Subscribable Event

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• using { /Verse.org/Concurrency }
  
  subscribable_event<public>(t:type) := class(subscribable(t), signalable(t)):
  
      var Callbacks<private>:[]tuple(type{_(:t):void}, int) = array{}
      var NextID<private>:int = 0
      
      Signal<override>(Payload:t):void =
          for (Callback : Callbacks) do Callback(0)(Payload)
  
      Subscribe<override>(Callback:type{_(:t):void})<transacts>:cancelable =
          ID := NextID
          set NextID += 1
          set Callbacks += array{(Callback, ID)}
          return basic_cancelable:
              ID := ID
              CancelCallback := CancelSubscription
  
      CancelAllSubscriptions<public>()<transacts>:void =
          set Callbacks = array{}
  
      CancelSubscription<private>(ID:int)<transacts>:void =
          set Callbacks = for (Callback : Callbacks, Callback(1) <> ID) do Callback
  
  listenable_event<public>(t:type) := class(subscribable_event(t), listenable(t)):