Q156739: BUG: Show Event Calls Refresh Despite NODEFAULT in Subclass

Article: Q156739
Product(s): Microsoft FoxPro
Version(s): WINDOWS:5.0,5.0a,6.0
Operating System(s): 
Keyword(s): kbprogramming kbvfp kbvfp500aBUG kbvfp500bugkbbuglist
Last Modified: 14-DEC-1999

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 5.0, 5.0a, 6.0 
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SYMPTOMS
========

When an instance of a form subclass has its Show() method called, if its
Refresh() method calls the parentclass Refresh(), a NODEFAULT in the subclass
Refresh() will be ignored and the Refresh() method for each control on the form
will be called twice.

WORKAROUND
==========

Conditionally issue a NODEFAULT in the parent class Refresh() method.

STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products listed at
the beginning of this article.

MORE INFORMATION
================

Normally, this behavior will not create much of a problem because it will result
in the Refresh() for each control getting called twice by the Show() only if the
conditions in SYMPTOMS section are met. In the rare case that this actually
causes a problem, the steps described below in Demonstration of Workaround can
be utilized.

Steps to Reproduce Problem
--------------------------

1. Run the following code from a program (.PRG) file:

     * Start of code example
     *
     CLEAR
     oForm = CREATE("FormChild")
     ACTIVATE SCREEN
     ?'Following lines from Show()'
     oForm.SHOW()
     ACTIVATE SCREEN
     ? ''
     ?'Following lines from Refresh()'
     oForm.REFRESH()
     ACTIVATE SCREEN
     ? ''
     ?'Following lines from Show()'
     oForm.SHOW()

     DEFINE CLASS FormChild AS FormParent

        AUTOCENTER = .T.

        ADD OBJECT text1 AS TEXTBOX

        PROCEDURE text1.REFRESH
           ACTIVATE SCREEN
           ?'In Textbox Refresh'
        ENDPROC

        PROCEDURE REFRESH()
           ACTIVATE SCREEN
           ?'In FormChild Refresh'
           DODEFAULT()
           NODEFAULT
        ENDPROC
     ENDDEFINE

     DEFINE CLASS FormParent AS FORM
        PROCEDURE REFRESH()
           ACTIVATE SCREEN
           ?'In FormParent Refresh'
        ENDPROC
     ENDDEFINE
     *
     * End of code example

2. In spite of the NODEFAULT in the Refresh() of the FormChild class, when the
  Show() method is called, the Refresh() of Text1 will get called twice: once
  by the FormChild Refresh() and once by the FormParent Refresh(). This does
  not occur if the Refresh() is called directly, only if the Refresh() is
  called implicitly through the Show() method.

Demonstration of Workaround
---------------------------

Rationale - Set a flag in the Show() that causes the parent class Refresh() to
conditionally execute a NODEFAULT and not carry out its default behavior of
calling the refresh of all controls on the form.

1. Run the following code from a program (.PRG) file. This is a modification of
  the code above. Add a property of lIsShow to the FormParent class, set it to
  .T. (true) in the Show(), conditionally issue a NODEFAULT in the ParentClass
  Refresh(), and flip lIsShow back to .F.(false). Note that this workaround
  requires changes to the parent class only, so all subclasses will be [ASCII
  147]fixed[ASCII 148] by implementing the workaround in the parent class.

     * Start of code example
     *
     CLEAR
     oForm = CREATE("FormChild")
     ACTIVATE SCREEN
     ?'Following lines from Show()'
     oForm.SHOW()
     ACTIVATE SCREEN
     ? ''
     ?'Following lines from Refresh()'
     oForm.REFRESH()
     ACTIVATE SCREEN
     ? ''
     ?'Following lines from Show()'
     oForm.SHOW()

     DEFINE CLASS FormChild AS FormParent

        AUTOCENTER = .T.

        ADD OBJECT text1 AS TEXTBOX

        PROCEDURE text1.REFRESH
           ACTIVATE SCREEN
           ?'In Textbox Refresh'
        ENDPROC

        PROCEDURE REFRESH()
           ACTIVATE SCREEN
           ?'In FormChild Refresh'
           DODEFAULT()
           NODEFAULT
        ENDPROC
     ENDDEFINE

     DEFINE CLASS FormParent AS FORM

        lIsShow = .F.                  && This is the flag

        PROCEDURE SHOW()               && This is added
           LPARAMETERS nStyle
           THISFORM.lIsShow = .T.      && Set the flag, so you can use it the
                                       && next time the Refresh gets called.
        ENDPROC

        PROCEDURE REFRESH()
           IF THIS.lIsShow             && Here you read the flag and
              NODEFAULT                && conditionally issue NODEFAULT
              THIS.lIsShow = .F.       && Flip the flag back to .F.
           ENDIF
           ACTIVATE SCREEN
           ?'In FormParent Refresh'
        ENDPROC

     ENDDEFINE
     *
     * End of code example

2. Note that the Textbox Refresh() is called only once when the Show() method is
  invoked.

Additional query words: kbvfp500 kbvfp500a kbvfp600

======================================================================
Keywords          : kbprogramming kbvfp kbvfp500aBUG kbvfp500bug kbbuglist
Technology        : kbVFPsearch kbAudDeveloper kbVFP500 kbVFP600 kbVFP500a
Version           : WINDOWS:5.0,5.0a,6.0
Issue type        : kbbug

=============================================================================

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