polling_event_listener.dart

/**
 * An example of a singleton that does some polling for an event 
 * and calls a callback when the event is fired.
 */

import 'dart:async';

void main() {
  print("Running..");
  
  EventListener listener = EventListener()..addCallback(EventHandler(
    event: 3,
    callback: ()=>print('3 did up in this bitch')
  ));
  
  EventListener listener2 = EventListener()..addCallback(EventHandler(
    event: 5,
    callback: ()=>print('5 did up in this bitch')
  ));
  
  
}

class EventHandler{
  int event;
  Function callback;
  EventHandler({
    required this.event,
    required this.callback
  });
}

class EventListener{
  static final EventListener _instance = EventListener._internal();
  static int _counter = 0;
  static List<EventHandler> _handlers = [];
  static Timer? _timer;
  
  EventListener._internal();
  
  factory EventListener(){
    return _instance;
  }
  
  void addCallback(EventHandler listener){
    _handlers.add(listener);
    if(_timer == null) _pollTheThing();
  }
  
  void cancelPolling(){
    if(null != _timer){
      _timer?.cancel();
      _timer = null;
    }
  }
  
  void _pollTheThing(){
    _timer = Timer.periodic(Duration(seconds: 5), (timer) async {

      List<int> indexesToRemove = [];
      for(int i=0; i<_handlers.length; i++){
        if(_handlers[i].event == _counter){
          indexesToRemove.add(i);
          _handlers[i].callback();
        }
      }
      
      for(int i=0; i<indexesToRemove.length; i++){
        _handlers.removeAt(indexesToRemove[i]);
      }
      
      if(_handlers.isEmpty){
        cancelPolling();
      }
      
      print('Counter: $_counter');
      _counter++;
    });
  }
  
}