在我的应用程序中，我想要调用来自 firebase 不同集合的数据。首先，我想列出所有项目并获取 id。 使用该 id 我想从价格集合中检索价格。之后我想从折扣中检索数据。为了打折。 我在这里使用循环。 在下面的代码中，输出即将到来。第一次加载列表后调用第二次收集价格。 任何人都知道解决方案。 我想听叫三集。因为如果有任何数据变化我想更新。

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


    Future _loadItems() async {

            int price;
            int discount;

            //calling first collection for getting id and name
            firestore.collection("item").snapshots().listen((itemData)async{

                for(int i=0;i<itemData.documents.length;i++){

                // calling second collection for getting price
                firestore.collection("price").where("id",isEqualTo: itemData.documents[i].data["id"])
                .snapshots().listen((priceData) async{
                        price=priceData.documents[0].data['price'];
                        debugPrint("price showing before loading:"+price.toString());

                                //calling third collection for getting discount
                                firestore.collection("discount")
                                .where("id",isEqualTo: itemData.documents[i].data["id"])
                                .snapshots().listen((discountData) async{
                                    for(int j=0;j<discountData.documents.length;j++){
                                        discount=discountData.documents.data['discount'];
                                    }
                                });

                });

                    setState(() {
                    debugPrint("price showing after loading:"+price.toString());
                    this.documents.add(new CartProduct(
                        name:itemData.documents[i].data["id"],
                        label:itemData.documents[i].data["label"],
                        price:price,
                        discount:discount

                    ));
                    });

                }
        });


    }

当前输出

    price showing after loading:0
    price showing after loading:0
    price showing after loading:0
    price showing before loading:10.0
    price showing before loading:10.0
    price showing before loading:10.0

预期输出

    price showing before loading:10.0
    price showing before loading:10.0
    price showing before loading:10.0
    price showing after loading:10.0
    price showing after loading:10.0
    price showing after loading:10.0

最佳答案

我觉得你可以使用嵌套的 StreamBuilder

Widget getTripleCollectionFromFirebase() {
  return StreamBuilder<QuerySnapshot>(
    stream: Firestore.instance.collection("item").snapshots(),
    builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
      if (snapshot.hasError) return Text("Error: ${snapshot.error}");
      switch (snapshot.connectionState) {
        case ConnectionState.none:
          return Text("No data, yet.");
        case ConnectionState.waiting:
          return Text('Loading...');
        case ConnectionState.active:
        case ConnectionState.done:
          if (snapshot.data == null) {
            return Text("No record");
          } else {
            // Do your staff after first query then call the other collection
            return StreamBuilder<QuerySnapshot>(
              stream: Firestore.instance
                  .collection("price")
                  .where("id", isEqualTo: "fill_it_with_your_code")
                  .snapshots(),
              builder: (BuildContext context,
                  AsyncSnapshot<QuerySnapshot> snapshot) {
                if (snapshot.hasError) return Text("Error: ${snapshot.error}");
                switch (snapshot.connectionState) {
                  case ConnectionState.none:
                    return Text("No data, yet.");
                  case ConnectionState.waiting:
                    return Text('Loading...');
                  case ConnectionState.active:
                  case ConnectionState.done:
                    if (snapshot.data == null) {
                      return Text("No record");
                    } else {
                      // do your staff after second Query
                      return StreamBuilder<QuerySnapshot>(
                        stream: Firestore.instance
                            .collection("discount")
                            .where("id", isEqualTo: "something")
                            .snapshots(),
                        builder: (BuildContext context,
                            AsyncSnapshot<QuerySnapshot> snapshot) {
                          if (snapshot.hasError)
                            return Text("Error: ${snapshot.error}");
                          switch (snapshot.connectionState) {
                            case ConnectionState.none:
                              return Text("No data, yet.");
                            case ConnectionState.waiting:
                              return Text('Loading...');
                            case ConnectionState.active:
                            case ConnectionState.done:
                              if (snapshot.data == null) {
                                return Text("No record");
                              } else {
                                // do your staff after third Query
                                // return the widget which you want to build when all data comes.
                              }
                          }
                        },
                      );
                    }
                }
              },
            );
          }
      }
    },
  );
}

关于dart - firebase listen 中的 for 循环没有更新，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54806400/