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Member Classes Bring Flexibility

The new VFP 8 feature often referred to as "Member Classes" is a set of new properties and new ways to define classes that can bring much more flexibility when working with certain controls.


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eed to define several pages in a PageFrame with different properties and settings? No problem. How about better control of grid column headers? No problem. By changing the way you can define and work with member classes, Microsoft has given VFP developers tremendous flexibility in working with PageFrames and grids.

In Visual FoxPro, there are certain classes that have meaning only when they're in containers: for example, Pages and Headers can live only in PageFrame and Column objects. In versions before VFP 8, if we wanted to subclass these classes, we needed to manually add them using the AddObject method. But, there were problems with this approach. For example, this kept us from putting controls on a subclassed page in the Form Designer.

However, VFP8 adds the MemberClass and MemberClassLibrary properties, as well as the HeaderClass and HeaderClassLibrary properties for the Column object. This lets us tell the containers what kind of controls they should be using when we change the properties that tell how many controls they contain. For example:

   *!* Create Header class library
   TEXT TO lcMembHeader NOSHOW 
      DEFINE CLASS hdrMembClass as Header
         cSort = "ASCENDING"
         PROCEDURE DblClick
            IF This.cSort = "ASCENDING"
               This.cSort = "DESCENDING"
            ELSE
               This.cSort = "ASCENDING"
            ENDIF
            MESSAGEBOX(This.cSort)
         ENDPROC 
      ENDDEFINE 
   ENDTEXT 
   
   *!* Create Column class library, and pull 
   *!* from Header class library
   TEXT TO lcMembColumn NOSHOW 
      DEFINE CLASS colMembClass AS Column
         HeaderClass = "hdrMembClass"
         HeaderClassLibrary = "membHeader.prg"
      ENDDEFINE 
   ENDTEXT 
   
   *!* Create Grid class library, and pull 
   *!* from Column class library
   TEXT TO lcMembGrid NOSHOW 
      DEFINE CLASS grdMembClass as Grid
         MemberClass = "colMembClass"
         MemberClassLibrary = "membColumn.prg"
      ENDDEFINE 
   ENDTEXT 
   
   *!* Write the libraries to disk
   STRTOFILE(lcMembHeader, "membHeader.prg")
   STRTOFILE(lcMembColumn, "membColumn.prg")
   STRTOFILE(lcMembGrid, "membGrid.prg")
   USE HOME(1) + "labels"
   loform = CREATEOBJECT("Form")
   loForm.NewObject("Grid1", "grdMembClass", 
      "membGrid.prg")
   WITH loForm.Grid1
      .RecordSource = "labels"
      .RecordSourceType = 1
      .Visible = .t.
   ENDWITH 
   loForm.Show(1)

With every release of VFP, there is something added that's so obvious, it tends to go in everyone's defaults. With VFP3, it was Label.AutoSize = .T. VFP5 added ComboBox.BoundTo = .T. Now that we're up to version 8, I think we'll see something like the following:

   DEFINE CLASS pagRefresh as Page
      lRefresh = .T.
      PROCEDURE Activate
         IF This.lRefresh
            This.Refresh()
         ENDIF 
      ENDPROC
   ENDDEFINE

I added an lRefresh property to this class because of the possibility that you might not want a given page to refresh as soon as you switch to it.

Once you've defined the classes, you can use them in the Form Designer by setting the properties at design time. When you make a design-time change to the MemberClass property, all of the controls will use the same class, even those already defined at that point.

However, if you change the MemberClass and MemberClassLibrary properties at runtime, this will not affect existing controls.  This opens up some interesting possibilities for dynamically switching between MemberClasses as you instantiate various objects at runtime.

Listing 1 contains code to first change the MemberClass and MemberClassLibrary properties on a PageFrame in (simulated) design mode, then shows that at runtime the properties can be changed with just the new Pages being instantiated with the newly-defined MemberClass.

For a more detailed example, please see the "Member Classes" Solution Sample that ships with VFP8. The easiest way to do this is to use the "Solution Samples" pane in the Task Pane, but you can still run SOLUTION.APP as always to get to the samples.



   
Garrett Fitzgerald   of Donnael Consulting, has been working with various flavors of Fox since 1994, from FoxBASE to VFP 7. He received the Microsoft Most Valuable Professional award for online peer support in 1998 and 1999, and followed that up with a stint in Microsoft Product Support Services. He's currently working full-time as a contractor with Volt Information Sciences. garrett@donnael.com.
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