The LA Fox Developer Newsletter

October 1999

Communications (Con’t from page 4)

TYPE(<cVarName>) will return “0”, and ? oVarName will

NULL. Some Visual FoxPro BaseClasses support a Release()

method (Forms and FormSets), so you can experiment with this

without having to create a custom class-based object.

notice that in either case, a Release() method leaves the object

variable intact, with a TYPE() of “0” and a value of. NULL

Note that unlike the technique of storing the reference name in

code listing 5, it is necessary for an object to be instantiated

before a reference to it can be stored as another object’s

property.

Another point is that one could, if needed, actually instantiate

an object as a property of another object. Imagine also an

object that has an array property, whose elements are a series

of objects!

There is one last special-case, but extremely useful, technique

for referencing form objects without having to explicitly name the

form you wish to send a message to.

The Visual FoxPro screen is an exposed public object, and

based on a subclass of the Visual FoxPro BaseClass “Form”.

As such it has all of the members of the ForrnClass, and has a

couple of additional properties that can be queried or set

(FormCount and Forms). One of the properties the _SCREEN

object posesses (courtesy of the Form baseclass) is

ActiveForm. If you have several forms open on the FoxPro

desktop, another object can determine which one has focus and

communicate with that form by referencing the form as:

SCREEN.ActiveForm

So, for example, if you have a toolbar that wants to tell the

active form to skip to the next record, it could do so by execut-

ing:

SCREEN.ActiveForm.Next()

and the form’s Next() method code would run.

As a further example of the difficulties in determining an object’s

reference, and to illustrate the utility of the AciveForm property,

try this: Create a form (call it TESTI) using the form designer

and save it. Don’t bother adding any controls or anything, just a

simple blank form will do. From the command window,

DO FORM TESTI

Drag the form to the upper left of the screen, return to the

command window, and again:

DO FORM TESTI

Drag the second instance of the form to the upper right of the

screen and drag the command window until it is visible at the

bottom. In the command window, type:

Testi .BackColor=RGB(O,O,O)

You will observe that the background of the form in the upper left

of the screen becomes black.

Now, try to figure out how to do the same with the form in the

upper right, It’s easy if you do:

,SCREEN.ActiveForm.BackCoIorRGB(O,O,O)

However, this is the only way to access the properties of the

second instance of Test.SCX! For ways to get around this

particular problem, check the Visual FoxPro help file under DO

FORM and pay particular attention to the NAME and LINKED

clauses.

There is more than one way to skin a cat, and there is clearly

more than one way to establish communications between

objects, but as the last example above shows, doing so easily

and with some flexibility requires some careful forethought. All

the techniques described here are very simple, but vary consid-

erably in the advantages, disadvantages, capabilities and

limitations that they bring to your class design and development

efforts. No one method is “right” for all situations, or even one

particular situation, so it will benefit developers to become

familiar with all of them. Also, if any of you come up with other

methods, I’d sure appreciate hearing about it!

[Ed.Note: Steve Sawyer is Director of Technology for Kirtland

Associates, a company specializing in custom database

applications. Steve is a Microsoft Certified Professional, a

Microsoft MVP, and a contributing editor forAdvisor Media,

editing the monthly Tips Tricks and Traps column in FoxPro

Advisor. He is co-author (with Jim Booth) of “EffectWe Tech-

niques forApplication Development with Visual FoxPro 6.0

(Hentzenwerke Publishing, 1998). Steve is a founding member

and president of the Detroit Area Fox User Group, and resides

in suburban Detroit, Ml.]

[One last Ed.Note: For those of you who missed the first

installment, point your browser to the LA Fox website at hftp://

www.lafox.org and check out the downloads section for

“message.zip”J

Removing Obstacles (Con’t from page 2)

the text format obstacle, someone wrote a few hundred lines of

code to change the new text format to the old text format. So,

the new text got converted to the old text, which is then read by

low-level file commands. APPEND FROM ... TYPE SDF and

SCAN were too simple, I guess. I spent 3 days trying to map

where things came from and went to. By removing the text

format obstacle, I got to toss out a bunch of my ugliest code,

and now my poor addled brain can understand this stuff.

Con’t, page 9)

Page 5