Object Machine Demo Program

Introduction:

The Object Machine Demo program (OBJMCH.EXE) demonstrates a simple finite-
state machine (FSM).  The FSM monitors mouse button events (Up/Down) and 
produces "Click" and "Double Click" messages at the appropriate transitions.  
It models the method that Visual Basic uses to manage "Click" and "Double 
Click" events. (It is interesting to note that different Window's apps do not 
all recognize mouse clicks in a uniform way.) 

OBJMCH draws a state diagram and then monitors mouse button events.  As mouse 
events are recognized the program changes state colors indicating the current 
state of the FSM (see color codes below).  When a click or double click 
message transition occurs the FSM generates a message as a pop-up message box 
that disappears after a moment. 

OBJMCH uses an exaggerated time scale so that the state transitions occur slow 
enough to watch.  The user can adjust the timeout duration to slow down the 
FSM or to speed it up (see timer operation below).

Operation:

OBJMCH.EXE requires the Visual Basic run-time DLL VBRUN100.DLL  (It was not 
bundled with the program to save down-load time.)  Copy this DLL to either 
c:\windows\system or to the home directory of OBJMCH.EXE    With VRUN100.DLL 
installed, launch OBJMCH.EXE from Windows.  The program draws a state diagram.  
You can resize the program's window, however, at small scales the diagram will 
appear cluttered. 

Notice that the Startup state is green indicating that it is the current 
state.  Also, the initial state transition is blue indicating the path to the 
Startup state.

Move the mouse cursor anywhere over the program's state diagram and press any 
mouse button down (hold it down).  Observe the state transition that occurred 
from the Startup state to state FirstDown.  Hold the button down for two 
seconds and the FSM will transition to state LongDown.  Let the button up and 
the FSM returns to the Startup state. 

Now press a mouse button down and let it up.  This causes two state 
transitions ending up at state OneUp and producing a Click message.  (The click 
message disappears after a brief moment.)  Pressing a mouse button down again 
before time expires produces a Double Click message. 

The transition from state Startup to FirstDown starts a timer which counts 
down in a window at the bottom of the diagram.

If you study the Click and Double Click events produced by Visual Basic you 
will see that this state diagram models the method that Visual Basic uses to 
generate these events.  Note that there are many variations possible.  Test 
yourself with the following questions: (You may find these questions trivial,
however, they help to emphasize operational subtleties that can be difficult to 
specify without a state diagram). 

1) How would you change the state diagram so that there is no Click message
   generated before a Double Click message?  (Of course, there must still
   be a Click message generated when no Double Click occurs.)

2) Visual Basic doesn't recognize a click until the mouse button goes up.
   What changes are necessary to recognize a click when the mouse button
   first goes down?

3) Can you draw a state diagram to recognize the Morse code message "SOS"
   ( . . .  - - -  . . . )?  How about a state diagram to decipher any Morse
   code message?


State Diagram Color Codes: 

States and state transitions are color coded to indicate the current FSM 
state. The previous state is colored red, the current state is green, and the 
transition between them is colored blue. All other states and transition are
colored grey.

Timer Operation:

The FSM uses a timer to distinquish two clicks from a double click.  The timer
duration can be set by the user.  It defaults to two seconds.  Making the 
timeout duration longer gives the user more time to recognize a double click. 


For More Information:

For more information about finite-state machines (FSM) and applying FSMs to 
C++ objects, send your name and address plus $5 for handling to:

           The Aerosoft Company
           51 N. Cromwell
           Fairfield, IA 52556
Attention: Object Machines

Request article, "Object Machines".  This article describes the theory of FSMs 
and discusses their application to object-oriented programming.  It includes 
C++ examples.

You can mail comments or questions to the above address or send them via
CompuServe at 72260,3551.