StreamBuilder<QuerySnapshot>（脏，状态：_StreamBuilderBaseState<QuerySnapshot，AsyncSnapshot<QuerySnapshot>）：

Question

我无法从 Firestore 中检索数据并出现如下错误，

════════ 小部件库捕获的异常 ══════════════════════════════════════════════════ ═════以下 断言被抛出构建 StreamBuilder(dirty, 状态：_StreamBuilderBaseState#e568b): 构建函数返回 null。

有问题的小部件是：StreamBuilder 构建函数 绝不能返回 null。

返回导致建筑小部件填充的空白空间 可用空间，返回“Container()”。返回一个空白空间 占用尽可能少的空间，返回 "Container(width: 0.0, 高度：0.0)"。

相关的导致错误的小部件是：StreamBuilder file:...dart:140:15 抛出异常时，这是堆栈：

#0 debugWidgetBuilderValue。 （包：flutter/src/widgets/debug.dart:300:7） #1 _Closure.call (dart:core-patch/function.dart) #2 debugWidgetBuilderValue (package:flutter/src/widgets/debug.dart:321:4) #3 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4569:7) #4 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4737:11) ...

---

下面是我的代码。

StreamBuilder<QuerySnapshot>(
                  stream: Firestore.instance.collection("currency").snapshots(),
                  builder: (context, snapshot) {
                  if (!snapshot.hasData){
                      print('test pharse');
                       Text("Loading.....");}
                    else {
                      List<DropdownMenuItem> currencyItems = [];
                      for (int i = 0; i < snapshot.data.documents.length; i++) {
                        DocumentSnapshot snap = snapshot.data.documents[i];
                        currencyItems.add(
                          DropdownMenuItem(
                            child: Text(
                              snap.documentID,
                              style: TextStyle(color: Color(0xff11b719)),
                            ),
                            value: "${snap.documentID}",
                          ),
                        );
                      }
                      return Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: <Widget>[
                          Icon(Icons.mail,
                              size: 25.0, color: Color(0xff11b719)),
                          SizedBox(width: 50.0),
                          DropdownButton(
                            items: currencyItems,
                            onChanged: (currencyValue) {
                              final snackBar = SnackBar(
                                content: Text(
                                  'Selected Currency value is $currencyValue',
                                  style: TextStyle(color: Color(0xff11b719)),
                                ),
                              );
                              Scaffold.of(context).showSnackBar(snackBar);
                              setState(() {
                                selectedCurrency = currencyValue;
                              });
                            },
                            value: selectedCurrency,
                            isExpanded: false,
                            hint: new Text(
                              "Choose Currency Type",
                              style: TextStyle(color: Color(0xff11b719)),
                            ),
                          ),
                        ],
                      );
                    }
                  }),

Answer 1

您需要在StreamBuilder 的!snapshot.hasData 部分的Text 小部件之前添加return

if (!snapshot.hasData){
    print('test phrase');
    return Text("Loading.....");
}