非null字符串必须提供给文本窗口小部件

Question

List<Question> questionBank =[
    Question(b: 'You can lead a cow down stairs but not up stairs.', a: true) ,
    Question(b: 'Approximately one quarter of human bones are in the feet.',a: false),
    Question(b: 'A slug\'s blood is green.',a: false),
  ];
  int questionNum=0;
  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.end,
      children: <Widget>[
        Expanded(
          flex: 5,
          child: Padding(
            padding: EdgeInsets.all(15.0),
            child: Center(
              child: Text(
               questionBank[questionNum].questionText,
                style: TextStyle(
                  fontSize: 30.0,
                  fontWeight: FontWeight.bold,
                  color: Colors.white,
                ),
              ),
            ),
          ),
        ),

我尝试从“ Question”类获取Text，并且将“ questionNum”用作索引。 我曾尝试Text(questionBamk[questionnum].questionText??'default value显示“默认值”。 这是我一直在使用的班级代码

class Question {
  String questionText ;
  bool questionAnwsear;
  Question ({String b ,bool a}){
    b= questionText ;
    a= questionAnwsear;
  }
}

Answer 1

尝试

questionBank[questionNum].questionText!=null?questionBank[questionNum].questionText : 'loading ...'


//change your class like this,
class Question {
  String questionText ;
  bool questionAnwsear;
  Question({this.questionText ,this.questionAnwsear});
}

//then call like 

Question(questionText: 'You can lead a cow down stairs but not up stairs.', questionAnwsear : true);