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

Question

最初，我使用页面路由将数据发送到下一页（问题）。（数据为列表类型）

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

然后我使用构造函数在 Question Stateful class 中接收到值并将其值设置为另一个变量。

问题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())
      ],
    ),
  );}

当我尝试在问题状态下打印问题列表时，它运行良好。

但我需要另一个 class QuestionBuilder 中的值。因此，我使用问题 Class.

的实例访问了该值

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

现在，当我打印问题列表时，它是空的，而且当我在 PageView 或 ListView 中使用它时，错误显示 getter 长度是对空调用的。

我不知道是什么原因造成的。此外，当尝试手动将虚拟列表值分配给问题 class 中的问题列表时。它适用于 QuestionBuilder class

Answer 1

问题是您正在使用以下代码创建问题 class 的新实例：

questions = Question().questions;

其中一种简单的方法是将值传递给params。例如，问题 class 在您调用 QuestionBuilder() 的参数中传递数据，如下所示：

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

现在定义一个变量来访问参数中传递的这个值，如下所示：

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

现在在 QuestionBuilder 的 initState 中访问问题列表如下：

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

所以现在你更正的代码是，对于问题 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))
      ],
    ),
  );}

而 QuestionBuilder 将是：

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

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

getter length called on null when accessing a variable of a stateful widget using its instance from another class

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