Dart offers a FutureOr class, that allows writing:
FutureOr<int> future;
future = 42; // valid
future = Future.value(42); // also valid
I would assume that FutureOr would be useful to remove the unnecessary delay caused by the event loop if the value can be read synchronously.
But that doesn't seem to be the case, as showcased by:
import 'dart:async';
void main() async {
print('START');
futureOrExample();
print('END');
}
void futureOrExample() async {
FutureOr<int> futureOr = 42;
print('before await');
await futureOr;
print('end await');
}
which prints:
START
before await
END
end await
when I would expect:
START
before await
end await
END
In that case, why does FutureOr (or more generally await 42) work this way?
Similarly, what's the purpose of FutureOr in that situation since it produces the same result as Future?
I know that I could use SynchronousFuture to achieve the desired result, but I'm just trying to understand what's the use of FutureOr.
The use of FutureOr, as introduced with Dart 2, is to allow you to provide either a value or a future at a point where the existing Dart 1 API allowed the same thing for convenience, only in a way that can be statically typed.
The canonical example is Future.then. The signature on Future<T> is Future<R> then<R>(FutureOr<R> action(T value), {Function onError}).
The idea is that you can have an action on the future's value which is either synchronous or asynchronous. Originally there was a then function which took a synchronous callback and a chain function which took an asynchronous callback, but that was highly annoying to work with, and in good Dart 1 style, the API was reduced to one then method which took a function returning dynamic, and then it checked whether it was a future or not.
In Dart 1 it was easy to allow you to return either a value or a future. Dart 2 was not as lenient, so the FutureOr type was introduced to allow the existing API to keep working. If we had written the API from scratch, we'd probably have done something else, but migrating the existing asynchronous code base to something completely different was not an option, so the FutureOr type was introduced as a type-level hack.
The await operation was also originally defined to work on any object, long before FutureOr existed. For consistency and smaller code, an await e where e evaluated to a non-future would wrap that value in a future and await that. It means that there is only one quick and reusable check on a value (is it a future, if not wrap it), and then the remaining code is the same. There is only one code-path.
If the await worked synchronously on non-Future values, there would have to be a synchronous code path running through the await, as well as an asynchronous path waiting for a future. That would potentially double the code size, for example when compiling to JavaScript (or worse, if there were more awaits in the same control flow, you could get exponential blow-up for a naive implementation). Even if you avoided that by just calling the continuation function synchronously, it would likely be confusing to some readers that an await would not introduce an asynchronous gap. A mistake around that can cause race conditions or things happening in the wrong order.
So, the original design, predating FutureOr, was to make all await operations actually wait.
The introduction of FutureOr did not change this reasoning, and even if it did, it would now be a breaking change to not wait in places where people expect their code to actually give time for other microtasks to run.
The await keyword always lock the function execution.
Writing:
await 42
Is equivalent to:
await Future.value(42)
The reason being:
This is how await works in Javascript
it makes the behavior of await consistent.
Now, what's the purpose of FutureOr then?
FutureOr was never intended as a way to potentially make await synchronous.
Instead, it is an implementation detail of Future.
Without FutureOr, writing the following would not compile:
Future(() {
return 42; // compile error, not a Future
});
Future<int> future;
future.then((value) {
return value * 2; // compile error, not a Future
});
Instead, we would have to wrap all values in a Future.value like so:
Future<int> future;
future.then((value) {
return Future.value(value * 2);
});
for those who are still confused
I found good explanation https://itnext.io/what-is-futureor-in-dart-flutter-681091162c57 without diving into details
This piece of code could explain the target and real use cases of FutureOr
abstract class IDBService {
FutureOr<String> fetch();
}
class FirebaseRemoteService extends IDBService {
#override
Future<String> fetch() async => await 'data';
}
class LocalHiveDbService extends IDBService {
#override
String fetch() => 'data';
}
so in implementations of IDBService
the return type can be Future or String at the same time now!
Coming late to the discussion.
Updating my Dart comprehension - pardon my C++/JS -ish approach.
Seems like this would be useful for singleton initiation. Consider following:
import 'dart:async';
class AClass {
static String _info = '';
static FutureOr<String> get info async {
if (_info.isEmpty) {
print('--> is empty...');
_info = await Future.delayed(Duration(seconds:2),
() => "I'm alive!!");
}
else {
print('--> not empty');
}
return _info;
}
}
Future<void> main() async {
String info = await AClass.info;
print('Fist call: ' + info);
info = await AClass.info;
print('Second call: ' + info);
}
It works as expected - in either case, whether the _info member has been instantiated or not, the getter returns a valid string.
It works fine if I just use a Future<String> specifier in the getter, too. The current implementation makes FutureOr seem mostly like a self-documentation exercise (can return a Future<String> or a String...)
But, even if await currently always locks the execution, a future update may allow it to work as expected, in which case using the FutureOr construct would anticipate updates.
(Aside: I imagine this example could be condensed using an Optional wrapping the _info member, but that's a different exercise...)
I needed to use FutureOr today. I wanted to call a function that might be asynchronously (not always).
String callbackOne() => "hello";
Future<String> callbackTwo() async => (await Future.delayed(Duration(seconds: 1),() => "This is a sentence"));
Problem
I can do getLengthOfResult(callbackOne), but not getLengthOfResult(callbackTwo). Conversely, if accept an async callback, I can't use the sync callback.
DartPad
Future<int> getLengthOfResult(String Function() callback) async {
return callback().length;
}
Solution
DartPad
Future<int> getLengthOfResult(FutureOr<String> Function() callback) async {
// I can await on callbackOne, even though it returns a String.
final result = await callback();
return result.length;
}
main() {
getLengthOfResult(callbackOne);
getLengthOfResult(callbackTwo);
}
Is there an existing API to group a list of Future object into one so that I can wait for all to complete? Like in javascript's Promise.all.
You can use the Future.wait method documented here:
https://api.dart.dev/stable/2.5.0/dart-async/Future/wait.html
You can use Future.wait
Example
var futures = await Future.wait([
getAddress(),
getTeam(),
getEvents(),
]);
With the release of the new dart:io libraries, in order to read the data from an HttpRequest we now need to:
Listen to the body to handle data and notified of it's completion.
In order to accomplish this, I've been using something similar to the following to get all of the data from the request (taken from the dart:io tests):
List<int> body = new List<int>();
request.listen(body.addAll, onDone: () {
var str = new String.fromCharCodes(body);
// Now do something with the string of data
});
Is there some way of using transform and/or StringDecoders to already provide a constructed result. The only way I can think of, I would still need to create a StringBuffer and use writeAll to ensure all data is passed in the event it doesn't all arrive at once, and still call the onDone before using the string in the buffer.
So I guess ultimately the question is: is there some way I can use a HttpRequest (or in all actuality any Stream) without needing to buffer/build the results and can just pass a callback or handler which receives the entire contents?
I haven't tried this, but I think you should be able to use StringDecoder, toList() and join() to get the whole body:
request.transform(new StringDecoder()).toList().then((data) {
var body = data.join('');
print(body);
}
In Dart, there is a concept of Isolates. I have an application (that I'm experimenting in Dart) that has lots of asynchronous IO where each call (they are database calls) are dependent on the previous one. So I have ended up in a nested callback hell.
I was wondering if Isolates could solve that nested callback soup, but it looks a bit verbose and I'm not sure if it fits it well.
There are also Generators proposed in the next ECMAScript Harmony which could solve these things, but how would you currently do lots of asynchronous IO in Dart in a clean way?
You can use Future's and Completers to chain work together. The following future returns the result of a 'ls' command from a process:
Future<String> fetch(String dir) {
final completer = new Completer();
Process process = new Process.start('ls', [dir]);
process.exitHandler = (int exitCode) {
StringInputStream stringStream = new StringInputStream(process.stdout);
stringStream.dataHandler = () {
String content = stringStream.read();
completer.complete(content);
process.close();
};
};
process.errorHandler = (var error) {
completer.completeException(error);
return true;
};
return completer.future;
};
which you can then chain together like this:
fetch('/').then((val) => fetch("/usr").then((val) => fetch("/tmp")));
Not the most pretty solution but this is what I get by with now.
There is no F# async wrapper (in the PowerPack) for neither
TcpListener.BeginAcceptTcpClient() nor EndAcceptTcpClient().
How do I write my own wrapper around these, so that I can use the let! and async keywords and run it in parallel?
Have you checked out the Async.BuildPrimitive function? I think you can do something like:
type TcpListener with
member x.AsyncAcceptClient() =
Async.BuildPrimitive(x.BeginAcceptTcpClient, x.EndAcceptTcpClient)
to create an extension method returning an appropriate async result.