Migrating to null safety: The argument type 'Object?' can't be assigned to the parameter type 'List'

Question

After migrating to null safety I'm getting an error on ListView as "The argument type 'Object?' can't be assigned to the parameter type 'List'." I'm getting error on return ListView(children: snapshot.data,); Can anyone help me to fix this error and build a ListView for activityfeeditem in my app?

Here is my code for activity_feed.dart,

class ActivityFeed extends StatefulWidget {
@override
_ActivityFeedState createState() => _ActivityFeedState();
}

class _ActivityFeedState extends State<ActivityFeed> {

getActivityFeed() async {
 QuerySnapshot snapshot = await activityFeedRef
     .doc(currentUser!.id)
     .collection('feedItems')
     .orderBy('timestamp', descending: true)
     .limit(50)
     .get();
 List<ActivityFeedItem> feedItems = [];
 snapshot.docs.forEach((doc) {
   feedItems.add(ActivityFeedItem.fromDocument(doc));
   print('Activity Feed Item: ${doc.data}');
 });

 return feedItems;
}

@override
Widget build(BuildContext context) {
 return Scaffold(
   backgroundColor: Colors.deepPurple[50],
   appBar: header(context, titleText: "Activity Feed"),
   body: Container(
     child: FutureBuilder(
       future: getActivityFeed(),
       builder: (context, snapshot) {
         if (!snapshot.hasData) {
           return circularProgress();
         }
         return ListView(
           children: snapshot.data,

// Here I'm getting error on `snapshot.data`

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

Widget? mediaPreview;
String? activityItemText;

class ActivityFeedItem extends StatelessWidget {
final String? username;
final String? userId;
final String? type; // 'like', 'follow', 'comment'
final String? mediaUrl;
final String? postId;
final String? userProfileImg;
final String? commentData;
final Timestamp? timestamp;

ActivityFeedItem({
 this.username,
 this.userId,
 this.type,
 this.mediaUrl,
 this.postId,
 this.userProfileImg,
 this.commentData,
 this.timestamp,
});

factory ActivityFeedItem.fromDocument(DocumentSnapshot doc) {
 return ActivityFeedItem(
   username: doc['username'],
   userId: doc['userId'],
   type: doc['type'],
   postId: doc['postId'],
   userProfileImg: doc['userProfileImg'],
   commentData: doc['commentData'],
   timestamp: doc['timestamp'],
   mediaUrl: doc['mediaUrl'],
 );
}

showPost(context) {
 Navigator.push(
     context,
     MaterialPageRoute(
         builder: (context) => PostScreen(postId: postId, userId: userId)));
}

configureMediaPreview(context) {
 if (type == "like" || type == 'comment') {
   mediaPreview = GestureDetector(
     onTap: () => showPost(context),
     child: Container(
       height: 50.0,
       width: 50.0,
       child: AspectRatio(
           aspectRatio: 16 / 9,
           child: Container(
             decoration: BoxDecoration(
               image: DecorationImage(
                 fit: BoxFit.cover,
                 image: CachedNetworkImageProvider(mediaUrl!),
               ),
             ),
           )),
     ),
   );
 } else {
   mediaPreview = Text('');
 }

 if (type == 'like') {
   activityItemText = "liked your post";
 } else if (type == 'follow') {
   activityItemText = "is following you";
 } else if (type == 'comment') {
   activityItemText = 'replied: $commentData';
 } else {
   activityItemText = "Error: Unknown type '$type'";
 }
}

@override
Widget build(BuildContext context) {
 configureMediaPreview(context);

 return Padding(
   padding: EdgeInsets.only(bottom: 2.0),
   child: Container(
     color: Colors.white54,
     child: ListTile(
       title: GestureDetector(
         onTap: () => showProfile(context, profileId: userId),
         child: RichText(
           overflow: TextOverflow.ellipsis,
           text: TextSpan(
               style: TextStyle(
                 fontSize: 14.0,
                 color: Colors.black,
               ),
               children: [
                 TextSpan(
                   text: username,
                   style: TextStyle(fontWeight: FontWeight.bold),
                 ),
                 TextSpan(
                   text: ' $activityItemText',
                 ),
               ]),
         ),
       ),
       leading: CircleAvatar(
         backgroundImage: CachedNetworkImageProvider(userProfileImg!),
       ),
       subtitle: Text(
         timeago.format(timestamp!.toDate()),
         overflow: TextOverflow.ellipsis,
       ),
       trailing: mediaPreview,
     ),
   ),
 );
}
}

showProfile(BuildContext context, {String? profileId}) {
Navigator.push(
 context,
 MaterialPageRoute(
   builder: (context) => Profile(
     profileId: profileId,
   ),
 ),
);
}

I have tried many ways but I counldn't figure out how I can fix this

New code for list

Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Colors.deepPurple[50],
        appBar: header(context, titleText: "Activity Feed"),
        body: Container(
          child: StreamBuilder<QuerySnapshot>(
              stream: activityFeedRef
                  .doc(currentUser!.id)
                  .collection('feedItems')
                  .orderBy('timestamp', descending: true)
                  .limit(50)
                  .snapshots(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return Center(
                    child: circularProgress(),
                  );
                } else
                  return ListView(
                    children: snapshot.data!.docs.map((doc) {
                      return Card(
                        child: ListTile(
                          title: GestureDetector(
                            onTap: () =>
                                showProfile(context, profileId: doc['userId']),
                            child: RichText(
                              overflow: TextOverflow.ellipsis,
                              text: TextSpan(
                                  style: TextStyle(
                                    fontSize: 14.0,
                                    color: Colors.black,
                                  ),
                                  children: [
                                    TextSpan(
                                      text: doc['username'],
                                      style: TextStyle(
                                          fontWeight: FontWeight.bold),
                                    ),
                                    TextSpan(
                                      text: ' $activityItemText',
                                    ),
                                  ]),
                            ),
                          ),
                          leading: CircleAvatar(
                            backgroundImage: CachedNetworkImageProvider(
                                doc['userProfileImg']!),
                          ),
                          subtitle: Text(
                            timeago.format(doc['timestamp']!.toDate()),
                            overflow: TextOverflow.ellipsis,
                          ),
                          trailing: mediaPreview,
                        ),
                      );
                    }).toList(),
                  );
              }),
        ));
  }

Lakmal Fernando · Accepted Answer · 2021-06-13T04:41:15.837

2

Chage your getActivityFeed

 Future<List<ActivityFeedItem>>   getActivityFeed() async {
 try{
   QuerySnapshot snapshot = await activityFeedRef
     .doc(currentUser!.id)
     .collection('feedItems')
     .orderBy('timestamp', descending: true)
     .limit(50)
     .get();
 List<ActivityFeedItem> feedItems = [];
 snapshot.docs.forEach((doc) {
   feedItems.add(ActivityFeedItem.fromDocument(doc));
   print('Activity Feed Item: ${doc.data}');
 });

 return feedItems;
}

catch (error) {
   print(error);
   return <ActivityFeedItem>[];
  }}

change you FutureBuilder as follows

FutureBuilder<List<ActivityFeedItem>>(
       future: getActivityFeed(),
       builder: (BuildContextcontext, AsyncSnapshot<List<ActivityFeedItem>> snapshot) {
         if (snapshot.hasError){
          return Center(child: Text("You have an error in loading 
          data"));   
          }
         if (snapshot.hasData) {
           return ListView(
           children: snapshot.data!,
         );
         }
         return CirclularProgressIndicator();

edited Jun 13 '21 at 04:41

answered Jun 13 '21 at 02:39

Lakmal Fernando

1,420
6
14

getting error as The argument type 'List?' can't be assigned to the parameter type 'List'. – Maldives Hunt Jun 13 '21 at 02:50
Thanks, but still its not working even though it shows no error when i run app that page keeps showing circularprogress and does not load any items, on debug console it shows as Activity Feed Item: Closure: () => Map from Function 'data':. – Maldives Hunt Jun 13 '21 at 03:07
Are you sure that you are returning `ListView` when `snapsot.hasData` is true. – Lakmal Fernando Jun 13 '21 at 03:16
@MaldivesHunt Can you add a `print(snapshot.data)` before the `if (snapshot.hasData)` to see what it's printed? – enzo Jun 13 '21 at 03:31
@enzo print output shows as null, how can i fix it? – Maldives Hunt Jun 13 '21 at 03:48
1

That means your `getActivityFeed()` function is either returning null or throwing an error (most likely). Does "Activity Feed Item: ..." gets printed in your console? – enzo Jun 13 '21 at 03:51
It shows as " Activity Feed Item: Closure: () => Map from Function 'data':." – Maldives Hunt Jun 13 '21 at 03:58
@enzo thank you. I have updated the answer check whether it complete with error – Lakmal Fernando Jun 13 '21 at 04:03
https://prnt.sc/157yig7 shows an error as shown in screenshot, The argument type 'Object? Function()' can't be assigned to the parameter type 'DocumentSnapshot – Maldives Hunt Jun 13 '21 at 04:22
change `doc.data` to only `doc` – Lakmal Fernando Jun 13 '21 at 04:26
then I get You have an error in loadingdata on my screen, if i add `for (var feedItems in snapshot.docs) {print(feedItems.data()); }` in my getActivityFeed() function then it prints my feed items on my console, how can i use it to fetch data to my list? https://prnt.sc/157zzfz – Maldives Hunt Jun 13 '21 at 04:35
I have changed the answer. the error should logged to console. what is the error. – Lakmal Fernando Jun 13 '21 at 04:42
Bad state: field does not exist within the DocumentSnapshotPlatform – Maldives Hunt Jun 13 '21 at 04:50
check whether you are correctly named the `timestamp` in the order method – Lakmal Fernando Jun 13 '21 at 04:53
i think it is correct, do you think i should change it? – Maldives Hunt Jun 13 '21 at 05:00
just try again by removing that order method – Lakmal Fernando Jun 13 '21 at 05:02
it gives same error as Bad state: field does not exist within the DocumentSnapshotPlatform, – Maldives Hunt Jun 13 '21 at 05:13
you may try to access non existing field in `ActivityFeedItem.fromDocument` constructor. – Lakmal Fernando Jun 13 '21 at 05:24
how can i fix it? – Maldives Hunt Jun 13 '21 at 05:45
Thank you @sajithlakmal your code actually fix the error i got due to migration, i think the error due to which feeds were not loading was not related to migration, now can load the list but still i can not get ' $activityItemText', it shows null – Maldives Hunt Jun 17 '21 at 12:55
can you show the out put of the `print('Activity Feed Item: ${doc.data}');` – Lakmal Fernando Jun 17 '21 at 15:43
@sajithlakmal i have updated and added new code i'm using, now i'm adding a `StreamBuilder` instead of `FutureBuilder` – Maldives Hunt Jun 17 '21 at 18:22

score 0 · Answer 2 · answered Jun 22 '21 at 17:31

0

You can also use as.

ListView(
  children: object as List<Widget>,
)

answered Jun 22 '21 at 17:31

iDecode

22,623
19
99
186

Russ Westwood · Answer 3 · 2021-10-11T18:33:37.110

I see that you are using a StreamBuilder instead of FutureBuilder, but for what its worth, I believe I have found a solution to the original FutureBuilder problem.

First of all: Using the following print statements, you can troubleshoot the issue better, I found that I was Querying for paths that didnt exist with combinations of wrong .doc(userId) and .doc(ownerId) in posts.dart so the deserialization process wasn't working correctly for me when switching between users during debugging (used a provider package to remedy this eventually) but the below print statements did help me identify some issues that I had (that may or may not have contributed to the problem for you, but worth the look).

getActivityFeed() async {
QuerySnapshot snapshot = await FirebaseFirestore.instance
    .collection('feed')
    .doc(_auth.currentUser!.uid)
    .collection('feedItems')
    .orderBy('timestamp', descending: true)
    .limit(50)
    .get();

List<ActivityFeedItem> feedItems = [];
snapshot.docs.forEach((doc) {
  feedItems.add(ActivityFeedItem.fromDocument(doc));
  print('Activity Feed Item: ${doc.id}');
  print('Activity Feed Item: ${doc.data()}');
});

// return feedItems;
return snapshot.docs;

Then I found that the deserialization process wasn't working correctly due to the difference between 'likes' and 'comments', due to the 'likes' having 7 inputs and 'comments' having 8 inputs. Comments have the extra 'commentData' input which I set up manually for the 'likes' in the addLikeToActivityFeed() part, and set it to an empty string as such:

'commentData': '',

Finally, I added a Dynamic type to the FutureBuilder to get rid of the => argument type 'Object?' can't be assigned to the parameter type 'List' error...

Container(
            child: FutureBuilder<dynamic>(
              future: getActivityFeed(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return circularProgress();
                }
                return ListView(
                  children: snapshot.data,
                );
              },
            ),
          ),

Migrating to null safety: The argument type 'Object?' can't be assigned to the parameter type 'List'

3 Answers3