在带有颤振的Firebase中使用流检索数据

Question

也许这是一个新手问题，但我正在学习颤振和一些东西，如异步、等待、未来，还不适合我的想法。无论如何，我要做的是从“field.documents[index][“name”]”中获取值，并构建一个列表。这是我的代码：

String productName;

Stream<QuerySnapshot> productRef = Firestore.instance
    .collection("stores")
    .document(name)
    .collection("products")
    .snapshots();
productRef.forEach((field) {
  field.documents.asMap().forEach((index, data) {
    productName = field.documents[index]["name"];
    //IF I PRINT HERE, IT SHOWS THE PRODUCTS. 
  });
});
BUT IF I PRINT HERE, I GOT A NULL VALUE

我想让productName在foreach之外。当我打印时，它首先打印null。我也会发布我的整个函数：

List mapToList({DocumentSnapshot doc, List<DocumentSnapshot> docList}) {
    if (docList != null) {
      List<Store> storeList = [];
      docList.forEach((document) {
        String name = document.data[StringConstant.nameField];
        num score = document.data[StringConstant.scoreField];
        String delivery = document.data[StringConstant.deliveryField];
        String photo = document.data[StringConstant.photoField];
        String description = document.data[StringConstant.descriptionField];
        String open = document.data[StringConstant.openField];
        String close = document.data[StringConstant.closeField];
        GeoPoint geoPoint = document.data[StringConstant.positionField]
            [StringConstant.geopointField];

        bool isOpen = document.data[StringConstant.isOpenField];
        final currentHour = DateTime.now();
        final openHour = DateTime.parse(open).hour;
        final closeHour = DateTime.parse(close).hour;
        int openMin = DateTime.parse(open).minute;
        int closeMin = DateTime.parse(close).minute;
        if (openHour <= currentHour.hour && currentHour.hour <= closeHour) {
          isOpen = true;
          if ((currentHour.hour == openHour && currentHour.minute < openMin) ||
              (currentHour.hour == closeHour &&
                  currentHour.minute > closeMin)) {
            isOpen = false;
          }
        } else {
          isOpen = false;
        }
        final double meter = distance(
          LatLng(latitude, longitude),
          LatLng(geoPoint.latitude, geoPoint.longitude),
        );

        String productName;

        Stream<QuerySnapshot> productRef = Firestore.instance
            .collection("stores")
            .document(name)
            .collection("products")
            .snapshots();
        productRef.forEach((field) {
          field.documents.asMap().forEach((index, data) {
            productName = field.documents[index]["name"];
          });
        });

        if (meter <= range && isOpen == true) {
          Store otherStore = Store(name, photo, score.toDouble(), delivery,
              meter, description, productName, 10.0, "");
          storeList.add(otherStore);
        }
      });
      return storeList;
    } else {
      return null;
    }
  }

谁能帮帮我吗？我知道它与异步编程有关，但我正在学习。谢谢

Answer 1

我要做的是：

List<String> productName= [];

Stream<QuerySnapshot> productRef = Firestore.instance
    .collection("stores")
    .document(name)
    .collection("products")
    .snapshots();
productRef.forEach((field) {
  field.documents.asMap().forEach((index, data) {
    productName.add(field.documents[index]["name"]);
  });
});

您可以在查询外部初始化一个数组，以便每次“.”时都可以向其中添加一个字符串。forEach’迭代。除了在函数中手动输入每个数据外，您还可以创建一个数据模型，将数据库中的数据存储到一个局部变量中，该变量比手动字段更灵活。文档['dataField']。以下是一些有用的链接：https://medium.com/flutter-community/parsing-complex-json-in-flutter-747c46655f51