Before 'm3' you could check if a Future was completed with 'completer.future.isComplete' this seems to be gone. Is there a replacement? or do I need to save it myself then
(it seems inside the _CompleterImpl there is still a field '_isComplete' but its not exposed
With M3 Dart, it's best to just use your own flag.
future.whenComplete(() {
tweenCompleted = true;
});
Dart is a single threaded language so there is no race condition here.
Note that the [action] function is called when this future completes, whether it does so with a value or with an error.
An alternative to #Cutch's solution is to wrap the Future in a Completer:
Completer<T> wrapInCompleter<T>(Future<T> future) {
final completer = Completer<T>();
future.then(completer.complete).catchError(completer.completeError);
return completer;
}
Future<void> main() async {
final completer = wrapInCompleter(asyncComputation());
if (completer.isCompleted) {
final result = await completer.future;
// do your stuff
}
}
This approach is more resourceful since you can both await for the completion asynchronously and check whether the future is completed synchronously.
Using an extension on Future and building on Hugo Passos' answer:
extension FutureExtension<T> on Future<T> {
/// Checks if the future has returned a value, using a Completer.
bool isCompleted() {
final completer = Completer<T>();
then(completer.complete).catchError(completer.completeError);
return completer.isCompleted;
}
}
Related
I have a simple function that retrieves some UserData from our backend if its cache has expired. The commented return below created some odd behaviours. On a whim, I wrapped my _cache value in a Future and voila, though I'm perplexed why. I had assumed that Dart would wrap my return in a Future, but clearly there's something fundamental I do not understand. Can someone explain to me the difference between the two return values from within an async function?
UserData _cache;
Future<UserData> getUserData() async {
if (isCacheStale) {
_cache = await _endpoint.getUserData();
}
return Future(() => _cache); // <- Works
// return _cache; // <- Does not work
}
Is there is a synchronous way to check if a Future has completed in Dart?
Cheers!
No.
There is deliberately no way to check synchronously whether a future has already completed.
You can obviously keep track yourself:
Foo result = null;
fooFuture.then((value) { result = value; });
...
/// later
if (result != null) ...
That still only works after the first microtask, because your callback won't get called any sooner than that.
You can use Completer class.
final completer = Completer();
myFuture
.then(completer.complete)
.catchError(completer.completeError);
if (completer.isCompleted) {
// Future has completed
}
Scenario
If I want to read from a file and store the data in a Map, and if that map is being used multiple times for validation.
Is it possible for me to do this without having to change the return type of all methods, that use the above mentioned map, to Future?
Example:
Map metadata = null
Future readFromFile async {
.... metadata = await File.readingfromFile(...);
}
Future getRegion(..) async {
if(metadata == null) { await readFromFile() }
return metadata["region"]
}
Using the above code if a method(like isValidRegion,etc) that uses and needs getRegion(..) to complete, then the return type of isValidRegion should be converted to Future.
Future<bool> isValidRegion(..) async {
return ((await getRegionData(...)) != null )
}
If that isValidRegion is present within another methods, then the return type of them have to be changed to Future as well.
Future<String> parse(...) async {
....
if(await isValidRegion()) {
...
}
...
}
What is an elegant way to avoid this chain of futures as return types?
Async execution is contagious, there is nothing you can do to get back from async to sync execution.
What you can do is to do the read from the file synchronous to avoid the problem in the first place (if this is possible, if you read it from a network connection, this might not be possible).
I have a Dart function that looks like:
Future beAwesome() {
if (notActuallySupported) {
return new Future.error(new UnsupportedError('uh oh'));
}
return new Future.value(42);
}
// ...
beAwesome().then((answer) => print(answer));
I want to use the new async/await functionality. How do I change my function?
In general, add the word async after your function's signature and before the {. Also, return raw values instead of wrapping those values in futures. Also, throw actual exceptions instead of wrapping the errors with a future.
Here's the new version:
Future beAwesome() async {
if (notActuallySupported) {
throw new UnsupportedError('uh oh');
}
return 42;
}
// ...
var answer = await beAwesome();
print(answer);
Note that you should still use Future as the return-type annotation.
I would like to know how to define the data type and how to return the object (record) using getObject(). Currently, the only way that I have been able to use the result (record) outside of the function that obtains it is to call another function with the result. That way, the data-type does not need to be specified. However if I want to return the value, I need to define the data-type and I can't find what it is. I tried "dynamic" but that didn't appear to work. For example ":
fDbSelectOneClient(String sKey, Function fSuccess, String sErmes) {
try {
idb.Transaction oDbTxn = ogDb1.transaction(sgTblClient, 'readwrite');
idb.ObjectStore oDbTable = oDbTxn.objectStore(sgTblClient);
idb.Request oDbReqGet = oDbTable.getObject(sKey);
oDbReqGet.onSuccess.listen((val){
if (oDbReqGet.result == null) {
window.alert("Record $sKey was not found - $sErmes");
} else {
///////return oDbReqGet.result; /// THIS IS WHAT i WANT TO DO
fSuccess(oDbReqGet.result); /// THIS IS WHAT i'm HAVING TO DO
}});
oDbReqGet.onError.first.then((e){window.alert(
"Error reading single Client. Key = $sKey. Error = ${e}");});
} catch (oError) {
window.alert("Error attempting to read record for Client $sKey.
Error = ${oError}");
}
}
fAfterAddOrUpdateClient(oDbRec) {
/// this is one of the functions used as "fSuccess above
As someone else once said (can't remember who), once you start using an async API, everything needs to be async.
A typical "Dart" pattern to do this would be to use a Future + Completer pair (although there's nothing inherently wrong with what you've done in your question above - it's more a question of style...).
Conceptually, the fDbSelectOneClient function creates a completer object, and the function returns the completer.future. Then, when the async call completes, you call completer.complete, passing the value in.
A user of the function would call fDbSelectOneClient(...).then((result) => print(result)); to make use of the result in an async way
Your code above could be refactored as follows:
import 'dart:async'; // required for Completer
Future fDbSelectOneClient(String sKey) {
var completer = new Completer();
try {
idb.Transaction oDbTxn = ogDb1.transaction(sgTblClient, 'readwrite');
idb.ObjectStore oDbTable = oDbTxn.objectStore(sgTblClient);
idb.Request oDbReqGet = oDbTable.getObject(sKey);
oDbReqGet.onSuccess.listen((val) => completer.complete(oDbReqGet.result));
oDbReqGet.onError.first.then((err) => completer.completeError(err));
}
catch (oError) {
completer.completeError(oError);
}
return completer.future; // return the future
}
// calling code elsewhere
foo() {
var key = "Mr Blue";
fDbSelectOneClient(key)
.then((result) {
// do something with result (note, may be null)
})
..catchError((err) { // note method chaining ..
// do something with error
};
}
This future/completer pair only works for one shot (ie, if the onSuccess.listen is called multiple times, then the second time you will get a "Future already completed" error. (I've made an assumption on the basis of the function name fDbSelectOneClient that you are only expecting to select a single record.
To return a value from a single future multiple times, you'll probably have to use the new streams feature of the Future - see here for more details: http://news.dartlang.org/2012/11/introducing-new-streams-api.html
Note also, that Futures and Completers support generics, so you can strongly type the return type as follows:
// strongly typed future
Future<SomeReturnType> fDbSelectOneClient(String sKey) {
var completer = new Completer<SomeReturnType>();
completer.complete(new SomeReturnType());
}
foo() {
// strongly typed result
fDbSelectOneClient("Mr Blue").then((SomeReturnType result) => print(result));
}