asynchronous - Getter "Snapshot"在 Flutter 中使用 StreamBuilder 调用 null

Question

我正在使用 Flutter Streambuilder，我的流在加载之前调用了 null 一秒钟。我正在尝试加载“正在加载...”卡片，但它不起作用。在显示我想要的卡片列表之前，我的模拟器短暂地显示了一个红色错误屏幕。我怎样才能让它停止这样做？这是我的代码...

new Expanded(
                  child: new StreamBuilder(
                      stream: streamQuery,
                      builder:
                          (BuildContext context, AsyncSnapshot<Event> event) {
                        if (event.data.snapshot.value == null) {
                          return new Card(
                            child: new Text('Loading...',
                                style: new TextStyle(
                                    fontSize: 12.0,
                                    fontWeight: FontWeight.bold,
                                    fontStyle: FontStyle.italic)),
                          );
                        }

                        Map myMap = event.data.snapshot.value; //store each map
                        var titles = myMap.values;

                        List onesTitles = new List();
                        List onesIds = new List();
                        List onesImages = new List();
                        List onesRank = new List();

                        for (var items in titles) {
                          onesTitles.add(items['vidTitle']);
                          onesIds.add(items['vidId']);
                          onesImages.add(items['vidImage']);
                          onesRank.add(items['Value']);
                        }

                        names = onesTitles;
                        ids = onesIds;
                        numbers = onesRank;
                        vidImages = onesImages;

                        switch (event.connectionState) {
                          case ConnectionState.none:
                            return new Card(
                              child: new Text('Loading...',
                                  style: new TextStyle(
                                      fontSize: 12.0,
                                      fontWeight: FontWeight.bold,
                                      fontStyle: FontStyle.italic)),
                            );
                          case ConnectionState.waiting:
                            return new Card(
                              child: new Text('Awaiting Results...',
                                  style: new TextStyle(
                                      fontSize: 12.0,
                                      fontWeight: FontWeight.bold,
                                      fontStyle: FontStyle.italic)),
                            );
                          default:
                            if (event.hasError)
                              return new Card(
                                child: new Text('Error: ${event.error}',
                                    style: new TextStyle(
                                        fontSize: 12.0,
                                        fontWeight: FontWeight.bold,
                                        fontStyle: FontStyle.italic)),
                              );
                            else
                              return new InkWell(

Answer 1

您需要在 else 中明确声明您的所有数据操作，它具有您的 if (snapshot!=null) {//do something} 的构建布局

像这样:

else //event.data.snapshot.value != null
 { 
Map myMap = event.data.snapshot.value; //store each map
                        var titles = myMap.values;

                        List onesTitles = new List();
                        List onesIds = new List();
                        List onesImages = new List();
                        List onesRank = new List();

                        for (var items in titles) {
                          onesTitles.add(items['vidTitle']);
                          onesIds.add(items['vidId']);
                          onesImages.add(items['vidImage']);
                          onesRank.add(items['Value']);
                        }

                        names = onesTitles;
                        ids = onesIds;
                        numbers = onesRank;
                        vidImages = onesImages;

//return my layout 
}

因此，不要在 snapshot!=null 时有效的 if 语句之外留下任何正在使用您的 snapshot.value 的内容。