Limitations of the AINSTANCE Function

ID: Q129865

3.00 WINDOWS

 FxprgGeneral

The information in this article applies to:

Microsoft Visual FoxPro for Windows, version 3.0

SUMMARY

The AINSTANCE function places all instances of a class into a memory variable array. The function also returns the number of instances of a class. The function does not, however, report information about "member objects" added by the ADD OBJECT method. "Member objects" are not, in themselves, instances; they are a part (member) of a container.

This article provides an explanation of this function to clarify what the AINSTANCE function does and does not do.

MORE INFORMATION

The AINSTANCE function places all instances of a class into a memory variable array:

   =AINSTANCE(aMyArray,"Form") &&  Places all instances of the base class
                               &&  Form into an array called "aMyArray"

In the following example, AINSTANCE places all "created" instances into an array. The WAIT WINDOWS will show how many "created" instances exist. The list box will show "0" because it is added to the Form by the ADD OBJECT method and is not "created." As an alternative to the AINSTANCE function, you can use the AMEMBERS function to return information about whether or not an object exists. The AMEMBERS function is used to populate the list box in the following example:

   && Start of the Example:

   frmMyForm=CREATEOBJECT("FORM")
      WITH frmMyForm
          .Left = 200
          .Top  = 100
      ENDWITH

   cmdMyQuitButton=CREATEOBJECT("cmdQuitButton")

   && If the above line is changed to a comment, the form and button
   && still work the same but the AINSTANCE function will not
   && return information for the commandbutton.

   frmMyForm.ADDOBJECT("cmdMyQuitButton", "cmdQuitButton")
   frmMyform.ADDOBJECT("cmdMyButton","commandbutton")
   &&The above button will not be visible on the Form

   frmMyForm.ADDOBJECT("lstMyList","ListBox")

   frmMyForm.SHOW

   nMvar=AINSTANCE(aMyArray,"Form")
   WAIT WINDOW "Forms = "+ALLTRIM(str(nMvar))

   nMvar=AINSTANCE(aMyArray,"cmdQuitButton")
   WAIT WINDOW "QuitButtons = "+ALLTRIM(str(nMvar))

   nMvar=AINSTANCE(aMyArray,"lstMyList")
   WAIT WINDOW "List Boxes = "+ALLTRIM(str(nMvar))

   =AMEMBERS(aMarray,frmMyForm,1)

   nX=ASUBSCRIPT(aMarray,(ASCAN(aMarray,"Object")),1)

   nMPos   = 1
   nMySize = 2

   FOR i = 1 to ALEN(aMarray)*2
      nMPos    = ASCAN(aMarray,"Object",nMPos)
      IF nMPos = 0
          EXIT
      ELSE
          DIMENSION aMarray2(nMySize/2,2)
      ENDIF

      WAIT WINDOW aMarray(ASUBSCRIPT(aMarray,;
      (ASCAN(aMarray,"Object",nMPos)),1),1)+"  "+;
      aMarray(ASUBSCRIPT(aMarray,;
      (ASCAN(aMarray,"Object",nMPos)),1),2)

      =ACOPY(aMarray,aMarray2,nMPos-1,2,nMySize-1)

      nMPos   = nMPos+1
      nMySize = nMySize+2

   ENDFOR

   WAIT WINDOW "No Other Objects Found"

   && The list box displays the members of the Form or rather the objects
   && that are contained on the Form.

   WITH frmMyForm.lstMyList
        .visible = .T.
        .width   = 300
        .height  = 150
        .top     = 30
        .left    = 30
        .rowsourcetype = 5
        .rowsource     = "aMarray2"
        .columncount   = 2
        .columnwidths  = "150"
   ENDWITH

   READ EVENTS

   DEFINE CLASS cmdQuitButton AS CommandButton
      PROCEDURE Click
         RELEASE ThisForm
         CLEAR EVENTS
      ENDPROC

      Visible = .T.
      Caption = "Quit"

      Top    = 200
      Left   = 300
      Width  = 60
      Height = 30
   ENDDEFINE

   && End of the Example.

The AINSTANCE function returns object information only on those objects that have been instantiated programmatically by the CREATEOBJECT function or through the interface, even though the object is a member of a defined class. The AINSTANCE function can not return object information on "member objects."

Additional reference words: 3.00 VFoxWin KBCategory: KBSubcategory: FxprgGeneral

Keywords          : kbcode FxprgGeneral 
Version           : 3.00
Platform          : WINDOWS