Threads test - Elevators

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Here is a program exploring Delphi's TThreads class as the mechanism to run independent processes.  In this case, I've implemented the rudiments of an elevator simulator.  TElevator is a TThreads descendant which visually moves from floor to to floor, opening its doors at each stop.  

This is the basis for an Elevator Simulation program currently being developed.   Several days trying to implement elevator controls using queues and lists convinced me that perhaps I have finally found an application suitable for thread processing.     

Here are the tricks I've learned so far about using threads.

bulletAnything outside of it's own instance variables which is updated must be synchronized.  There is a Synchronize method to which schedules such routines to run serially from the main VCL unit's thread.    In this case, there are three methods (MoveIt, OpenDoorStep, CloseDoorStep)  that change the elevator display and therefore must run synchronized. 
bulletThe Execute method where all processing is performed is only called one time.  It is up to the programmer to not exit  until termination time.  In this case, I nested a "While moving"  loop inside of a "While not terminated" loop.   When the elevator is stopped at a floor,  moving flag is set to false, the elevator is suspended and I sleep 1/10th of a second for each iteration to minimized CPU usage while we're not doing anything.   When it's time to move again, the outside  force (button click) just  sets Moving to true and calls the  Resume method for the elevator to start it up again.  
bulletCalls to the Suspend method are counted and Resume calls must match Suspend calls before the thread is actually resumed , so it is important not to call Suspend redundantly.  
bulletWhenever the number of floors or elevators changes, I free and recreate all of the elevators.  Gracefully terminating the elevator's thread before destroying it was a little tricky.    There is a "Waitfor" method that  waits for the thread to actually terminate before continuing.  So the logical   sequence of method calls to free an elevators seems like it should be "Terminate; Waitfor; Free;"     If the elevator happens to be stopped (Suspended) when that sequence is executed, the call to Waitfor never returns.   The version that seem to work is :   Terminate;  If not suspended then waitfor; free;

The rest of the code is not trivial, but not too complex either.  I made an array of buttons to move the elevators one floor per push.   Only the buttons corresponding to the current number of elevators are made visible.    The TImage, Image1, is used only to provide a convenient way to define the field for floors and elevators.   It remains invisible in order to speed up the drawing the other elements.    A timer is used to periodically check how long each elevators doors have been open to close them when the specified "Door open" time has passed.

I've learned quite a bit in the process of coding this.  Perhaps  you will too!

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Created: July 29, 2017

Modified: July 29, 2017

 

 

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