PRB: Order of Read Level Clauses Different in Visual FoxPro

MORE INFORMATION

When developing applications in Visual FoxPro it is very important to learn the order of events. Because the event model in Visual FoxPro is greatly enhanced over previous versions, items may not fire when expected if compared to earlier versions of FoxPro.

Code Sample

The following example demonstrates how Visual FoxPro's READ DEACTIVATE clause fires in two different places depending on which version of FoxPro runs the program. Create the following program, and name it Readordr.prg:

****Begin ReadOrdr.PRG****

   SET PRINT TO readordr.log
   SET PRINT ON
   COUNT = 1
   CLEAR
   SET TALK OFF
   mquit = .F.
   ON KEY LABEL F5 mquit = .T. && Use F5 to clear the READ

   ***Define & Activate Windows, Gets and READ
   DEFINE WINDOW window1 FROM 1,1 TO 20,60 SYSTEM CLOSE
   ACTIVATE WINDOW window1

   @1,1 GET X1 ;
     DEFAULT "1234567890" ;
     SIZE 1,10 ;
     FONT "FIXEDSYS",10 ;
     WHEN ret("@get x1 when clause") ;
     VALID ret("@get x1 valid clause")

   @1, 40 GET Y1 ;
     DEFAULT "1234567890" ;
     SIZE 1,10 ;
     FONT "FIXEDSYS",10 ;
     WHEN ret("@get y1 when clause") ;
     VALID ret("@get y1 valid clause")

   DEFINE WINDOW window2 FROM 21,1 TO 40,60 SYSTEM CLOSE
   ACTIVATE WINDOW window2

   @1,1 GET X2 ;
     DEFAULT "1234567890" ;
     SIZE 1,10 ;
     FONT "FIXEDSYS",10 ;
     WHEN ret("@get x2 when clause") ;
     VALID ret("@get x2 valid clause")

   @1, 40 GET Y2 ;
     DEFAULT "1234567890" ;
     SIZE 1,10 ;
     FONT "FIXEDSYS",10 ;
     WHEN ret("@get y2 when clause") ;
     VALID ret("@get y2 valid clause")

   READ ;
     CYCLE ;
     MODAL ;
     ACTIVATE ret("read activate clause") ;
     DEACTIVATE ret("read deactivate clause") ;
     VALID ret("read valid clause", IIF(mquit = .T., .T.,.F.)) ;
     WHEN ret("read when clause") ;
     SHOW ret("read show clause")

   ***CleanUp Code
   RELEASE WINDOW window1
   RELEASE WINDOW window2
   SET PRINT OFF
   SET PRINT TO

   ***************************
   *  Function to print return values to Readordr.log file
   *  This function is called every time an event occurs, events found
   *  in this program include:
   *       READ WHEN
   *            ACTIVATE
   *            SHOW
   *            DEACTIVATE
   *            VALID
   *       SHOW GET WHEN
   *                VALID
   **************************

   FUNCTION ret
   PARAMETERS clause, retval
   IF param() < 2
      retval = .T.
   ENDIF
   ? ALLTRIM(STR(count)) + ": "
   ?? clause
   count = count + 1
   RETURN retval

   ***End of Program Readordr.Prg
				

When you run the Readordr.prg program, each of the events that fires is written to a log file name Readordr.log. You can use this file to check the order of events in Visual FoxPro or in a previous version of FoxPro.

Steps to Demonstrate Behavior

To illustrate this point, execute the Readordr.prg program in Visual FoxPro, and then follow these steps:

1. When the screen is run, these events occur in this order:

   READ WHEN (determines if we may enter the READ)
   READ ACTIVATE (fires when READ is executed)
   READ SHOW (occurs when @GET fields are shown on the screen)
   @GET X1 WHEN (occurs when the cursor moves to the first GET object)

   This behavior is identical to the behavior of previous FoxPro versions.
2. From the first GET field (X1), press the TAB key to move the focus to the second field (Y1) on the screen. The following events occur:

   @GET X1 VALID (occurs when we attempt to leave the object)
   @GET Y1 WHEN

   This behavior is identical to the behavior of previous FoxPro versions.
3. From the second GET field (Y1), press the TAB key again, this moves the focus from Y1 to X2, but also moves from Window1 to Window2. These events occur:

   @GET Y1 VALID
   @GET X2 WHEN
   READ DEACTIVATE (Occurs when we move from one window to the next)

   This behavior is different from previous versions of FoxPro. Notice the WHEN clause for the first object on window2 is fired before the READ DEACTIVATE. In previous versions, the order of events was:

   @GET Y1 VALID
   READ DEACTIVATE

4. To exit the Readordr.prg program, press the F5 key and then the ESC key.
5. Run the same code in a previous version of FoxPro, and compare the log files.

Why Visual FoxPro Uses a Different Firing Order

The reason why Visual FoxPro differs is because of the current event model. The Visual FoxPro order of events is described in the Visual FoxPro Help file in the "Visual FoxPro Event Sequence" topic.

In the example, you press TAB to move from window1 to window2. The events are firing in the correct order. First, the VALID clause of Y1 determines if you may leave the object. Second, the WHEN clause is executed for the first object (X2) on window2. This occurs because Visual FoxPro's order of events executes the WHEN clause for the next object in the TAB order, before the form object (window1) loses the focus. Visual FoxPro has to first determine if the next object will get the focus before the current object loses the focus. When the form object (window1) loses the focus, the READ DEACTIVATE is executed.

If for example, the WHEN clause for X2 fails, the form object (window1) will not lose focus, so the DEACTIVATE clause should not fire.