Question

我是该语言的初学者，我不知道该如何处理。我想要的这段代码是getDocs（）函数将从firestore中取出数据。发生这种情况后，statefull小部件将从列表中构建文本，该列表将包含来自Firestore的数据。错误显而易见，表明我的列表为空：Error from device

我的代码如下：

class TestBasic extends StatefulWidget {
  int time;
  int amount;
  TestBasic({this.time, this.amount});

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

class _TestBasicState extends State<TestBasic> {

  List<Map> listOfQuestionsMap = [];
  List<Question> listOfQuestions = [];

  Future getDocs() async {
    QuerySnapshot querySnapshot =
        await Firestore.instance.collection("questions").getDocuments();
    for (int i = 0; i < widget.amount; i++) {
      var a = querySnapshot.documents[i];
      listOfQuestionsMap.add(a.data);
    }
     List<Question> listOfQuestionsQ = listOfQuestionsMap.map((doc) {
      return Question(
        answer: doc['answer'] ?? '',
        questionText: doc['questionText'] ?? '',
        type: doc['type'] ?? '',
        substantiation: doc['substantiation'] ?? '',
        article: doc['article'] ?? ''
        );
    }).toList();
    print(listOfQuestionsQ[1].answer);
    setState(() {
      listOfQuestions.addAll(listOfQuestionsQ);
    });
  }

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

  @override

  Widget build(BuildContext context) {
    return Padding(
        padding: const EdgeInsets.symmetric(horizontal: 40.0),
        child: Container(
                alignment: Alignment.center,
                height: Responsive.height(40, context),
                decoration: BoxDecoration(
                    border: Border.all(color: Colors.black, width: 1.0)),
                child: Text(listOfQuestions[0].answer,
                    textAlign: TextAlign.center,
                    style: TextStyle(
                      decoration: TextDecoration.none,
                      color: Colors.black,
                      fontSize: 20.0,
                    )),
              ),
            );

有问题模型：

class Question {
  final String type;
  final String questionText;
  final String answer;
  final String substantiation;
  final String article;

  Question({this.answer, this.questionText, this.type, this.substantiation, this.article});
}

有人可以向我解释如何获得我想要的结果吗？

Answer 1

您收到该错误是因为build是在getDocs更新列表之前被调用的，而您的listOfQuestions为空，并且您尝试在索引（0）处获取了一个值。

尝试使用：

Text(
 listOfQuestions.isEmpty ? "Loading" : listOfQuestions[0].answer
)

这样，当listOfQuestions为空时，Text显示正在加载，而当listOfQuestions有数据时，它将显示listOfQuestions[0].answer的值

我无法从未来功能内的列表中取出数据。颤振/飞镖

1 个答案: