同步返回值并在dart中等待 [英] returning a value in sync and await in dart

问题描述

我试图了解Dart中async和await的用法。不知何故，我在某些方法中无法返回值。

I am trying to understand the usage of async and await in Dart. Somehow I am having issues returning values in certain methods.

请考虑以下代码

Future<int> getMrn() async {
  var mrnRef = await firebaseClient.child('mrn');

  DataSnapshot ss;
  StreamSubscription<Event> onValueSubscription = await mrnRef.onValue
      .listen((event) {
    ss = event.snapshot;

    return ss.val();
  });

  //return Future<int> ss.val();
}

mrn 是键入 int ，应通过 getMrn 方法返回。但是，每次返回的 ss.val（）返回 null 。似乎在最后返回的值中没有看到 ss = event.snapshot

mrn is of type int which should be returned by getMrn method. However each time the returned ss.val() returns null. It seems that ss = event.snapshot is not seen in the last returned value

正确的处理方法是这样做。
谢谢

What is the correct way of doing this. Thanks

推荐答案

在上面的代码中，您声明了匿名函数（事件） {..} 作为回调，而您的 return 语句与此相关，而您的意图是 return 来自 getMrn（）。

In the code above, you're declaring anonymous function (event){..} as a callback, and your return statement relates to it, while your intention was to return from getMrn().

您实际需要的是完成您要从回调中的 getMrn（）返回的未来。

What are you actually need, is to complete a Future you're returning from getMrn() inside your callback.

就像这样：

Future<int> getMrn() async {
  var mrnRef = await firebaseClient.child('mrn');

  Completer<int> c = new Completer<int>();
  StreamSubscription<Event> onValueSubscription = await mrnRef.onValue
      .listen((event) {
    DataSnapshot ss = event.snapshot;
    c.complete(ss.val());
  });

  return c.future;
}

但如果<$中出现第二个事件，该代码将无法正常工作c $ c> mrnRef.onValue 流。因此，假设 mrnRef.onValue 是 Stream ，并且只需要第一个事件，最好重写它

but that code wouldn't work good if there second event appear in mrnRef.onValue stream. So, assuming mrnRef.onValue is a Stream, and you need only first event, it would be better to rewrite it this way:

Future<int> getMrn() async {
  var mrnRef = await firebaseClient.child('mrn');

  Event event = await mrnRef.onValue.first;
  DataSnapshot ss = event.snapshot;
  // note, you're implicitly returning a Future<int> here,
  // because our function is asyncronous
  return ss.val();
}

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