I have an array of JSON objects. I want to iterate through the array, and for each object check some parameters, and if certain conditions are true, make a request to create a record using the Air-table module.
Because of the rate limit, I want to throttle how quickly i make those requests, essentially calling the functions in series, and also implementing a delay in between them.
Promises have always confused the crap out of me. How would Iterate over an array to achieve this with promises. Can I do this with Async Await? I know that some promise libraries like Bluebird have both a delay and a concurrent function, but I am pretty confused about how to use them correctly.
If you want to iterate through array synchronously you can use for ... of construction:
async function doSomethingWithArray(array, delayTime) {
for (const item of array) {
await someMethod(item);
await delay(delayTime);
}
}
the simplest delay function is:
function delay(delayTime) {
return new Promise(resolve => setTimeout(resolve, delayTime));
}
Related
I'm still struggeling with the async/await pattern so I'm here to ask you some precisions.
I saw this page explaining the async/await pattern pretty well. I'm posting here the example that bother me :
import 'dart:async';
Future<String> firstAsync() async {
await Future<String>.delayed(const Duration(seconds: 2));
return "First!";
}
Future<String> secondAsync() async {
await Future<String>.delayed(const Duration(seconds: 2));
return "Second!";
}
Future<String> thirdAsync() async {
await Future<String>.delayed(const Duration(seconds: 2));
return "Third!";
}
void main() async {
var f = await firstAsync();
print(f);
var s = await secondAsync();
print(s);
var t = await thirdAsync();
print(t);
print('done');
}
In this example, each async method is called one after another, so the execution time for the main function is 6 seconds (3 x 2 seconds). However, I don't understand what's the point of asynchronous function if they are executed one after another.
Are async functions not supposed to execute in the background ? Is it not the point of multiple async functions to fastens the process with parrallel execution ?
I think I'm missing something about asynchronous functions and async/await pattern in flutter so if you could explain me that, it would be very appreciated.
Best
Waiting on multiple Futures to complete using Future.wait()
If the order of execution of the functions is not important, you can use Future.wait().
The functions get triggered in quick succession; when all of them complete with a value, Future.wait() returns a new Future. This Future completes with a list containing the values produced by each function.
Future
.wait([firstAsync(), secondAsync(), thirdAsyncC()])
.then((List responses) => chooseBestResponse(responses))
.catchError((e) => handleError(e));
or with async/await
try {
List responses = await Future.wait([firstAsync(), secondAsync(), thirdAsyncC()]);
} catch (e) {
handleError(e)
}
If any of the invoked functions completes with an error, the Future returned by Future.wait() also completes with an error. Use catchError() to handle the error.
Resource:https://v1-dartlang-org.firebaseapp.com/tutorials/language/futures#waiting-on-multiple-futures-to-complete-using-futurewait
The example is designed to show how you can wait for a long-running process without actually blocking the thread. In practice, if you have several of those that you want to run in parallel (for example: independent network calls), you could optimize things.
Calling await stops the execution of the method until the future completes, so the call to secondAsync will not happen until firstAsync finishes, and so on. If you do this instead:
void main() async {
var f = firstAsync();
var s = secondAsync();
var t = thirdAsync();
print(await f);
print(await s);
print(await t);
print('done');
}
then all three futures are started right away, and then you wait for them to finish in a specific order.
It is worth highlighting that now f, s, and t have type Future<String>. You can experiment with different durations for each future, or changing the order of the statements.
If anyone new in this problem use the async . Dart has a function called FutureGroup. You can use it to run futures in parallel.
Sample:
final futureGroup = FutureGroup();//instantiate it
void runAllFutures() {
/// add all the futures , this is not the best way u can create an extension method to add all at the same time
futureGroup.add(hello());
futureGroup.add(checkLocalAuth());
futureGroup.add(hello1());
futureGroup.add(hello2());
futureGroup.add(hello3());
// call the `.close` of the group to fire all the futures,
// once u call `.close` this group cant be used again
futureGroup.close();
// await for future group to finish (all futures inside it to finish)
await futureGroup.future;
}
This futureGroup has some useful methods which can help you ie. .future etc.. check the documentation to get more info.
Here's a sample usage Example One using await/async and Example Two using Future.then.
you can always use them in a single future
final results = await Future.wait([
firstAsync();
secondAsync();
thirdAsync();
]);
results will be an array of you return type. in this case array of strings.
cheers.
Try this resolve.
final List<Future<dynamic>> featureList = <Future<dynamic>>[];
for (final Partner partner in partnerList) {
featureList.add(repository.fetchAvatar(partner.uid));
}
await Future.wait<dynamic>(featureList);
If want parallel execution you should switch to multi thread concept called Isolates
mix this with async/await concepts . You can also check this website for more
https://buildflutter.com/flutter-threading-isolates-future-async-and-await/
Using async / await like that is useful when you need a resource before executing the next task.
In your example you don't do really useful things, but imagine you call firstAsync, that gives you a stored authorization token in your phone, then you call secondAsync giving this token get asynchronously and execute an HTTP request and then checking the result of this request.
In this case you don't block the UI thread (user can interact with your app) and other tasks (get token, HTTP request...) are done in background.
i think you miss understood how flutter works first flutter is not multi threaded.....!
second if it isn't multi threaded how can it executes parallel tasks, which doesnt happen....! here is some links that will help you understand more https://webdev.dartlang.org/articles/performance/event-loop
https://www.dartlang.org/tutorials/language/futures
flutter doesn't put futures on another thread but what happens that they are added to a queue the links that i added are for event loop and how future works. hope you get it , feel free to ask me :)
Say we have an asynchronous function that may resolve within any duration. However fast it may be, we want to ensure it never resolves before t time as passed. To achieve this in a reusable way, we create a function (here in JavaScript):
async function stall(task, minDuration) {
// Wait until both `task` and `delay(minDuration)` resolves
// and return the results from `task`
const results = await Promise.all([task, delay(minDuration)])
return results[0]
}
where delay() is a simple function that resolves after a given amount of time.
Is there a canonical name for the stall() function?
Currently having a simple funtion :
async function getItem(id) {
const item = await ... doing a bunch of time consuming stuff
return item;
}
Coming from synchronous languages, and still understanding async/awaits from the surface; i'd expected the following to return the resolved item :
const item = getItem('items-8');
console.log(item); // Just getting a promise
The question is a bit 'cosmetic', but as async/await kind of solves callbacks, i'd be looking to get further and even avoid thenables things.
Is it possible to get such one-liner variable assignment with async/awaits ? What would be the syntax/code structure ?
async..await is syntactic sugar for ES6 promises, it allows to write asynchronous code in synchrounous-like manner. The code that uses promises cannot be synchronous, because promises are asynchronous.
In order to be written in synchronous manner, the code that uses getItem should reside in async function, too. If it is top-level and doesn't reside in another function (like application initialization code), it can reside in async IIFE.
It's either
getItem('items-8').then(item => {
...
});
Or
// inside `async` function
const item = await getItem('items-8');
...
I am trying to implement Rx stream/observable merging with Hack async, and a core step is described by the title. A code version of this step would look something like this:
<?hh // strict
async function foo(Awaitable<Iterable<T>> $collection): Awaitable<void> {
$ordered_generator = async_collection_to_gen($collection) // (**)
foreach($ordered_generator await as $v) {
// do something with each awaited value in the time-order they are resolved
}
}
However, after mulling it over, I don't think I can write the starred (**) function. I've found that at some point or another, the implementations I've tried require functionality akin to JS's Promise.race, which resolves when the first of a collection Promises resolves/rejects. However, all of Hack's Awaitable collection helpers create an Awaitable of a fully resolved collection. Furthermore, Hack doesn't permit that we don't await async calls from async functions, which I've also found to be necessary.
Is it possible to anyone's knowledge?
This is possible actually! I dug around and stumbled upon a fork of asio-utilities by #jano implementing an AsyncPoll class. See PR for usage. It does exactly as I hoped.
So it turns out, there is an Awaitable called ConditionWaitHandle with succeed and fail methods* that can be invoked by any context (so long as the underlying WaitHandle hasn't expired yet), forcing the ConditionWaitHandle to resolve with the passed values.
I gave the code a hard look, and underneath it all, it works by successive Awaitable races, which ConditionWaitHandle permits. More specifically, the collection of Awaitables is compressed via AwaitAllWaitHandles (aka \HH\Asio\v) which resolves as slowly as the slowest Awaitable, then nested within a ConditionWaitHandle. Each Awaitable is awaited in an async function that triggers the common ConditionWaitHandle, concluding the race. This is repeated until the Awaitables have all resolved.
Here's a more compact implementation of a race using the same philosophy:
<?hh
function wait(int $i): Awaitable<void> {
return Race::wrap(async { await HH\Asio\usleep($i); return $i; });
}
// $wait_handle = null;
class Race {
public static ?ConditionWaitHandle $handle = null;
public static async function wrap<T>(Awaitable<T> $v): Awaitable<void> {
$ret = await $v;
$handle = self::$handle;
invariant(!is_null($handle), '');
$handle->succeed($ret);
}
}
Race::$handle = ConditionWaitHandle::create(
\HH\Asio\v(
Vector{
(wait(1))->getWaitHandle(),
(wait(1000000))->getWaitHandle()
}
)
);
printf("%d microsecond `wait` wins!", \HH\Asio\join(Race::$handle));
Very elegant solution, thanks #jano!
*(the semblance to promises/deferred intensifies)
I am curious how premature completion via ConditionWaitHandle meshes with the philosophy that all Awaitables should run to completion.
What is the correct pattern to consume a highland stream using an async function? It seems the .each method does not work with an node-style async function wrapped using wrapCallback.
I would like to do something like the following. Note that I would like to catch errors generated in the .each function as well and trigger a .done function when all async consumption is done.
function asyncConsume(item,cb) {
// perform async operation based on data stream here
return cb(null,item);
}
var wrappedFunction = _.wrapCallback(asyncConsume);
highlandStream.each(wrappedFunction).done('All successfully done!')
.stopOnError(function(err) {
console.log('This error handler catches errors in .each() as well.');
});
Thank you.
As far as I know, you can only use an async function with Highland map function (after the function has been wrapCallbacked). The map does not actually call the function, so you need a Highland series or parallel call to follow and actually handle the calls.
I think you want flatMap here because you are transforming your items into a new stream:
highlandStream
.flatMap(function(item) {
return wrappedFunction(item);
}).each(someFn).done('All successfully done!')
.stopOnError(function(err) {
console.log('This error handler catches errors in .each() as well.');
});