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?
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');
}
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().
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;
}
Related
Aye Aye good people,
I'm experiencing a weird behavior
when using the top level function of an isolate asynchronously;
you can find example code HERE, but in short
as top level function of an isolate this works:
String _syncHandle(int data) {
return 'done';
}
and this doesn't:
Future<String> _syncHandle(int data) async {
return 'done';
}
can anybody explain me why?
(or if should work, why isn't doing so in my code?)
thank you in advance
Francesco
...
[edit: just noticed that a similar question has been asked,
nevertheless it is still unanswered
Call async function from Isolate function,
plus issue open on github ]
forgot to update this :/
if you look at the code linked in the question
isolates_logging/lib/provider/test_isolate.dart
Future<void> _handle(int _m) async {
final response = ReceivePort();
isolateTest = await Isolate.spawn(_isolate, response.sendPort);
final sendPort = await response.first as SendPort;
final answer = ReceivePort();
sendPort.send([_m, answer.sendPort]);
await answer.first.then((p) {
_outbound.sink.add(p);});
}
static void _isolate(SendPort _initialReplyTo) {
final port = ReceivePort();
_initialReplyTo.send(port.sendPort);
port.listen((message) {
final data = message[0] as int;
final send = message[1] as SendPort;
send.send(_syncHandle(data));
});
}
}
Future<String> _syncHandle(int data) async {
return 'done';
}
note the send.send(_syncHandle(data)); part
if you do so, you can send only primitives and not futures,
basically that's it
I am still a beginner on dart flutter, now I am trying to retrieve data from the REST API and socket.IO. at this time I have a confusing problem, I have tried searching on the internet for 3 days, but there is no solution. I have async and await scripts, but the function I added await doesn't give any response and still pause.
it is assumed that I have two different files, the first is the main file and the second is the helper file.
main.dart
Future<List<ChatTile>> fetchChat(socketutil,id) async {
socketutil.join(id); //STACK IN HERE
SharedPreferences prefs = await SharedPreferences.getInstance();
String messagePrefs = prefs.getString('messagePrefs');
print("DUA");
return await compute(parseListChat, messagePrefs);
}
helper.dart
Future<void> join(String id_room) async {
String jsonData ='{"room_id" : "$id_room","user_id" : "5a91687811138e74009839c9","user_name" : "Denis Muhammad Ramdan","user_photo" : "photo.jpg","user_status" : "1"}';
socketIO.sendMessage("join", jsonData, null);
//subscribe event
return await socketIO.subscribe("updateMessageList", (result) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setString('messagePrefs', result);
print('SATU');
return await result;
});
}
my question is there something wrong with my code, and how is the best way?
many thanks,
I suggest you to add await_only_futures to your analyzer config
analysis_options.yaml
lint:
rules:
- await_only_futures
You also don't need to do return await something since your function already return a future, this is redondant.
And from what I see of the socketio subscribe method, it does not return the result like you expect but use a callback and does not return it (https://pub.dartlang.org/documentation/flutter_socket_io/latest/flutter_socket_io/SocketIO/subscribe.html)
to handle this you should use a Completer
final completer = Completer<String>()
socketIO.subscribe("updateMessageList", (result) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setString('messagePrefs', result);
socketIO.unSubscribe("updateMessageList");
completer.complete(result);
});
return completer.future;
you probably want to handle error when there is using completer.completeError(error)
Update
You can alos convert the subscription to a Dart Stream to handle more case.
StreamController<String> controller;
Stream<String> get onUpdateMessageList {
if (controller != null) return controller.stream;
constroller = StreamController<String>.broadcast(
onCancel: () => socketIO.unSubscribe("updateMessageList"),
);
socketIO.subscribe("updateMessageList", constroller.add);
return controller.stream;
}
Future<StreamSubscription> join(String id_room) async {
...
return onUpdateMessageList.listen((result) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
await prefs.setString('messagePrefs', result);
});
}
I write a lot of async code that uses await to handle Futures.
If I have
() async {
var result = await someFuture();
}
what would be the preferred way to catch errors. Wraping the code in try/catch or doing
() async {
var result = await someFuture().catch(_errorHandler);
}
EDIT:
Also, if I have many await calls in one async method, what is the preferred catch all the errors instead of writing .catchError for eachOne.
() async {
var result = await someFuture();
var result2 = await someFuture2();
var result3 = await someFuture3();
}
According to the Dart docs if you use await wrap it in a try-catch
According to the Dart docs if you use await wrap it in a try-catch
https://dart.dev/codelabs/async-await#handling-errors
The Docs suggest just wrapping in a try-catch
Example code:
try {
print('Awaiting user order...');
var order = await fetchUserOrder();
} catch (err) {
print('Caught error: $err');
}
Reference also has a runnable example https://dart.dev/codelabs/async-await#handling-errors
I am trying to understand the usage of async and await in Dart. Somehow I am having issues returning values in certain methods.
Consider the code below
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 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
What is the correct way of doing this.
Thanks
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().
What are you actually need, is to complete a Future you're returning from getMrn() inside your callback.
Like this:
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;
}
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();
}
I'm working with two functions, both of them should return a future. A third function gets called when both of them are done. Right now the future is returned too early, so that my third function is called before my second function is completed.
Function1:
static var getObjectDataCompleter = new Completer();
static var fillObjectCompleter = new Completer();
static Future getObjectData(List jsonMap) {
for (int i = 0; i < jsonMap.length; i++) {
fillObjectCompleter = new Completer();
var request = buildRequest("GET", resourceUrl);
request.send();
request.onLoadEnd.listen((event) => fillObject(request));
}
if(fillObjectCompleter.isCompleted) {
getObjectDataCompleter.complete(null);
}
return getObjectDataCompleter.future;
}
Function2:
static Future fillObject(HttpRequest request) {
String responseText = request.response;
List stringJson = JSON.decode(responseText);
fillObjectCompleter.complete(null);
return fillObjectCompleter.future;
}
Function1 is returning the future before the call "fillObject()" is completed.
What am I doing wrong?
The function1-future should be returned when the "for-loop" is done and all "fillObject-calls" are completed.
Async code is just scheduled for later execution and the sync code continues executing without waiting for the async code. The method you pass ot Future.then(...) is executed when the scheduled async code is finished. You find a lot of such questions and examples tagged [:dart-async:] here on StackOverflow.
I have a hard time figuring out what you actually try to accomplish. Can you please explain in prosa what you actually try to accomplish, then I can try to fix your code example to do what you want it to do.
Usually there is no need to use a Completer in custom async functions. You just have to ensure that nested async calls are properly chained by always returning the future of the call.
See these two lines of the following code as example. The returns are important for the example to work.
return async.Future.forEach(jsonMap, (item) {
return request.onLoadEnd.first.then((event) => fillObject(event.target));
The Future returned from getObjectData completes after the response of all requests are processed.
import 'dart:html' as dom;
import 'dart:async' as async;
import 'dart:convert' show JSON;
class Xxx {
static async.Future getObjectData(List jsonMap) {
return async.Future.forEach(jsonMap, (item) {
//var request = new dom.HttpRequest();
//request.open("GET", "https://www.googleapis.com/discovery/v1/apis?fields=");
var request = buildRequest("GET", resourceUrl);
request.send();
return request.onLoadEnd.first.then((event) => fillObject(event.target));
});
}
static fillObject(dom.HttpRequest request) {
print('fillObject');
String responseText = request.response;
List stringJson = JSON.decode(responseText);
}
}
void main() {
var json = ['a', 'b', 'c'];
Xxx.getObjectData(json).then((_) => print('done'));
}
See https://www.dartlang.org/articles/event-loop for more details about async execution.