如何在 Dart 中使用 await 而不是 .then()

Question

I'm having the following lines in a Flutter app. _devicesRef refers to some node in a Firebase Realtime Database.

_devicesRef.child(deviceId).once().then((DataSnapshot data) async {
    print(data.key);
    var a = await ...
    print(a);
}

These lines work fine. Now I want to use await instead of .then() . But somehow, once() never returns.

var data = await _devicesRef.child(deviceId).once();
print(data.key);
var a = await ...
print (a);

So print(data.key) is never called.

What's wrong here?

Answer 1

It could be explained by the code following your snippet. Perhaps the future completion is trigger by something after your code and transforming your code with await will wait until a completion that never happens.

For instance, the following code works:

main() async {
  final c = Completer<String>();
  final future = c.future;
  future.then((message) => print(message));
  c.complete('hello');
}

but not this async/await version:

main() async {
  final c = Completer<String>();
  final future = c.future;
  final message = await future;
  print(message);
  c.complete('hello');
}

Answer 2

If you intend to use await as a replacement of .then() in your snippet, this is how you can accomplish it:

() async {
    var data = await _devicesRef.child(deviceId).once();
    print(data.key);
    var a = await ...
    print(a);
}();

By placing the code in the asynchronous closure () async {}() , we are not preventing execution of the code that comes after, in a similar fashion to using .then() .

Answer 3

it should be encased in an async function like this to use await

Furtre<T> myFunction() async {
  var data = await _devicesRef.child(deviceId).once();
  return data;
}