My future builder widget keep returns null value?

Question

This is my Train Client class and getName method returns me a list of Train objects. I check it by using printing statement it worked successfully, but when I am returning I am not sure whether it's returning properly

class TrainClient {
  

  static Future<List<Train>> getName() async {

    final uri = Uri.parse("http://localhost:3000/search?from=54&to=61&date=2021-08-11");

    final response = await get(uri);



       final data = json.decode(response.body);

       final result = data["RESULTS"]["directTrains"]["trainsList"];
       final list =  result.map((json) => Train.fromJson(json)).toList();
       print("TrainClient");
       print(list);
       return list;





  }

}

Output got when I called this method

[Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train', Instance of 'Train']

But when I use a future builder here I am always getting null value, first, the screen is red, and after it changes to green and after that nothing returns. But I want DataTable to return

Here is the code of my future builder

class TimeTable extends StatefulWidget {
  final url;
  const TimeTable({Key? key,required this.url}) : super(key: key);

  @override
  _TimeTableState createState() => _TimeTableState();
}

class _TimeTableState extends State<TimeTable> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(body:FutureBuilder(
      future:TrainClient.getName() ,
      builder: (context,AsyncSnapshot<List<Train>> snapshot){
        if(snapshot.connectionState==ConnectionState.done){
          print(snapshot.data);
          if(snapshot.data != null) {
            return buildDataTable(trains: snapshot.data!);
          }
          return Container(color:Colors.green);
        }
        return Container(color: Colors.red,);
      },
    ) );

  }}

I also got snapshot.data== null when I print it. Why is that??? Why it's keep returning null

If you get data from API go to my answer here https://stackoverflow.com/a/68533647/13997210 or here https://stackoverflow.com/a/68594656/13997210 — Ravindra S. Patil, Aug 05 '21 at 05:10
@RavindraS.Patil snapshot.hasData returns me false. My snapshot has no data but future method prints correctly the list of output and returning the list, but snapshot saying no data is retuning by the that method — Vinasirajan Vilakshan, Aug 05 '21 at 05:22
Yes when I call print(list) however that returns me list which has Train objects, and I printed and checked is that returning object correctly. — Vinasirajan Vilakshan, Aug 05 '21 at 05:25
Can you show one of the data on the list? Try `print(list[0]."variable name")`. Its only to check whether the data is saved correctly to each of Train object — Ananda Pramono, Aug 05 '21 at 05:34
remove static keyword before your Future call and try `if(snapshot.hasData)` insted of `if(snapshot.connectionState==ConnectionState.done)` — Ravindra S. Patil, Aug 05 '21 at 05:34
COLOMBO COMMUTER it prints me successfully , here is the out put of that — Vinasirajan Vilakshan, Aug 05 '21 at 05:35
@AnandaPramono After using this I found that my function returns me a List rather List, how can I solve that, but if I print the list it's having train objects inside it — Vinasirajan Vilakshan, Aug 05 '21 at 05:58

Ananda Pramono · Accepted Answer · 2021-08-05T06:54:23.113

1

You can try this. It also used to check whether the snapshot has error

 Widget build(BuildContext context) {
        return Scaffold<List<Train>>(body:FutureBuilder(
          future:TrainClient.getName() ,
          builder: (BuildContext context, AsyncSnapshot snapshot){
            if(snapshot.hasError){
                print(snapshot)
            } else
            if(snapshot.hasData){
                print(snapshot.data);
                return buildDataTable(trains: snapshot.data!);
            }
              return Container(color:Colors.green);
            }
            return Container(color: Colors.red,);
          },
        ) );

And define the return type in the TrainClient

return List<Train>.from(result.map((item) => Train.fromJson(item)));

edited Aug 05 '21 at 06:54

answered Aug 05 '21 at 05:41

Ananda Pramono

899
1
6
18

After using this I found that my function returns me a List rather List, how can I solve that, but if I print the list it's having train objects inside it – Vinasirajan Vilakshan Aug 05 '21 at 05:58
1

Try to define the return type in the TrainClient `return List.from(result.map((item) => Train.fromJson(item)));` – Ananda Pramono Aug 05 '21 at 06:00
1

@VinasirajanVilakshan Sure, please mark this as the correct answer so it can help other people too. – Ananda Pramono Aug 05 '21 at 06:51

score 0 · Answer 2 · answered Aug 05 '21 at 05:24

what about using dio for api integrations and this json converter to handle your responses? by the way. The getNamefunction will be called every time the widget is rebuilt. To avoid the function to be called every time the widget is rebuilt, you must not create the Future inside State.build or StatelessWidget.build method. You must create the future outside the future builder earlier. So create a local var _listName and int State.didChangeDependencies, or State.didUpdateWidget.

class TimeTable extends StatefulWidget {
  final url;
  const TimeTable({Key? key,required this.url}) : super(key: key);

  @override
  _TimeTableState createState() => _TimeTableState();
}

class _TimeTableState extends State<TimeTable> {
Future<List<Train>> _value;
 @override
  initState() {
    super.initState();
    _value = getName();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(body:FutureBuilder(
      future:_value ,
      builder: (context,AsyncSnapshot<List<Train>> snapshot){
        if(snapshot.connectionState==ConnectionState.done){
          print(snapshot.data);
          if(snapshot.data != null) {
            return buildDataTable(trains: snapshot.data!);
          }
          return Container(color:Colors.green);
        }
        return Container(color: Colors.red,);
      },
    ) );

  }}

My future builder widget keep returns null value?

2 Answers2