使用来自另一个 class 的实例访问有状态小部件的变量时，在 null 上调用的 getter 长度

Question

Initially, I used the page route to send the data to the next page(Question). (The data is a List Type)

Navigator.pushReplacement(context,MaterialPageRoute(
                                        builder: (context) => Question(
                                            questions: this.questions)));

Then I received the value in the Question Stateful class using the constructor and set its value to another variable.

Question Class:

class Question extends StatefulWidget {
  final List<QuestionModel> questions;

    const Question({this.questions});

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

class _QuestionState extends State<Question> {
  @override
  void initState() {
    super.initState();
    print(widget.questions); //<----------working
  }

  @override
  Widget build(BuildContext context){
  return Scaffold(
    body: Column(
      children: <Widget>[
        Timer(),
        SizedBox(height: 40),
        Expanded(child: QuestionBuilder())
      ],
    ),
  );}

when I tried to print the questions List in the Question state, it is perfectly working.

But I need that value in another class QuestionBuilder. So, I accessed the value using an instance of the Question Class.

 class QuestionBuilder extends StatefulWidget {
      @override
      _QuestionBuilderState createState() => _QuestionBuilderState();
    }

        class _QuestionBuilderState extends State<QuestionBuilder> {
          List<QuestionModel> questions;

          @override
          void initState() {
            super.initState();
            questions = Question().questions;
            print(questions); //<---------------not working
          } 

         @override
       Widget build(BuildContext context) {
    return PageView.builder(
      itemCount: questions.length ?? 0,
      itemBuilder: (context, index) {
        return SingleChildScrollView(
          child: Column(
            children: <Widget>[
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 20),
                child: Row(
                  crossAxisAlignment: CrossAxisAlignment.end,
                  children: <Widget>[
                    Text(
                      "Question  ${index + 1} / ",
                      style: Styles.questionNumberTextStyle,
                    ),
                    Text(
                      "7",
                      style:
                          Styles.questionNumberTextStyle.copyWith(fontSize: 20),
                    ),
                  ],
                ),
              ),
              SizedBox(height: 20),
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 20),
                child: Text(
                  questions[index].question,
                  style: Styles.questionTextStyle,
                  textAlign: TextAlign.left,
                ),
              ),
              SizedBox(height: 20),
              option(option: questions[index].option_1, onTap: () {}),
              option(option: questions[index].option_2, onTap: () {}),
              option(option: questions[index].option_3, onTap: () {}),
              option(option: questions[index].option_4, onTap: () {}),
              SizedBox(height: 20),
            ],
          ),
        );
      },
    );
  }}

Now When I print the questions List it is null and further when I used it in a PageView or ListView, the error shows up that the getter length was called on null.

I don't know what causes the problem. Moreover, when tried to manually assign a dummy list value to the question list in Question class. It works in the QuestionBuilder class

Answer 1

The problem is you are creating a new instance of Question class by using this code:

questions = Question().questions;

One of the simple way is to pass the value in params. For example in question class pass the data in parameter where you are calling QuestionBuilder() like this:

 @override
  Widget build(BuildContext context){
  return Scaffold(
    body: Column(
      children: <Widget>[
        Timer(),
        SizedBox(height: 40),
        Expanded(child: QuestionBuilder(widget.questions),
)
      ],
    ),
  );}

Now define a variable to access this value passed in the param as follows:

 class QuestionBuilder extends StatefulWidget {
final List<QuestionModel> questions;
QuestionBuilder(this.questions);
      @override
      _QuestionBuilderState createState() => _QuestionBuilderState();
    }

Now in initState of QuestionBuilder access the questionList as follows:

 @override
          void initState() {
            super.initState();
            questions = widget.questions;
            print(questions); //<---------------this will work now
          } 

So now your corrected code would be, For Question class:

class Question extends StatefulWidget {
  final List<QuestionModel> questions;

    const Question({this.questions});

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

class _QuestionState extends State<Question> {
  @override
  void initState() {
    super.initState();
    print(widget.questions); 
  }

  @override
  Widget build(BuildContext context){
  return Scaffold(
    body: Column(
      children: <Widget>[
        Timer(),
        SizedBox(height: 40),
        Expanded(child: QuestionBuilder(widget.questions))
      ],
    ),
  );}

and QuestionBuilder would be:

class QuestionBuilder extends StatefulWidget {
final List<QuestionModel> questions;
QuestionBuilder(this.questions);
      @override
      _QuestionBuilderState createState() => _QuestionBuilderState();
    }

        class _QuestionBuilderState extends State<QuestionBuilder> {
          List<QuestionModel> questions;

          @override
          void initState() {
            super.initState();
            questions = widget.questions;
            print(questions); //<---------------not working
          } 

         @override
       Widget build(BuildContext context) {
    return PageView.builder(
      itemCount: questions.length ?? 0,
      itemBuilder: (context, index) {
        return SingleChildScrollView(
          child: Column(
            children: <Widget>[
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 20),
                child: Row(
                  crossAxisAlignment: CrossAxisAlignment.end,
                  children: <Widget>[
                    Text(
                      "Question  ${index + 1} / ",
                      style: Styles.questionNumberTextStyle,
                    ),
                    Text(
                      "7",
                      style:
                          Styles.questionNumberTextStyle.copyWith(fontSize: 20),
                    ),
                  ],
                ),
              ),
              SizedBox(height: 20),
              Padding(
                padding: const EdgeInsets.symmetric(horizontal: 20),
                child: Text(
                  questions[index].question,
                  style: Styles.questionTextStyle,
                  textAlign: TextAlign.left,
                ),
              ),
              SizedBox(height: 20),
              option(option: questions[index].option_1, onTap: () {}),
              option(option: questions[index].option_2, onTap: () {}),
              option(option: questions[index].option_3, onTap: () {}),
              option(option: questions[index].option_4, onTap: () {}),
              SizedBox(height: 20),
            ],
          ),
        );
      },
    );
  }}