Run multiple future-returning & async functions with limited concurrency

Motivation

This is a port of p-limit from javascript to dart.

Install

flutter pub add p_limit

Usage

import 'package:p_limit/p_limit.dart';

void main() async {
  final limit = PLimit<string>(1);
  final input = [
    limit(() => fetchSomething('foo')),
    limit(() => fetchSomething('bar')),
    limit(() => doSomething())
  ];
  // Only one future is run at once
  final results = await Future.wait(input);
  print(results);
}

API

PLimit(concurrency)

Returns a limit function.

concurrency

Type: number
Minimum: 1
Default: Infinity

Concurrency limit.

limit(fn)

Returns the future returned by calling fn().

fn

Type: Function

Future-returning/async function.

limit.activeCount

The number of futures that are currently running.

limit.pendingCount

The number of futures that are waiting to run (i.e. their internal fn was not called yet).

limit.clearQueue()

Discard pending futures that are waiting to run.

This might be useful if you want to teardown the queue at the end of your program's lifecycle or discard any function calls referencing an intermediary state of your app.

Note: This does not cancel futures that are already running.

FAQ

How is this different from the async_task package?

This package is only about limiting the number of concurrent executions, while async_task is a fully featured queue implementation similar to classic thread pools and with lots of different options.

But what about args?

You probably don't need this optimization unless you're pushing a lot of functions. If you are in need of this, make a pull request or an issue on Github.

Related

• async_task - Asynchronous tasks and parallel executors
• concurrent_queue - Priority queue with concurrency control

Additional information

If you have an issue, let me know on Github issues

Special Thanks

Since this is a port of p-limit, I'd like to thank the creator for the amazing work and maintenance.