在 Flutter 中从 Firebase FireCloud 接收数据时，StreamBuilder 中的快照代码不会被执行

Posted 2023-02-23

【中文标题】在 Flutter 中从 Firebase FireCloud 接收数据时，StreamBuilder 中的快照代码不会被执行

【英文标题】：Snapshot code inside StreamBuilder does not get executed when receiving data from Firebase FireCloud in Flutter

【发布时间】：2020-12-24 03:36:53

【问题描述】：

我正在尝试从 Firecloud 数据库中获取描述文本并在 3 个不同的小部件中使用它们，将这些小部件添加到列表中，然后将该列表传递给 ListView。 为此，我在StreamBuilder 中执行了以下代码：

StreamBuilder(
              stream: FirebaseFirestore.instance.collection('humansOfLpc').snapshots(),
              builder: (BuildContext context,snapshot)
                if(!snapshot.hasData)
                  return Center(
                    child: Text('Loading...'),
                  );
                
                print('outside title and desc widget');
                List<TitleAndDescription> titleAndDescriptions=[
                ];
                snapshot.data.docs.map((document)
                    final about = document.data()["about"];
                    final aftercampusProgress =document.data()["afterCampusPlacement"];
                    final incampusPlacement = document.data()["incampusProgress"];
                    print('inside snapshots');
                    print(about);
                    final aboutDesc = TitleAndDescription(
                      title: 'About',
                      description: about,
                    );
                    final incampusProgressDesc = TitleAndDescription(
                      title: 'In-Campus Progress',
                      description: incampusProgress,
                    );
                    final afterCampusPlacementDesc=TitleAndDescription(
                      title: 'After Campus Placements',
                      description: afterCampusPlacement,
                    );
                    titleAndDescriptions.add(aboutDesc);
                    titleAndDescriptions.add(incampusProgressDesc);
                    titleAndDescriptions.add(afterCampusPlacementDesc);

                );

                return Expanded(
                  child: ListView(
                    children: titleAndDescriptions,
                  ),
                );

              ,
            ),

我面临的主要问题是它没有阅读以下代码：

snapshot.data.docs.map((document)
                    final about = document.data()["about"];
                    final aftercampusProgress =document.data()["afterCampusPlacement"];
                    final incampusPlacement = document.data()["incampusProgress"];
                    print('inside snapshots');
                    print(about);
                    final aboutDesc = TitleAndDescription(
                      title: 'About',
                      description: about,
                    );
                    final incampusProgressDesc = TitleAndDescription(
                      title: 'In-Campus Progress',
                      description: incampusProgress,
                    );
                    final afterCampusPlacementDesc=TitleAndDescription(
                      title: 'After Campus Placements',
                      description: afterCampusPlacement,
                    );
                    titleAndDescriptions.add(aboutDesc);
                    titleAndDescriptions.add(incampusProgressDesc);
                    titleAndDescriptions.add(afterCampusPlacementDesc);

                );

我可以这样说，因为控制台正在打印 'outside title and desc widget' 但不打印 'inside snapshots'。 因此，我的titleAndDescriptions 列表为空，我的 UI 上没有 3 个小部件和数据。请告诉我我做错了什么以及我有什么替代方案？ 提前致谢。

【参考方案1】：

使用forEach 而不是map

snapshot.data.docs.forEach((document)
                    final about = document.data()["about"];
                    final aftercampusProgress =document.data()["afterCampusPlacement"];
                    final incampusPlacement = document.data()["incampusProgress"];
                    print('inside snapshots');
                    print(about);
                    final aboutDesc = TitleAndDescription(
                      title: 'About',
                      description: about,
                    );
                    final incampusProgressDesc = TitleAndDescription(
                      title: 'In-Campus Progress',
                      description: incampusProgress,
                    );
                    final afterCampusPlacementDesc=TitleAndDescription(
                      title: 'After Campus Placements',
                      description: afterCampusPlacement,
                    );
                    titleAndDescriptions.add(aboutDesc);
                    titleAndDescriptions.add(incampusProgressDesc);
                    titleAndDescriptions.add(afterCampusPlacementDesc);

                );

在此处查看Why map doesn't work but forEach does。

【讨论】：

谢谢。这真的帮助了我。由于某些未知原因，map 不起作用，但 foreach 可以！