Q131357: How to Reference Objects in the Container Hierarchy
Article: Q131357
Product(s): Microsoft FoxPro
Version(s): 3.00
Operating System(s):
Keyword(s): kbcode
Last Modified: 25-AUG-1999
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The information in this article applies to:
- Microsoft Visual FoxPro for Windows, version 3.0
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SUMMARY
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This article describes how to reference objects and manipulate them in the
container hierarchy. It shows how to use the generic object references (THIS,
THISFORM, THISFORMSET, and Parent) in Visual FoxPro.
MORE INFORMATION
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To manipulate an object, you must identify it in relation to the container
hierarchy. When referencing objects, you can use the names of the objects to
move through the container hierarchy. However, hard coding the names does not
allow the code to be reusable.
There are four keywords that make it easier to reference an object from within
the container hierarchy: THIS, THISFORM, THISFORMSET, and Parent. Visual FoxPro
compiles THIS, THISFORM, and THISFORMSET into tokens that resolve to their
actual object names at run time. By using these four generic keywords, you
ensure that the code is reusable.
THIS Keyword
------------
The keyword THIS allows you to reference a class's property or method. If there
are multiple instances of an object, THIS always refers to the instance in which
the code is executing. THIS is a generic reference and always refers to the
correct object, thus making the code reusable.
For example, if you want the caption of a command button when clicked to change
to "OK," place the following code in the Click event of the command button:
THIS.Caption="OK"
Sending Messages Without Naming the Container
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In a method within a container, you can send a message to another object without
using the name of the container. In fact, it's better (more generic) not to name
it explicitly. There are two techniques for referring to a contained object
indirectly:
- Use the token THISFORM or THISFORMSET keyword.
-or-
- Use the Parent keyword.
THISFORM Keyword
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THISFORM is a generic object reference to the form in which the code is
executing. You can manipulate the form or any object on the form container by
using THISFORM.
For example, say you have a form named myform with two command buttons named
cmdBtn1 and cmdBtn2. You can manipulate the form and the cmdBtn2 button by
clicking the cmdBtn1 button. In this example, to change the background color of
the form to red and the caption of cmdBtn2 to "Hello," place the following code
in the Click event of the cmdBtn1 button:
THISFORM.BackColor = RGB(255,0,0)
THISFORM.cmdBtn2.Caption = "Hello"
THISFORMSET Keyword
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THISFORMSET is a generic object reference to the form set in which the code is
executing. You can manipulate the form set or any object in the form set
container by using THISFORMSET.
For example, say you have a form set named frsMyfrs with two forms named
frmMyfrm1 and frmMyfrm2. The frmMyfrm1 form contains a command button named
cmdBtn1, and the frmMyfrm2 form contains a label named lblMylb1. You can change
the background color of the label on frmMyfrm2 to blue when you click cmdBtn1 in
frmMyfrm1 by placing the following code in the Click event of the cmdBtn1
button.
THISFORMSET.frmMyfrm2.lblMylb1.BackColor = RGB(0,0,255)
Parent Keyword
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Parent is a generic keyword that refers to the immediate container of the object.
It is another way to reference objects without hard-coding their names. By
evaluating the expression THIS.PARENT, you get the name of the container. These
can be chained together, as in THIS.PARENT.PARENT. The only disadvantage of
PARENT is that you need to know how many levels of containers to point through
to get the name of the one that contains your target object.
For example, say you have a form with a page frame named pgfPgframe1 that has two
pages named pagPage1 and pagPage2 on it. The pagPage1 page has a grid named
grdMygrid1 and a command button named cmdCommand1 on it. To change the
background color of the grid to blue when you click the cmdCommand1 button, you
could place the following code in the cmdCommand1 button's Click event:
THISFORMSET.pgfPgframe1.pagPage1.grdMygrid1.BackColor = RGB(0,0,255)
But this hard-codes the names of many objects, so the code is less reusable. You
can use the Parent keyword to refer to the container of an object. Because
cmdCommand1 and grdMygrid1 are both in the same container, OR have the same
Parent (pagPage1), you can use the Parent keyword. Therefore, you can accomplish
the same result by placing the following code in the cmdCommand1 button's Click
event:
THIS.Parent.grdMygrid1.BackColor = RGB(0,0,255)
REFERENCES
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Microsoft Visual FoxPro Developer's Guide, Chapter 3.
FoxPro Advisor June 1995 issue, "I hear a Symphony," by Alan Schwartz.
Additional query words: VFoxWin
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Keywords : kbcode
Technology : kbVFPsearch kbAudDeveloper kbVFP300
Version : 3.00
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