Flutter Firebase A value of type 'List<Future<CustomClass>>' can't be assigned to a variable of type 'List< CustomClass >?'

Question

I am having an issue with my FutureBuilder in flutter. It's supposed to map the custom class 'GeoPost' to a 'List' from a firebase query but somewhere in my code it is not reading the Geopost list as a non Future

here is the code where the error occurs:

FutureBuilder(
            future: postsRef.get().then((doc) {
              geoPosts = doc.docs.map((doc) async => await GeoPost.fromDoc(doc)).toList();
            }),
            builder: (context, snapshot) { }

custom class and functions:

    class GeoPost {
      final String caption;
      final String postId;
      final String authorId;
      final int likesCount;
      final int commentCount;
      final Position position;
      final Timestamp timeStamp;
      final String? imageUrl;
      final Userdata authordata;
    
      GeoPost(this.caption, this.postId, this.authorId, this.likesCount, this.commentCount, this.position, this.timeStamp, this.imageUrl, this.authordata);
    
      static Future<GeoPost> fromDoc(DocumentSnapshot doc) async {
        return GeoPost.fromDocument(doc, await getPostUserDataFromFirestore(doc['userId']));
    
      }
    
      factory GeoPost.fromDocument(DocumentSnapshot doc, Userdata author) {
        return GeoPost(
            doc['caption'],
            doc['postId'],
            doc['userId'],
            doc['likeCount'] ?? 0,
            doc['commentCount'] ?? 0,
            doc['position'],
            doc['timeStamp'],
            doc['imageUrl'],
            author
        );
      }

Future<void> getPostUserDataFromFirestore (String did) async {
    return Userdata.fromDoc(await usersRef.doc(uid).get());
  }
    }

error:

Error: A value of type 'List<Future<GeoPost>>' can't be assigned to a variable of type 'List<GeoPost>?'.

Answer 1

• You need to wait for all the Futures in the List<Future<GeoPost> (which converts them into a List<GeoPost>) before assiging them to a variable of type List<GeoPost>.

  Update your FutureBuilder code to this:

  FutureBuilder<List<GeoPost>>(
    future: () async { 
      var doc = await FirebaseFirestore.instance.collection('collectionPath').get();
      var data = doc.docs.map((doc) async => await GeoPost.fromDoc(doc)).toList();
      geoPosts = await Future.wait(data);
      return geoPosts;
    }(),
    builder: (context, snapshot) { }
  

• Also, you need to change the return type of the getPostUserDataFromFirestore method from Future<void> to Future<Userdata>. void is used when there is no value returned.

  Update the getPostUserDataFromFirestore method to this:

  Future<Userdata> getPostUserDataFromFirestore (String did) async {
    return Userdata.fromDoc(await usersRef.doc(uid).get());
  }