| Inheritance tutorial |
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| Summary: | This is a step-by-step tutorial to show inheritance, specifically in Visual FoxPro forms, as a guidance for people who are not familiar with inheritance in general, or who don’t know how to implement it in Visual FoxPro.
The basic idea of inheritance is that all your forms, or several of your forms, obtain their properties and methods from a "base form". It is possible to do changes only once, in your "base form"; the changes will be propagated to all your forms.
| | Rating: | Rate this entry | | Created by: |  | | Hilmar Zonneveld 
Independent Consultant
Cochabamba |
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Introduction
The following should work in all versions of Visual FoxPro (i.e., version 3 and later).
The examples are not useful by themselves, they only serve to show how to use inheritance.
Create a new project
modify project inheritance
Create the base class
Select “Classes”, click on “New”. Name your class cMyForm, and base it on the class “Form”. Store it in class library MyClasses.
Give your form some color, selecting the property “BackColor” in the Property Sheet.
Save it.
Create a derived form
Create a form that is based on the class, with the command:
create form MyForm as cMyForm from MyClasses
Note that the form has the same BackColor as the one you defined in the class.
Save your form.
Propagating changes
Go back to the class (open class library MyClasses in the Project Manager, click on the “+” sign, and open class cMyForm).
Change the background color (change property BackColor in the Property Sheet).
Save the class, and open the form:
modify form MyForm
Note that the form’s background color changed.
Breaking inheritance
Change the BackColor directly in the form. If you do this, inheritance is “broken”, that is, any change in the BackColor of the class will no longer be applied in the form.
To restore inheritance, right-click on the BackColor in the Property Sheet, and select “Reset to Default”.
Inheriting behavior
Until now, we have only seen how to inherit properties.
Now, go to the base class (cMyForm), add a button, and put the following command in the Button’s Click() Event:
MessageBox("Hello")
Run your form:
do form MyForm
then click on the button, and confirm that the message appears on screen.
Adding to the inherited behavior
If you write anything in the Click() Event of the button on the form, inheritance is broken – the code from the parent class is not executed.
For example, write the following in the Click() Event of the form:
MessageBox("This is the second command.")
The original code is no longer executed.
Usually you will want to include the original commands, and just add something in a derived form. Change the button’s Click() Event to:
DoDefault()
MessageBox("This is the second command.")
Run the form again, and verify that both MessageBoxes show, first one, then the other.
Additional considerations
The practical value of all this is that any change done in the class is propagated to all forms that derive from the class. Of course, you can base several forms on the same form class.
A class can derive from another class; you can have a hierarchy of inheritance.
You can also use inheritance for Buttons, TextBoxes, etc. For example, you could define a common behavior for all TextBoxes that refer to money amounts.
In older versions of Visual FoxPro, a form class can not use a DataEnvironment. You have to open your tables with commands, in the Form’s Load() Event. Common commands include USE, SET ORDER, CursorSetProp() (for buffering), SELECT, SET RELATION.
For information about common problems, see also my other FAQ, on breaking inheritance – but first practice a while to get familiar with inheritance in general.More entries from this member - Breaking Inheritance December 6, 2001
(The latest update contains minor edits only.)
Five easy and fun ways to get yourself into trouble with inheritance.
A frequent source of problems in OOP is called "breaking inheritance". This document briefly describes what inheritance is, how it applies to properties and methods, and how it can be maintained or broken. Problems appear especially when inheritance is broken in methods. - CHM help file shows error messages instead of help pages October 6, 2005
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(The last update contains minor edits only.)
The idea is to have several controls on a form controlled with an array. Thus, you can quickly go through all the controls on the form, managing the array.
The sample code included will help you get started quickly. You can easily adapt it to manage arrays of any type of control on your forms (or other containers). - Least common multiple and greatest common factor May 30, 2004
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The following function will open any document, with its default association (the same application that will be called when you double-click on the file, in Windows Explorer). Use it to open a text-file, a Word or Excel document, an image, etc., with an external application. - Rushmore Optimization - not always optimal July 20, 2001
Rushmore Optimization can help make queries much faster. However, "Full Rushmore Optimization" is not always a desirable goal. "Partial Optimization" is sometimes much faster.
It is often believed that to speed things up, you need to have as many indices as possible. This article explains that some indices may actually make queries slower.
Also, some additional tips on query optimization. - Show seconds in a readable format June 7, 2002
If you need to check elapsed time with seconds() or a datetime value, this function allows you to display the elapsed time in a human-readable format, that is, hours:minutes:seconds, instead of the total number of seconds. Just pass a number of seconds as a parameter.
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