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\section{The Timer Class}
The Timer class is a straightforward periodic timer which increments its
clock in its own thread. It provides operations for starting, and stopping
timing, and reading the current time.
\begin{vdm_al}
class Timer
\end{vdm_al}
The Timer has two instance variables the current time and a flag indicating
whether the Timer is active or not (the current time is only incremented
if the Timer is active).
\begin{vdm_al}
instance variables
curTime : nat := 0;
active : bool := false;
\end{vdm_al}
The Timer provides straightforward operations which need no further explanation.
\begin{vdm_al}
operations
public Start : () ==> ()
Start() ==
(active := true;
curTime := 0);
public Stop : () ==> ()
Stop() ==
active := false;
pure public GetTime : () ==> nat
GetTime() ==
return curTime;
IncTime: () ==> ()
IncTime() ==
if active
then curTime := curTime + 100;
\end{vdm_al}
The Timer's thread ensures that the current time is incremented. We take one
time unit for our Timer to correspond to 10 system time units.
\begin{vdm_al}
-- FIXME: Example used RT constructs must be rewritten using threads
--thread
-- periodic(1000)(IncTime)
end Timer
\end{vdm_al}
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