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\begin{vdm_al}
class AbstractTask
instance variables
-- keep the name of the task for easy logging
name : seq of char := [];
-- the queue for normal events to be handled by this task
events : seq of NetworkEvent := [];
-- the queue of high-priority events to be handled by this task
interrupts : seq of InterruptEvent := [];
-- a link to the dispatcher for out-going messages (events)
dispatcher : EventDispatcher
operations
public AbstractTask: seq of char * EventDispatcher ==> AbstractTask
AbstractTask (pnm, ped) == atomic ( name := pnm; dispatcher := ped; );
pure public getName: () ==> seq of char
getName () == return name;
-- setEvent is a call-back used by the EventDispatcher to insert
-- events into the appropriate event queue of this AbstractTask instance
public setEvent: Event ==> ()
setEvent (pe) ==
if isofclass(NetworkEvent,pe)
then events := events ^ [pe]
else interrupts := interrupts ^ [pe];
-- getEvent is called by the event loop of this AbstractTask instance to
-- process incoming events when they are available. note that getEvent
-- is blocked by a permission predicate (see sync) when no events are
-- available and also note that getEvent gives interrupts priority over
-- other events
protected getEvent: () ==> Event
getEvent () ==
if len interrupts > 0
then ( dcl res: Event := hd interrupts;
interrupts := tl interrupts;
return res )
else ( dcl res: Event := hd events;
events := tl events;
return res );
-- handleEvent shall be overloaded by the derived classes to implement
-- the actual event loop handling. a typical event loop handler would be
-- thread while (true) do handleEvent(getEvent())
--protected handleEvent: Event ==> ()
--handleEvent (-) == is subclass responsibility;
-- sendMessage is used to send a message to another task
-- typically used for inter process communication
protected sendMessage: seq of char * nat ==> ()
sendMessage (pnm, pid) == dispatcher.SendNetwork(name, pnm, pid);
-- raiseInterrupt is used to send a high-priority message
-- typically used to communicate from environment to the system or vice versa
protected raiseInterrupt: seq of char * nat ==> ()
raiseInterrupt (pnm, pid) == dispatcher.SendInterrupt(name, pnm, pid)
sync
-- setEvent and getEvent are mutually exclusive
mutex (setEvent, getEvent);
-- getEvent is blocked until at least one message is available
per getEvent => len events > 0 or len interrupts > 0
end AbstractTask
\end{vdm_al}
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