如何在 Flutter 中基于 Future 结果构建 Stream? [英] How to build a Stream based on a Future result in Flutter?

问题描述

我有一个 Flutter 应用，它使用 Firebase-storage 和 google-signin.我正在尝试执行的步骤非常简单:

I have a Flutter app which uses Firebase-storage and google-signin. the steps I am trying to do is so simple:

1- 使用 Google 登录(完成).

1- Sign-in using Google (Done).

2- 获取当前用户 ID(完成).

2- Get Current User Id (Done).

3- 在为流构建器构建流时使用用户 ID(问题).

3- Use the User Id when construct the stream for the stream builder (the problem).

到目前为止我所做的是使用 Future 来获取当前用户 ID，然后将用户 ID 注入 Where 子句

what I did so far is that I am using a Future to get the Current User Id, then to inject the user Id inside the Where clause

.where('userId', isEqualTo: userId)

这就是我的结局:

这是我应该创建流的部分:

this is the part where I should create the stream:

  // Get document's snapshots and return it as stream.
  Future<Stream> getDataStreamSnapshots() async {
    // Get current user.
    final User user = await FirebaseAuth().currentUser();
    String userId = user.uid;

    Stream<QuerySnapshot> snapshots =
      db
        .collection(db)
        .where("uid", isEqualTo: userId)
        .snapshots();

    try {
      return snapshots;
    } catch(e) {
      print(e);
      return null;
    }
  }

这是我应该在哪里调用和接收流的部分，

and this is the part where should I call and receive the stream,

...
children: <Widget>[
    StreamBuilder<QuerySnapshot>(
          stream: CALLING THE PREVIOUS FUNCTION,
            builder: (BuildContext context, 
                      AsyncSnapshot<QuerySnapshot> snapshot) {
                if (snapshot.hasData) {
                    ...
                }
              ...

但是这段代码不起作用，因为我无法获得 Future 应该返回的值?有什么想法吗?

But this code does not work, because I am not able to get the value that should returned by the Future? any idea?

非常感谢

推荐答案

你永远不应该有 Future，那是双异步，这是不必要的.只需返回一个 Stream，然后在准备好之前不必发出任何事件.

You should never have a Future<Stream>, that's double-asynchrony, which is unnecessary. Just return a Stream, and then you don't have to emit any events until you are ready to.

不清楚 try/catch 正在保护什么，因为非 Future 的返回不能抛出.如果您返回一个流，只需在该流上发出任何错误即可.

It's not clear what the try/catch is guarding because a return of a non-Future cannot throw. If you return a stream, just emit any error on the stream as well.

您可以将代码重写为:

Stream<QuerySnapshot> getDataStreamSnapshots() async* {
  // Get current user.
  final User user = await FirebaseAuth().currentUser();
  String userId = user.uid;

  yield* db
    .collection(db)
    .where("uid", isEqualTo: userId)
    .snapshots();
}

async* 函数是异步的，因此您可以使用 await.它返回一个 Stream，您可以使用 yield event; 或 yield* streamOfEvents; 在流上发出事件.

An async* function is asynchronous, so you can use await. It returns a Stream, and you emit events on the stream using yield event; or yield* streamOfEvents;.

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