How to Reference Objects in the Container Hierarchy

• Microsoft Visual FoxPro for Windows, version 3.0

SUMMARY

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

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

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

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

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

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)

   THIS.Parent.grdMygrid1.BackColor = RGB(0,0,255)

REFERENCES

Microsoft Visual FoxPro Developer's Guide, Chapter 3.

FoxPro Advisor June 1995 issue, "I hear a Symphony," by Alan Schwartz.