4 chat app?

In a chat app I’m trying to query Firestore and return the other user’s name as a String from a ‘Contacts’ collection. I’m still trying to wrap my head around async and Futures in dart, so when I return this, it prints ‘Instance of Future’ instead of the string that findName should query for.

return ListView(
                    children: List.generate(_conversations.length, (i) {
                      

                      DocumentReference? _conversationRef =
                          _conversations[i].reference;
                      Map _conversation = _conversations[i].data() ?? {};



                      String otherUser = _conversation["members"][0] != uid
                          ? _conversation["members"][0]
                          : _conversation["members"][1];



                      Future<String> findName(otherUser) async {
                        var recipName = await FirebaseFirestore.instance
                            .collection("contacts")
                            .where("phone_number", isEqualTo: otherUser)
                            .get().then(

                              (querySnapshot) {
                                 for (var docSnapshot in querySnapshot.docs) {

                                   return docSnapshot.data()['full_name'].toString();
                                 }
                              });


                            if (recipName != null) {
                              return recipName;
                            } else {
                              return otherUser;
                            }
                      }

                      
                      contactName() async {
                        var name = await findName(otherUser);
                        return name.toString();
                      }


                      return ConversationTile(
                          conversationReference: _conversationRef,
                          otherUserId: contactName() ?? "user number");
                    }),
Tags:

2

Answers


  1. 0

    If you are building UI off of a future, you want to use FutureBuilder, it makes controlling the state of the result of the future and rendering the ui once the future is complete. Below is a simple example of FutureBuilder, i can provide more details if needed.

    FutureBuilder(
 future: getMessages(),
 builder:(context, snapshot) {
  if (snapshot.hasData) { // Future Complete 
   return ListView.builder(itemBuilder:(context, index) {
    //build listview 
   },);
  }
 },
 return Center(child: CircularProgressIndicator()); // Future Not Complete / No Data
),
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  2. 0

    In Dart you can return a String from a Future by using the async and await keywords. Follow this link https://dart.dev/codelabs/async-await

    In your case updated code will be-

    return ListView(
  children: List.generate(_conversations.length, (i) {
    DocumentReference? _conversationRef = _conversations[i].reference;
    Map _conversation = _conversations[i].data() ?? {};

    String otherUser = _conversation["members"][0] != uid
        ? _conversation["members"][0]
        : _conversation["members"][1];

    Future<String> findName(String otherUser) async {
      var querySnapshot = await FirebaseFirestore.instance
          .collection("contacts")
          .where("phone_number", isEqualTo: otherUser)
          .get();

      for (var docSnapshot in querySnapshot.docs) {
        return docSnapshot.data()['full_name'].toString();
      }

      return otherUser;
    }

    return FutureBuilder<String>(
      future: findName(otherUser),
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          return ConversationTile(
            conversationReference: _conversationRef,
            otherUserId: snapshot.data!,
          );
        } else if (snapshot.hasError) {
          return Text("Error: ${snapshot.error}");
        } else {
          return CircularProgressIndicator();
        }
      },
    );
  }),
);
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