The LA Fox Developer Newsletter

August 1997

Implementation of ActiveX

Controls in Visual FoxPro

Part I

by Michael W Thomas, Rocky Mountain PUG

When you hear "ActiveX" your first thought may be “Internet”.

ActiveX is a technology that reaches far beyond the Internet and

into our Visual FoxPro applications. If you haven’t worked with

ActiveX yet, you will in your next application. We’ll take a brief

look at what ActiveX is all about and then study an example of

implementing ChartFX, an ActiveX graphing tool.

An ActiveX Primer

Terms

OLE Object - An object that can be linked or embedded. OLE

objects include OLE controls and insertable OLE objects

such as Excel spreadsheets and Word documents. (from

Visual FoxPro 3 Help)

ActiveX Control - “The new name for programmable elements

formerly known as OLE controls, OCXs, or OLE custom

controls. Controls previously built with the MFC Control

Developer’s Kit meet the ActiveX control specification.

“An ActiveX control, like a built-in control, is an object that

you place on a form to enable or enhance a user’s interac-

tion with an application. ActiveX controls have events and

can be incorporated into other controls. These controls have

an OCX file name extension.” (from Visual FoxPro 5 Help)

In-Process - The command execution thread runs within the

client application’s processing thread. Examples: ChartFX,

TreeView Control

Out-of-Process - The command execution thread runs outside

of the client application. The advantage is that the client

application can continue processing while the server works

independently. The disadvantage is that cross-process

communication is slow. Examples: Adobe Acrobat reader,

Graphics Server

MFC - Microsoft Foundation Class. ActiveX controls are created

using the MFC’s. Think of them a base classes for VC++

developers.

Acti veX Controls vs. ActiveXAutomation

There are two common, yet very distinct, implementations of

ActiveX. First is ActiveX Automation. This usually refers to

controlling one application within another. An example might be

automatically sending an e-mail message from within an

application. Another may be creating and formathng a Word

document from within VFP. This technology has been around

for a while and is often what was referred to as “doing OLE”.

Probably the most powerful component of ActiveX technology is

the capability to create reusable software components. As VFP

developers we’re familiar with the concepts of classes and

objects. ActiveX components offers the same capability, but

across development plafforms, including web page develop-

ment. An ActiveX control, is simply a class that can be instan-

tiated and reused. These controls can be created in VC++ or

VB5.

Flavors of ActiveX Projects

To add to the confusion, ActiveX projects come in five basic

types. In VFP, we commonly only deal with two of them

(sometimes a three), the others are just packaging.

ActiveX EXE. This is an application that exposes one or

more classes. ActiveX EXE’s expose their object model to

allow ActiveX automation. Examples are Word, Excel, your

own VFP5 application, or Maplnfo. ActiveX EXE’s always

run out-of-process with respect to the client.

An ActiveX Control is a self-contained class or set of

classes compiled into an OCX. ActiveX controls can be

instantiated within VB forms, VFP forms, web browsers,

etc. The great advantage of using these for web develop-

ment is that they are small and run locally. Internet Ex-

plorer 3.0 has facilities built-in to automatically download,

install, and register OCX’s as needed when viewing a web

page. Distribution of new updates happens automatically

when a new version is posted to the web page using the

OCX

An ActiveX DLL exposes one or more classes which allow

creation of objects running in-process. These can be

created by VFP5.

(For more information on this topic, see ActiveX and the Single

Programmer , Peter G. Aitken; Visual Developer Magazine; Apr!

May 1997; Vol 8, Nol; pp. 40-45)

16-bit vs. 32-bit Controls

ActiveX controls can be compiled as 16-bit and 32-bit controls,

much as other applications can. This is a very important

distinction because of incompatibilities between operating

systems and development environments.

To start, let’s say that 32-bit controls cannot run in a 16-bit

environment. However, there are now a few twists. VFP, being a

32-bit language, supports 32-bit (only, but I’ll come back to

this). VFP3 can run in a 16-bit operating system only with the

W1n32s thunk layer, but remember we already pointed out that

32-bit controls can’t run in 16-bit environments, including

Win32s.

[Ed. Note: Michael W. Thomas is a commercial application

developer forResource Data International, Inc. in Boulder, CO.

CIS: 103650,2175; email: mthomas@resdata.com.]

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