I have Flutter mobile app that is using Riverpod with hooks.
I have the following function that I would like to be called when the widget is disposed:
useEffect(
() {
final firestoreRepo =
ref.read(firebaseFirestoreRepositoryProvider);
return () async {
try {
// I get exception at this line.
// I need this future to be called when the
// widget is disposed.
// Calling this future earlier is not userful
// for my business logic.
final relationship =
await ref.read(relationshipWithProvider(pid).future);
if (relationship?.unseen ?? false) {
await firestoreRepo?.updateRelatinoship(pid: pid);
}
} catch (e, st) {
// print error
}
};
},
[],
);
I keep getting this error at the line shown in the comment above.
I/flutter ( 5967): Looking up a deactivated widget's ancestor is unsafe.
I/flutter ( 5967): At this point the state of the widget's element tree is no longer stable.
How can I sold this problem
We can initially get our relationship and then await and use it:
useEffect(
() {
final firestoreRepo = ref.read(firebaseFirestoreRepositoryProvider);
final relationship = ref.read(relationshipWithProvider(pid).future);
return () async {
try {
if (await relationship?.unseen ?? false) {
await firestoreRepo?.updateRelatinoship(pid: pid);
}
} catch (e, st) {
// print error
}
};
},
[],
);
As far as I can tell, this won't contradict the logic of the business process, because one way or another, we'll have to make the relationshipWithProvider(pid) request early (when we initialize the widget) or late (when we delete the widget).
Related
I am failing to understand, why the error thrown from addItem method in below code is not caught in the try-catch block
void main() async {
var executor = Executor();
var stream = Stream.fromIterable([0, 1, 2, 3, 4, 5, 6, 7]);
try {
await for (var _ in stream) {
executor.submit(() => demoMethod());
}
await executor.execute();
} catch (e) {
print(e);
}
}
Future<void> demoMethod() async {
var list = [1, 2, 3, 1, 4, 5];
var executor = Executor();
var test = Test();
for (var element in list) {
executor.submit(() => test.addItem(element));
}
await executor.execute();
test.list.forEach(print);
}
class Test {
var list = <int>[];
Future<void> addItem(int i) async {
if (list.contains(i)) {
throw Exception('Item exists');
}
list.add(i);
}
}
class Executor {
final List<Future<void>> _futures = [];
bool _disposed = false;
void submit(Future<void> Function() computation) {
if (!_disposed) {
_futures.add(computation());
} else {
throw Exception('Executor is already disposed');
}
}
Future<void> execute() async {
await Future.wait(_futures, eagerError: true);
_disposed = true;
}
}
but below code is able to catch the error properly
void main() async {
var executor = Executor();
try {
for (var i = 0; i < 10; i++) {
executor.submit(() => demoMethod());
}
await executor.execute();
} catch (e) {
print(e);
}
}
I am guessing it has something to do with the stream processing.
It's the stream.
In your other examples, you synchronously run through a loop a and call Executor.submit with all the computations, then immediately call executor.execute().
There is no asychronous gap between calling the function which returns a future, and Future.wait starting to wait for that future.
In the stream code, each stream events starts an asynchronous computation by calling Executor.submit. That creates a future, stores it in a list, and goes back to waiting for the stream.
If that future completes, with an error, before the stream ends and Future.wait gets called, then there is no error handler attached to the future yet. The error is then considered unhandled, and is reported to the current Zone's uncaught error handler. Here that's the root zone, which means it's a global uncaught error, which may crash your entire program.
You need to make sure the future doesn't consider its error unhandled.
The easiest way to do that is to change submit to:
void submit(Future<void> Function() computation) {
if (!_disposed) {
_futures.add(computation()..ignore());
} else {
throw StateError('Executor is already disposed');
}
}
The ..ignore() tells the future that it's OK to not have an error handler.
You know, because the code will later come back and call executor.execute, that any errors will still be reported, so it should be safe to just postpone them a little. That's what Future.ignore is for.
(Also changed Exception to StateError, because that's what you should use to report people using objects that have been disposed or otherwise decommissioned.)
I want to load with flutter a list from realtime firebase database and wait for finished loading. I expect that
FirebaseDatabase database = new FirebaseDatabase();
Query _newsQuery = database
.reference()
.child('news')
.orderByChild('published')
.limitToFirst(10);
Future<List<News>> loadNews() async {
List<News> list = new List<News>();
_newsQuery.onChildAdded.listen(onNewsAdded);
return list;
}
Future<News> onNewsAdded(Event event) async {
News n = News.fromSnapshot(event.snapshot);
print(n.title);
return n;
}
gives the result 2 with two database-entries when i excecute it with
loadNews().then((List<News> newsList) {
print(newsList.length);
});
But i get
I/flutter (19206): 0
I/flutter (19206): Title 1
I/flutter (19206): Title 2
That means that the listener is not async waiting. I tried a lot of different lines, but not found a solution to wait for the last query result.
I NOT want to use setState() in any widget, because i want to use a own singleton databaseHandler for all the widgets. For that reason is want to load the data async and wait for it in the widget.
Ok, i think i found a solution. After a lot of trial and errors i get an idea with https://webdev.dartlang.org/articles/performance/event-loop. Used and forced with that a nested future to fill the list. Used then Completer to close the list.
Future<List<News>> loadNews() async {
Completer c = new Completer<List<News>>();
List<News> list = new List<News>();
Stream<Event> sse = _newsQuery.onChildAdded;
sse.listen((Event event) {
onNewsAdded(event, list).then((List<News> newsList) {
return new Future.delayed(new Duration(seconds: 0), ()=> newsList);
}).then((_) {
if (!c.isCompleted) {
c.complete(list);
}
});
});
return c.future;
}
Future<List<News>> onNewsAdded(Event event, List<News> newsList) async {
News n = News.fromSnapshot(event.snapshot);
print("ADD: "+n.title);
newsList.add(n);
return newsList;
}
With that i get the result
I/flutter ( 5419): ADD: Title 1
I/flutter ( 5419): ADD: Title 2
I/flutter ( 5419): ADD: Title 3
I/flutter ( 5419): Size: 3
I need to load a string from a file. The following code always returns null:
static String l( String name ) {
String contents;
rootBundle
.loadString( 'i10n/de.yaml' )
.then( (String r) { contents = 'found'; print( 'then()' ); })
.catchError( (e) { contents = '#Error#'; print( 'catchError()' ); })
.whenComplete(() { contents = 'dd'; print( 'whenComplete()' ); })
;
print( 'after' );
if ( null == contents ) {
return '#null#';
}
String doc = loadYaml( contents );
return doc;
}
I have added this to the flutter: section in pupspec.yaml section:
assets:
- i10n/de.yaml
- i10n/en.yaml
The file i10n/de.yaml exists.
I'm aware, that rootBundle.loadString() is async. Therefore I appended the then() call - assuming that
(String r) { contents = 'found'; }
only gets executed if the Future returned by rootBundle.loadString() is able to return a value.
Actually, the method always returns '#null#'. Therefore, I added the print() statements, which output this:
I/flutter (22382): after
I/flutter (22382): then()
I/flutter (22382): whenComplete()
OK, obviously the future of loadString() executes later than the final print() statement.
Q: But how do I force the future to execute, so that I may retrieve its value?
In other words: How do I wrap some async stuff in certain code to retrieve its value immediately?
PS: First day of flutter/dart. Probably a trivial question...
the .then() is getting executed, but after the rest of the body. As you mention loadString() returns a Future, so completes in the future. To wait for a Future to complete use await. (Note that when you mark the function as async, the function must now return a Future itself - as it has to wait for loadString to complete in the future, so it itself has to complete in the future...) When you call l('something') you will have to await the result.
Future<String> l(String name) async {
try {
String contents = await rootBundle.loadString('i10n/de.yaml');
return contents == null ? '#null#' : loadYaml(contents);
} catch (e) {
return 'oops $e';
}
}
It's no big deal that lots of your utility functions become async as a result of having to wait for things (there's a lot of waiting - for files to read, http requests to complete, etc). You'll eventually end up at the top with something like (call this from initState)
refresh(String s) {
l(s).then((r) {
setState(() {
i18nStuff = r;
});
});
}
to set the state of your Widget when the i18nStuff is ready, and a Widget that has this in its build to switch between a dummy ui for the few milliseconds until it's ready, and then the real UI.
Widget build() {
if (i18nStuff == null) {
return new Container();
}
return new Column(
// build the real UI here
);
}
I am trying to use dart isolate library to improve my application performance.
Look at following code:
import 'dart:isolate';
import 'package:dbcrypt/dbcrypt.dart';
main() {
var pwConPort = new ReceivePort();
pwConPort.listen((data) {
print(data);
pwConPort.close();
}, onError: (err) {
print(err);
});
Isolate.spawn(generatePasswordConcurrency, pwConPort.sendPort);
}
void generatePasswordConcurrency(SendPort sendPort) {
sendPort.send(_generateHashPassword('Passsowr1222!'));
}
String _generateHashPassword(String password) {
var regex = new RegExp(r'^.*(?=.{7,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[^a-zA-Z0-9]).*$');
if (!regex.hasMatch(password)) {
throw new StateError('Errors');
}
return new DBCrypt().hashpw(password, new DBCrypt().gensalt());
}
Everything works fine but i can only pass a static password, or better to say, i don't know, how to pass something dynamically. Here you can see, password is hardcoded, but i want to pass a variable for example.
void generatePasswordConcurrency(SendPort sendPort) {
sendPort.send(_generateHashPassword('Passsowr1222!'));
}
If the method _generateHashPassword will throw an error, how can I handling this error? I try to catch the error on listen method from ReceivePort
pwConPort.listen((data) {
print(data);
pwConPort.close();
}, onError: (err) {
print(err);
});
but still got unhandling exceptions message.
Observatory listening on http://127.0.0.1:51433
in ShutdownIsolate: Unhandled exception:
Bad state: Errors
#0 _generateHashPassword (file:///D:/Dart/samples/bin/isolate_error.dart:26:9)
#1 generatePasswordConcurrency (file:///D:/Dart/samples/bin/isolate_error.dart:19:40)
#2 _startIsolate.isolateStartHandler (dart:isolate-patch/isolate_patch.dart:221)
#3 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:124)
Conclusion my question:
How can I pass a variable to called method on isolate?
How can I handling error on isolate?
First of all,
Isolate are not thread, they are independant process more like a fork() than a thread
dartApi: Isolate
Concurrent programming using isolates:
independent workers that are similar to threads but don't share memory, communicating only via
messages.
So, you can't access to the same variable than your parent process. It's a choice made by the dart team, because it's a mechanism usable when you compile your dart code in js. So it need to be possible in JS
How can I pass a variable to called method on isolate?
To do this, you need to see ReceivePort() like a unidirectionnal way of communication, so to pass variable in two way, you need two.
So on you main process:
pwConPort.listen((data) {
if (isolateSendPort == null && data is SendPort) {
isolateSendPort = data; // Receive the communication object of the isolate
isolateSendPort.send("Passsowr1222!");
} else {
print("Generated password: ${data}");
pwConPort.close();
}
}, onError: (err) {
print("SendPortError: ${err}");
});
});
In you isolate entry point :
sendPort.send(isolateConPort.sendPort);
isolateConPort.listen((data) {
// code ....
});
Note: be careful of what message you send. message send between one process and another need to respect some rules
DartApi: SendPort
The content of message can be: primitive values (null, num, bool,
double, String), instances of SendPort, and lists and maps whose
elements are any of these. List and maps are also allowed to be
cyclic.
How can I handling error on isolate?
Isolate get one method to listen throw error send by the isolate : addErrorListner
That is a useful function.
BUT ! this method is not implement in every plate-forme, so you need to do this in a others.
The way i chose is to send 2 SendPort in the entry point function :
One for the communication
One for the error.
So the spawn function looks like :
Isolate.spawn(generatePasswordConcurrency, [pwConPort.sendPort, errorPort.sendPort])
and the generatePasswordConcurrency :
void generatePasswordConcurrency(List<SendPort> commList) {
var sendPort = commList[0];
var errorPort = commList[1];
var isolateConPort = new ReceivePort();
sendPort.send(isolateConPort.sendPort);
isolateConPort.listen((data) {
try {
sendPort.send(_generateHashPassword(data));
} catch (e) {
errorPort.send("error: ${e.toString()}");
}
});
}
Here the full code :
import 'dart:isolate';
import 'package:dbcrypt/dbcrypt.dart';
main() {
var pwConPort = new ReceivePort();
var errorPort = new ReceivePort();
SendPort isolateSendPort = null;
Isolate.spawn(generatePasswordConcurrency, [pwConPort.sendPort, errorPort.sendPort])
.then((Isolate pcs) {
errorPort.listen((err) {
print("Error: ${err}");
pwConPort.close();
errorPort.close();
});
print(pcs);
pwConPort.listen((data) {
if (isolateSendPort == null && data is SendPort) {
isolateSendPort = data;
isolateSendPort.send("Passsowr1222!");
} else {
print("Generated password: ${data}");
pwConPort.close();
errorPort.close();
//pcs.kill();
}
}, onError: (err) {
print("SendPortError: ${err}");
});
});
}
void generatePasswordConcurrency(List<SendPort> commList) {
var sendPort = commList[0];
var errorPort = commList[1];
var isolateConPort = new ReceivePort();
sendPort.send(isolateConPort.sendPort);
isolateConPort.listen((data) {
try {
sendPort.send(_generateHashPassword(data));
} catch (e) {
errorPort.send("error: ${e.toString()}");
}
});
}
String _generateHashPassword(String password) {
var regex = new RegExp(r'^.*(?=.{7,})(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[^a-zA-Z0-9]).*$');
if (!regex.hasMatch(password)) {
throw new StateError('Errors');
}
return new DBCrypt().hashpw(password, new DBCrypt().gensalt());
}
I launch a request to a server with a future "requestServer".
I would like to poll a system for a specific value (passed from false to true, when request is done) and return when finished.
Code could be like that, but "while" synchronous and "checkOperation" is asynchronous?
return requestServer().then((operation) {
var done = false;
while (done)
return checkOperation(operation).then((result) {
done = (result == true);
});
sleep(10);
}
});
Any ideas ?
I guess this is not exactly what you want but as far as I know there is no way to block execution so you have to use callbacks.
void main(List<String> args) {
// polling
new Timer.periodic(new Duration(microseconds: 100), (t) {
if(isDone) {
t.cancel();
someCallback();
}
});
// set isDone to true sometimes in the future
new Future.delayed(new Duration(seconds: 10), () => isDone = true);
}
bool isDone = false;
void someCallback() {
print('isDone: $isDone');
// continue processing
}
You can of course pass the callback as parameter instead of hardcode it, because functions are first class members in Dart.
Polling doesn't work very well for async. It is better to wait for a signal from the thing that must complete.
Günter Zöchbauer's answer shows you how to poll anyway, by sampling with a timer.
As an alternative, it would be better to not have a boolean done, but instead complete another future when you are ready. This is busy-polling, which polls again as soon as a result comes back, which may be more intensive than you need. Using timer based polling can be more efficient if you don't need the result as soon as possible.
return requestServer().then((operation) {
var completer = new Completer();
void poll(result) {
if (!result) {
operation.then(poll, onError: completer.completeError);
} else {
completer.complete();
}
}
poll(false);
return completer.future;
});
(Code not really tested, since I don't have your requestServer).
When you want build functions that return Futures, it is sometimes useful to use Completers. Think that requestServer() is living in the Future too, so you will have threat the result as a Future.
return requestServer().then((operation) {
// This is necessary then you want to control async
// funcions.
Completer completer = new Completer();
//
new Timer.periodic(const Duration(seconds: 10), (_) {
checkOperation(operation).then((result) {
// Only when the result is true, you pass the signal
// that the operation has finished.
// You can alse use `completer.complete(result)` if you want
// to pass data inside of the future.
if (result == true) completer.complete();
});
});
// You return the future straight away.
// It will be returned by requestServer();
return completer.future;
});
I use a function like this in a TestUtil library:
static Future<bool> waitUntilTrue(bool Function() callback,
{Duration timeout: const Duration(seconds: 2),
Duration pollInterval: const Duration(milliseconds: 50)}) {
var completer = new Completer<bool>();
var started = DateTime.now();
poll() {
var now = DateTime.now();
if (now.difference(started) >= timeout) {
completer.completeError(Exception('timed out in waitUntilTrue'));
return;
}
if (callback()) {
completer.complete(true);
} else {
new Timer(Duration(milliseconds: 100), () {
poll();
});
}
}
poll();
return completer.future;
}
And then in my test code I'll do something like:
await TestUtil.waitUntilTrue(() => someObj.isDone);
Edit:
Note that if you're using this in a testWidgets test, you have to do a little extra, since it relies on real async work happening:
await tester.runAsync<bool>(
() => TestUtil.waitUntilTrue(() => myObj.isLoaded),
additionalTime: Duration(seconds: 5));