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Я изучаю, как анализировать файлы JSON в flutter, но когда я получаю данные на экране, они исчезают через секунду. и когда я обновляю экран, он появляется с 4-го или 5-го числа, а затем снова исчезает. Я не знаю причины. пожалуйста, помогите. ниже приведен код.

 import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:convert';



class Loadjsonagain extends StatelessWidget {
  List<String> selectedAnsr;
  Loadjsonagain({Key key, this.selectedAnsr})
      : super(key: key);


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: FutureBuilder(
        future: DefaultAssetBundle.of(context).loadString(
            'assets/question_file.json'),
        builder: (context, snapshot) {
          List mydata = json.decode(snapshot.data.toString());
          if (mydata == null) {
            return Center(
              child: Text('Loading'),
            );
          }
          else {
            return AnswersExplanation(mydata: mydata, myanswers: selectedAnsr,);
          }
        },
      ),
    );
  }
}






class AnswersExplanation extends StatefulWidget {

  final mydata;
  final myanswers;

  AnswersExplanation({Key key, @required this.mydata, this.myanswers})
      : super(key: key);

  @override
  _AnswersExplanationState createState() =>
      _AnswersExplanationState(mydata: mydata, myanswers: myanswers);
}

class _AnswersExplanationState extends State<AnswersExplanation> {

  List mydata;
  List myanswers;
  _AnswersExplanationState({this.mydata, this.myanswers});


  int i = 1;



question with timer

  Widget nextquestion(){
        if(i <= mydata[0].length){
          print(i);
          i  ;
          return buildCard(i);
        }
        else{
          return Container();
        }
  }





  @override
  void initState() {
    super.initState();
  }









  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body:
        ListView.separated(
          itemCount: mydata[0].length,
          itemBuilder: (context, index) {
            return nextquestion();
          },
          separatorBuilder: (context, index) {
            return Divider();
          },
        ),
    );
  }



  Widget buildCard(int k){
      return Padding(
        padding: EdgeInsets.only(top: 15, right: 15, left: 15),
        child: Card(
          elevation: 10,
          child: Column(
            children: [



              Text('hello'),

              Text(
                'Q$k. ${mydata[0][k.toString()]}',
              ),

              Text(
                mydata[2][k.toString()],
              ),

              Text(
                myanswers[i],
              ),

              

              Text(
                mydata[3][k.toString()],
              ),


            ],
          ),
        ),
      );
  }

}


 

и ниже приведен пример файла JSON, который я использую -:

 [
  {
    "1": "What Will Be The Output Of the Following Code ?na = "p" * 3nprint(a)",
    "2": "Which function finds out the Variable type in Python ?",
    "3": "Which of the following keyword is used to define a function in Python ?",
    "4": "Which of the following is a print Function in Python ?",
    "5": "Which function finds out the Variable type in Python ?",
    "6": "Which of the following keyword is used to define a function in Python ?",
    "7": "Which function finds out the Variable type in Python ?",
    "8": "Which of the following keyword is used to define a function in Python ?",
    "9": "Which function finds out the Variable type in Python ?",
    "10": "Which of the following keyword is used to define a function in Python ?"
  },

  {
    "1": {
      "a": "pp",
      "b": "ppp",
      "c": "3p",
      "d": "p3"
    },
    "2": {
      "a": "typedef",
      "b": "typeof",
      "c": "type",
      "d": "find"
    },
    "3": {
      "a": "func",
      "b": "def",
      "c": "void",
      "d": "function"
    },
    "4": {
      "a": "cout",
      "b": "print",
      "c": "println",
      "d": "stderr"
    },
    "5": {
      "a": "typedef",
      "b": "typeof",
      "c": "type",
      "d": "find"
    },
    "6": {
      "a": "func",
      "b": "def",
      "c": "void",
      "d": "function"
    },
    "7": {
      "a": "typedef",
      "b": "typeof",
      "c": "type",
      "d": "find"
    },
    "8": {
      "a": "func",
      "b": "def",
      "c": "void",
      "d": "function"
    },
    "9": {
      "a": "typedef",
      "b": "typeof",
      "c": "type",
      "d": "find"
    },
    "10": {
      "a": "func",
      "b": "def",
      "c": "void",
      "d": "function"
    }
  },


  {
    "1": "ppp",
    "2": "type",
    "3": "def",
    "4": "print",
    "5": "type",
    "6": "def",
    "7": "type",
    "8": "def",
    "9": "type",
    "10": "def"
  },


  {
    "1": "Explanation of 1st question",
    "2": "Explanation of 2nd question",
    "3": "Explanation of 3rd question",
    "4": "Explanation of 4th question",
    "5": "Explanation of 5st question",
    "6": "Explanation of 6st question",
    "7": "Explanation of 7st question",
    "8": "Explanation of 8st question",
    "9": "Explanation of 9st question",
    "10": "Explanation of 10st question"
  }
]

 

дайте мне знать, если есть другой способ сделать это…
а ниже приведена ошибка, которую я получаю -:

 ════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following assertion was thrown building:
A non-null String must be provided to a Text widget.
'package:flutter/src/widgets/text.dart':
Failed assertion: line 379 pos 10: 'data != null'

When the exception was thrown, this was the stack: 
#2      new Text (package:flutter/src/widgets/text.dart:379:10)
#3      _AnswersExplanationState.buildCard (package:education/Quiz_Screens/Answers_Explanations_Screen.dart:132:15)
#4      _AnswersExplanationState.nextquestion (package:education/Quiz_Screens/Answers_Explanations_Screen.dart:71:18)
#5      _AnswersExplanationState.build.<anonymous closure> (package:education/Quiz_Screens/Answers_Explanations_Screen.dart:103:20)
#6      new ListView.separated.<anonymous closure> (package:flutter/src/widgets/scroll_view.dart:1261:34)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
 

Сама ошибка является пояснительной. Вы должны проверить наличие TextView, особенно внутри buildCard. Там было много textview, вы должны отлаживать 1 на 1, если mydata содержит какие-либо данные. Также внутри Text() вы можете проверить, как это, чтобы увидеть, является ли оно null или нет, просто чтобы убедиться, что текст пустой или нет.

 Text(myanswers[i] != null ? myanswers[i] : ''),
 

Я полагаю, что во время future builder вы можете это сделать.

 if (snapshot.ConnectionState == ConnectionState.waiting) {
return Center(child: CircularLoadingIndicator); 
} else {
 List mydata = json.decode(snapshot.data.toString());
 return AnswersExplanation(mydata: mydata, myanswers: selectedAnsr);
}
 

Я получил это, удалив метод nexquestion() и поместив весь текст в перевернутую запятую, например

  Text(
                **'Q$i. ${mydata[0][i.toString()]}'**,
              ),

              Text(
                **'Correct Answer - ${mydata[2][i.toString()]}'**,
              ),
 

теперь данные сохраняются.