Firebase 实时数据库和 Flutter - 快照没有数据

Question

我尝试在 Flutter 中实现 Firebase 实时数据库，我想实时显示更新的值。我尝试使用StreamBuilder 来实现这一目标。

StreamBuilder 代码

StreamBuilder(
  stream: GuestbooksDatabase().getAllGuestbooksSync().asStream(),
  builder: (context, snapshot) {
    if (!snapshot.hasData || !snapshot.data.length) {
      return CircularProgressIndicator();
    } else {
      return ListView.builder(
          shrinkWrap: true,
          itemCount: snapshot.data.length,
          itemBuilder: (context, index) {
            return Text(snapshot.data[index].title);
          });
    }
  }),

流函数

Future<List<Guestbook>> getAllGuestbooksSync() async {
    List<Guestbook> guestbooks = [];
    databaseRef.onValue.listen((event) async {
      var dataSnapshot = event.snapshot;

      if (dataSnapshot.value != null) {
        dataSnapshot.value.forEach((key, value) async {
          Guestbook guestbook = await Guestbook.fromJson(value);
          guestbook.setId(key);
          guestbooks.add(guestbook);
        });

        await Future.delayed(Duration.zero);
        print(guestbooks); // Result: All Instances of Guestbook
        return guestbooks;
      }
    });
  }

我只看到CircularProgressIndicator() 这意味着快照没有数据。 有什么问题？

Answer 1

您可以为此使用StreamController。

创建一个新的控制器 -

final StreamController streamController = StreamController<List>.broadcast();

将Future<List> 转换为void 类型以用于您的getAllGuestbooksSync() 函数并且不返回任何内容。

它可以并且将会在initState()中被调用-

void getAllGuestbooksSync() {
    List<Guestbook> guestbooks = [];
    databaseRef.onValue.listen((event) async {
      var dataSnapshot = event.snapshot;

      if (dataSnapshot.value != null) {
        dataSnapshot.value.forEach((key, value) async {
          Guestbook guestbook = await Guestbook.fromJson(value);
          guestbook.setId(key);
          guestbooks.add(guestbook);
        });

        print(guestbooks); // Result: All Instances of Guestbook

        streamController.add(guestbooks); // Adding list to the stream
      }
    });
}

在您的StreamBuilder 中使用-

stream: streamController.stream,