How to run multiple async functions in order they were called (FIFO?)

Question

I've spent many hours looking for the solution, but since I am Dart begginer, I couldn't find it out by myself. What I want to achieve is to create something like queue for some of the async functions that are called randomly (let's say, when user is tapping a button in my app) from different points in code while the app is running. I want them to be executed in the order they were called, so basically I have async methods such as updateDate() and updatePoints() and when the user is tapping button X the updateDate() is going to be called (added to queue), and similar with Y and updatePoints(). When the user taps i. e. X, X, Y I want to run updateDate(), updateDate(), updatePoints() in this exact order. When one task is complete, another one is starting. I guess I can't use await to achieve that. Any hints would be appreciated!

Answer 1

import 'dart:async';
import 'dart:collection';
import 'dart:math';

Future<void> main() async {
  _simulateRealWork();
}

Scheduler _scheduler = Scheduler();

class Scheduler {
  bool _scheduled = false;

  Queue<Future Function()> _queue = Queue<Future Function()>();

  void schedule(Future Function() task) {
    _queue.add(task);
    if (!_scheduled) {
      _scheduled = true;
      Timer(Duration(seconds: 0), _execute);
    }
  }

  Future _execute() async {
    while (true) {
      if (_queue.isEmpty) {
        _scheduled = false;
        return;
      }

      var first = _queue.removeFirst();
      await first();
    }
  }
}

void _simulateRealWork() {
  var maxPeriod = 5;
  var count = 5;
  for (var i = 0; i < count; i++) {
    print('Timer $i');
    var random = Random();
    Timer(Duration(seconds: random.nextInt(maxPeriod)), () {
      print('Scheduled work $i');
      Future work() async {
        print('Started work $i');
        await Future.delayed(Duration(seconds: random.nextInt(maxPeriod)));
        print('Ended work $i');
      }

      _scheduler.schedule(work);
    });
  }
}

Result:

Timer 0 Timer 1 Timer 2 Timer 3 Timer 4 Scheduled work 2 Started work 2 Scheduled work 0 Scheduled work 3 Ended work 2 Started work 0 Scheduled work 1 Scheduled work 4 Ended work 0 Started work 3 Ended work 3 Started work 1 Ended work 1 Started work 4 Ended work 4