Evolution of ActiveX Controls
Chapter 15
What Is an ActiveX
Control?
ActiveX controls have evolved from object linking
and embedding technologies. They allow user access and interaction with shared documents
over the Internet or an enterprise's intranet. ActiveX, in general, consists of a number
of processes that allow many, varied technologies to operate over a distributed
environment such as the Internet (see Figure 15.1).
With object linking and embedding a spreadsheet from
an application such as Excel and a database from an application such as Access can be made
a part of the content of a document in Word. Without object linking and embedding, those
object would be kept separate and you would need to run all three applications to access
the data.
Figure 15.1. ActiveX is a
method for implementing various technologies over the Internet.
Before you can learn how to program with ActiveX
controls, there is a some background information with which you should be familiar. In
today's lesson, you will learn
- How ActiveX controls came to be
- How ActiveX controls are used
- Where to find ActiveX controls
- The system requirements for using ActiveX controls
Evolution of ActiveX
Controls
ActiveX controls come from half a century of
software development, beginning with the early research at companies such as Xerox/PARC
and Bell Labs. ActiveX controls are a marriage of an innovative method of allowing
applications to interact with each other (OLE) and Microsoft’s response to the demand
for a simple way to implement that interaction.
OLE
OLE
(Object Linking and Embedding) is what set the advanced programmer apart from the merely
experienced. To understand just what it means (no, it's not what the crowd says at a
bullfight!), let's dissect the phrase:
Object—Every "thing" on a
computer system is called an object. This includes everything such as the console,
applications, libraries (like many of the .DLL files in your \Windows\System directory),
and so on.
Linking —Sometimes a link is
created between two separate objects on a system that allows them to share each other's
information and resources. For instance, if you have a mailing list and a form letter, you
will create a link between the two so that the form letter can go out to everyone on the
list. If the list is modified in some way, the changes will be reflected in the mailings.
In Figure 15.2 you see a document from a doctor in a
remote location ordering new Yeti Restraints from his medical supplies company. Because he
wants to include a graph in his document, and because he is using the WordPad utility that
has no graphing support, he can create a link to a previously created Excel chart. This is
done through the menu option Insert | Object and then by selecting the Create from File
option.
Figure 15.2. The WordPad
dialog box for creating a link to external data or embedding a new object within a
document.
Referring again to Figure 15.2, if Dr. Kirbymeister
had not previously created an external graph, the doctor would choose the menu option
Insert | Object—but this time, he would select the Create New option instead of
Create from File. Then he could create and edit the chart from within WordPad much the
same as if he were using the Excel program itself.
The History of OLE
OOP (Object Oriented Programming) was developed
around the middle of the century. At that time, most of its use was limited to the
military, research labs, universities, and large corporations.
OLE , as a generally accepted technology, was not
introduced to the public until Apple Corporation developed and marketed the later versions
of the Macintosh. The Mac used the first widely accepted GUI (Graphical User Interface),
and its natural programming environment was object oriented.
It was a few more years before it found global
acceptance, when Microsoft incorporated OLE and OOP into millions of NT Servers and
Windows 95 Workstations. Now OLE is the state of the art in Programming—almost 50
years after it was first introduced.
Uses of OLE
OLE is the technology that allows applications on a
computer to interact with each other. An example of OLE is when you view an Excel
spreadsheet or a Word document in a PowerPoint presentation or an Office Binder. (See
Figure 15.3.)
The Office binder is utility that comes with MS
Office for Windows 95. It allows a user to place several, documents in one shell. This
shell then provides a method for linking the various object so that they can all reference
a common set of data. This is one of the first implementations of DCOM-type technologies
into a Windows95 product, since the different documents can reside on remote servers.
Figure 15.3. OLE between
Office95 applications.
Using OLE technology also simplifies programmers'
coding tasks. By referencing the procedures in external applications, a programmer need
not go through the development steps of re-creating something his users already have.
Programmers make use of this application-to-application interface to create front ends
for, and retrieve data from, other programs on the same computer.
You can see another instance of OLE by following
these steps:
- Start a new Word document.
- From the pull-down menu, select Insert and then
Object.
- You should see a list of many different objects you
can insert into the otherwise all-text document.
- From the list, select Bitmap Image.
- You should see that your menus and button bars have
switched from the standard MS Word features to tools from Microsoft Paint (or whatever OLE
Bitmap Editor your system uses).
- By clicking outside of the image area, you can return
to the standard Word setup.
OLE Components
The application whose features one wants to access
would need to support a certain set of functions. The presence or absence of these
features is what makes an application OLE-enabled or not. The host application is an OLE
Server and the system accessing it is the OLE client. Although the OLE Server
is usually a full-blown application, it could be as simple as DLL or two. As long as it
supports the interconnectivity features of OLE, other OLE-Enabled applications can use it.
The widest uses of OLE are based on COM (Component
Object Model). COM defines a basic structure and set of rules for developing
object-oriented programs. When a Windows OOP programmer develops their application, it is
the structure of the COM model upon which their program acts.
Note
I only gave a brief discussion of COM here. After you are more familiar with ActiveX and
OLE programing you may wish to reference the entire specification for the Component Object
Model, as well as information about its ongoing development, at Microsoft's OLE
Developer's Site at the URL http://www.microsoft.com/oledev
When creating an ActiveX control there are two
features that it must implement. These are the "IUnknown" interface, and that it
be self registering. The controls are then instantiated and uninstantiated through the
AddRef() and Release() methods in whatever programming language you use.
OLE also proved of great benefit to software
manufacturers who wanted to increase the value of their product by allowing other
applications to access the features of their software. A manufacturer would publish an API
(Application Programmers Interface) for its product, and developers would reference that
document to see how to make their programs interact with that product. Using this complex
API, the programmers would develop their own front ends for others' software, tailoring it
to the specific needs of their customers.
Sharing or Stealing?
The shared nature of OOP makes it very easy for a programmer to breach the barrier between
using and taking credit for another programmer's work.
When an OLE programmer creates an interface to another programmer's software, the features
add value to that application. If, however, the new application is nothing more than a
front end to another application, credit should be given to the original programmer.
The only real drawback to this is that the API can
be very difficult to understand for a programmer who is not familiar with the program.
Although OLE coding is complex, it is still not as complex as it would be without the
standardization of OLE features.
VBX—16-bit Custom
Control
The VBX was the first "custom
control." It incorporated the features of an API into a single item that could be
added to a program. This item would enable the programmer to access the features of a
given item without needing to know the more complex features of OLE. Each control enabled
users to interact with a specific OLE component or set of components.
Visual programming systems (like Visual Basic and
Access), which are OLE container applications, could now access complex features of an
external object with just a few lines of code. Although the processes the VBX uses are not
true OLE, in the strictest sense, this "mock OLE" is still quite powerful.
The development of custom controls brought an
additional benefit to programmers, many of whom were working with programming languages
that were not true OOP. Although the VBX was not a true OLE component, it broke ground in
enabling programmers to access functionality within external software components.
OCX—16/32-bit
Object Controls
OCX controls were developed in response to the need
for simpler, yet more powerful, access to control and OLE functionality. Lower-level VBXs
just did not do it. Although most of these controls are still not, by definition, true OLE
controls, many of them incorporate the more complex (and frequently needed) OLE functions.
The enhanced "mock OLE" of the first OCX’s filled most of this demand.
Custom controls like VBXs and OCXs provided an
excellent way of incorporating complex features into a simple package. As with any
technology that proves valuable, programmers began demanding more from these
tools—demanding that developers incorporate actual OLE features into them. Indeed,
OCXs met this demand so well that there is still some confusion and debate among
programmers about whether OCXs are true OLE controls. They were not yet, however, true OLE
controls.
ActiveX Controls
ActiveX controls were introduced to combine two
distinct areas in the evolution of computer technologies: custom controls and a rethinking
of the basic ideas behind OLE and OOP.
These new controls are the first DCOM (Distributed
Component Object Model)-based OLE controls. As I've mentioned, prior OLE interfaces were
based on COM. Therefore, ActiveX is the first true OLE control.
In DCOM, the programmer no longer programs for an
individual computer, but rather for the environment, or network, within which computers
operate. In the older COM model, a programmer need only follow a set of rules on how the
computer and the user would interact. With DCOM, the programmer is provided with a set of
rules by which computers, in general, interact with each other over the Internet or an
intranet.
How ActiveX Controls Are
Used
Two similar methods are used to interact with
ActiveX controls—programmatic and through script. When using VBScript, the line
between the two is blurred because it so closely follows the structure of the Visual Basic
programming language.
Programmatic
One
of the neatest things about ActiveX controls is the way they blur the line between
Internet controls and regular programming controls such as command buttons and drop-down
list boxes. ActiveX controls can be used within hypertext documents as well as within
full-blown programs. The major difference between the two uses is that scripting uses a
degree of restricted functionality for many controls when used in a network document. When
these same controls are used in an OLE container application (such as Visual Basic or
Visual C++), they can be programmed with all the power of the programming language used.
Using ActiveX with
Visual Basic/VBA
Although the programming of ActiveX controls is
possible today through Visual Basic 4.0, the integration of ActiveX into a programming
language is mature only in versions of Visual BASIC after version 4. Visual Basic and VBA
(Visual Basic for Applications) will be the first programming languages to incorporate
ActiveX into their environments.
Programmers who make use of Visual Basic to program
with ActiveX will find it simple to port their applications to the Web by replacing their
VB code with VBScript. Most of the syntax and other conventions will remain the same, and
most of the code will need very minor revisions to work the same as it did in the compiled
version. In fact, VBScript is actually just a subset of VBA.
This code will send 100 alerts to the user in Visual
Basic:
Sub cmdWarnUser
For x = 1 to 100
msgbox "Alert!"
Next X
End Sub
This code will do the same thing in VBScript:
Sub cmdWarnUser
For x = 1 to 100
alert "Alert!"
Nest x
End Sub
Using ActiveX in Other
Languages
Visual Basic and VBA are the programming
environments in which most of the ActiveX controls are designed to be used. They can be
used, however, in any programming environment that supports DCOM-based OLE. These
programming environments are called OLE Container applications. A few of these are
Visual Basic, Access, Visual FoxPro, Turbo C++, and Visual C++.
ActiveX Scripting
ActiveX
scripting adds a level of control to Internet documents and allows the programmer to
automate and activate Web pages. It does this through the use of server-side and
client-side scripts (which was covered earlier in this guide). The HTML page itself can
have bits of script (client-side) such as VBScript and JScript, which provide a level of
automation for use on the client machine. The server then can have its own scripts, such
as ISAPI and CGI, which provide a level of automation for use on the server machine.
Server Scripts
Without client scripting, the only way to program a
Web document is through a server script such as ActiveX ISAPI, HTTP CGI, or UNIX Perl.
These scripts are very powerful, but they require a Web page manager to have access to a
server's executable files.
Loading CGI Scripts on an ISP's
Server
Unless you operate your own HTTP server locally, it can be very time-consuming to upload,
test, and configure your own server scripts.
First you must select the language in which you are going to write your script. Most (if
not all) ISPs (Internet service providers) use UNIX as their server operating
system—and most users do not. Perl scripts usually work best on those systems. Some
systems also support CGI scripting, but those scripts must be written in C++. Some HTTP
servers support CGIs that are written in VB as well. MS Internet Information Servers
support the new ActiveX Server Scripting, called ISAPI. The language you choose depends on
the HTTP server software. Should your ISP change its system, you may need to rewrite and
re-compile your server scripts.
After you have written your script(s), you must put them into your HTTP server's script
directory (usually something like \CGI-BIN\). You will need special permission from your
ISP to access this directory because its other users will be using the same directory.
Consideration should be paid to those other users by not bothering the other scripts in
the directory.
Most scripts will need to be uploaded to the server (and made public to the world) just to
enable testing on them. Testing a server script on a local machine would result in some
unexpected results.
All this makes the capability to script within HTML documents (that is, client-side
scripting) easier on the Web page author and safer for the ISP and its users.
For many ISPs, this level of user access to their
servers is unacceptable and they disallow it (or charge a premium for it). Server
scripting gives the scriptwriter access to a large portion of the ISP's server
configuration. The malicious or uninformed writer could cause problems—not only on
the ISP's servers, but to its customers' machines as well.
HTML Script
The IETF (Internet Engineering Task Force) has
established a standardized method for coding script within an HTML (hypertext markup
language) document. This standard defines how all scripts are supposed to work within a
Web Document.
URL
This site contains the full text of all the current Internet Standards (RFCs) in their
final forms. You can also reference proposed specifications through this site:
IETF http://www.ietf.org
JavaScript
JavaScript is the first widely used language based
on the HTML scripting standard. It was developed by Sun Microsystems to support Java
applets. The similar naming of the scripts and the objects often causes confusion when
talking about either in conversation—but they are two completely different beasts.
Note
Sun Microsystems provides information on the Java specification (which it developed) on
its Web site at JavaSoft http://www.javasoft.com.
ActiveX honors the JavaScript standard and allows
interaction between JavaScript, VBScript, Java Controls, and ActiveX Controls.
VBScript
VBScript , which you learned about earlier, is
Microsoft's answer to JavaScript. It also allows a programmer to place his code and
objects in an HTML document. Because VBScript is part of the ActiveX line of technologies,
it supports programmatic control of both Java applets and ActiveX controls. Anyone with an
MSIE (Microsoft Internet Explorer) Web browser can run VBScript.
Note
Microsoft's VBScript site contains information on the VBScript language. You can find it
at http://www.microsoft.com/vbscript.
Security
Parents,
guardians, employers, and others provide those in their charge with Internet access in
order to increase their productivity and overall value. However, with this added power
comes a need to be protected from inappropriate, dangerous, or downright malicious
activity. ActiveX provides a framework over which a very effective security system can be
laid.
Threats
Because scripting is such a powerful tool, it is of
great value in distributing information according to a multitude of client system
requests. This punches a wide hole in most enterprises' security.
Security can be breached by allowing foreign
programs access to internal information systems. Dangerous or malicious activity by
scripts can be brought on through programmatic events such as file I/O. The threat is not
less for an individual system than it is for a larger network.
Protection
In order to protect yourself from malicious
programs, there are a couple of security policies you will want to implement on your
system. These should protect you against
- Users who wish to gain unauthorized access to your
system
- Malicious code, in the form of viri.
- Bad behavior on the part of downloaded ActiveX
Components.
Standard
Because most security boils down to three basic
levels (UserID, Password and/or physical location), the wise administrator will keep the
most sensitive information unavailable to the Net. Each user and system
administrator should determine the level of protection (and the resultant impact upon
interoperability) within which their particular system should operate.
AntiVirus
Any system that has a disk drive or network
connection or other way of accessing files (and being accessed for files) is exposed to
threats from viral infection. An AntiVirus program should be running on any system with
such interconnectivity. Microsoft AntiVirus, Norton AV, McAfee AntiVirus, and many other
utilities provide a basic level of security that should catch most threats from viral
infection.
Note
McAfee has dominated the anti-virus market throughout the last decade. Learn where to get
the latest copies of their anti-virus software at http://www.mcafee.com.
ActiveX
In response to the threat that scripting exposes, a
wide variety of methods have been developed with and without the ActiveX technologies to
protect systems against threats such as malignant code and corrupt controls. Some of these
methods include Internet ratings, code signing, trust verification, and user locator
services .
Where to Find ActiveX
Controls
ActiveX controls are available from many sources.
Controls that software manufacturers, corporate marketers, and even hobbyists post on
their Web pages can automagically find their way to systems all over the world. The nature
of ActiveX controls makes it almost too simple for users to load them onto, and install
them into, their machines.
World Wide Web
The
easiest way to obtain new software of any kind, especially ActiveX controls, is by way of
the World Wide Web (a.k.a. "WWW" or "The Web"). As ActiveX is being
implemented all over the world, the Web is turning out to be the most effective
distribution channel as well as a way to communicate. (Why order something through the
mail, with all of its overhead, cost, and inefficiencies, when it can be downloaded as
needed and with immediacy?)
Automatic Installation
from Web Pages
When a user points his browser to a standard Web
page (or HTML document), the page of text is displayed in the browser window. This is the
quickest, cheapest, easiest way to acquire a new ActiveX control. When a user views a page
containing ActiveX controls, and using an ActiveX-enabled browser, activities to load and
use those controls are launched. Part of this process is to prompt the users for whether
they want to load and install the controls (and, of course, expose themselves to the
threat those controls may pose).
Note
If the control does not require a user license, you can immediately start programming with
it. In fact, you can program with it immediately anyway. You just cannot distribute
your application until you have a license to do so.
During the installation, the control is loaded onto
the user's system. If the Web page creator referenced an .INF file, that file will tell
the user's system what files to get, where to get them, and how to add them to the system
registry as necessary.
At this point, a number of different security
activities are performed to assure the integrity of the control.
Software Distributors
At the time of this writing, Microsoft has ActiveX
in the beta test phase of its development. Still, many software developers have already
released their ActiveX applications. It's probably safe to say that ActiveX (or DCOM),
however it matures or who takes it the farthest, is a standard that will be developed upon
heavily—even if Microsoft never releases it in a final form!
ActiveX Web Browsers
There
are a wide variety of Web browsers on the market—each capable of viewing hypertext
documents.
In the earliest days of the Web, text-only browsers
such as Lynx were the standard. These browsers allow users to read the content of a Web
page over a slow (less than 9600 baud) connection. Any graphic content (which can swell
the size of a document significantly) is not retrieved unless the user specifically asks
for it.
In the early 1990’s, the National Center for
Supercomputing at the University of Illinois at Urbana-Champaign developed a graphical Web
browser called Mosaic that proved very popular on the net. Soon afterward, Netscape jumped
on the WWW bandwagon, developing and releasing its Navigator Web browser. By this time,
use of the Web by previous nonusers was increasing by leaps and bounds, and a wide variety
of (sometimes incompatible) Internet utilities was being released to the general market.
To maintain dominance in the computing industry,
Microsoft followed suit in a big way with the development of ActiveX browsers that not
only allow text and graphics, but a plethora of other types of content, such as music,
video, and a special Internet flavor of SQL (Structured Query Language).
Internet
Explorer
Beginning with version 5.0 of MSIE, Microsoft has
chosen its free Web browser as the developmental and presentation platform for the
introduction of ActiveX technologies. Needing no plug-ins or additional software, it is
the most effective tool for applying the features of activated HTML pages.
The term "ActiveX" comes from Bill Gates’ directive to his staff to
"Activate the Internet". Internet programmers will often refer to a standard web
feature, such as text browsing and file transfer, as "activated", if it has the
OLE or other advanced features of ActiveX.
Note
The Internet Explorer home page is found at http://www.microsoft.com/ie.
Information on MSIE and its related technologies can be found here and is updated
regularly.
The Netscape home page is found at http://www.netscape.com.
Information on Netscape Navigator and its plug-ins and accessories can be found here, and
is updated regularly:
Netscape Navigator
There is a wide variety of Web browsers used to
access the World Wide Web, and Netscape Communicastor is currently the Web browser of
choice for most users. In spite of this wide use, however, it does not internally support
ActiveX.
Installing Netscape and MSIE on
the Same Machine
As a developer, you will need to be able to see how others will view your pages. As long
as competition is alive and well, users will access your pages with a variety of different
browsers. Netscape and MSIE are the two most popular (for now).
On a Windows98 machine, you can enable the use of both browsers by following these
instructions:
- Obtain the most recent copy of MSIE from its Web
site. (The CD-ROM that came with this guide has a copy of the version that was current at
the time of printing.)
- Obtain the most recent copy of Netscape Navigator
from its Web site.
- Run the installation process for MSIE. When it is
done, reboot your machine.
- Run MSIE by double-clicking on the Internet icon on
your desktop. (You can take this opportunity to customize the settings from the View |
Options menu.) Exit the program and continue to the next step.
- Run the installation process for Netscape Navigator.
When it is done, reboot your machine.
- Run Netscape by double-clicking the Netscape
Navigator icon on your desktop. (You can take this opportunity to customize the settings
from the Options menu.)
- At this point, Netscape will ask you if you want to
use it as your default browser. If you will be doing your development work in ActiveX, you
should select No and check the box that says Do not ask this question again". Exit
the program and continue to the next step.
- Run MSIE one more time by double-clicking on the
desktop icon.
- At this point, Windows will ask you if you want to
use MSIE as your default browser. If you will be doing your development work in ActiveX,
you should select Yes and check the box that says Do not ask this question again. Leave
MSIE running and continue to the next step.
- Select the View | Options menu item. Then select the
File Types tab, select the Internet Document (HTML) item, and press the Edit button. This
should bring up a window with a section titled Actions.
- Press the New button.
- For Action, enter View with Netscape
- For Application", enter netscape.exe
Now, when you right-click an Internet document, you will have the option of viewing it
with Netscape or opening it with MSIE .
NCompass Labs
NCompass Labs makes a plug-in for Netscape that adds
some ActiveX functionality to it. Because Netscape does not yet have ActiveX
functionality, a plug-in such as this is necessary to browse ActiveX documents with that
browser.
Note
Find the NCompass Labs home page at http://www.ncompasslabs.com.
You can find the company's ActiveX plug-in as well as its other products here.
ActiveX Controls
Microsoft has released a number of ActiveX controls
into the public domain. Any developer may use most of these tools, royalty free, to
develop and distribute his applications. Some of these include the HTML Layout Control,
the Active Marquee Control, and (possibly the most important of all) the ICP (Internet
Control Pack).
The ICP is a set of ActiveX controls , incorporating
the standard Internet protocols. These controls include
| FTP Control |
File Transfer Protocol |
| NNTP Control |
Network News Transfer Protocol |
| POP3 Control |
Post Office Protocol |
| SMTP Control |
Simple Mail Transfer Protocol |
| HTTP Control |
Hypertext Transfer Protocol |
| HTML Control |
Hypertext Markup Language |
| WinSock Control |
Windows Sockets API |
Microsoft licensed the controls in the ICP from
NetManage, who has also has placed them in the public domain. You will learn more about
the ICP later .
ActiveX System
Requirements
The DCOM model behind ActiveX is designed to be
usable across a variety of systems. Cross-platform compatibility has always been the holy
grail of systems development. The variety of systems and the exponentially higher number
of possible configurations and software packages installed on those systems makes this
task all but impossible.
Operating Systems
ActiveX controls are usable on any system that
supports DCOM. The first to incorporate this technology is Windows 95, with Windows NT
version 4.0 following close behind. Microsoft is working with software vendors to enable
ActiveX on UNIX and Macintosh in the near future.
Required Files
In the following lessons, you will be introduced to
some specific ActiveX controls; any files required to run them will be noted. A few are
basic to the use of ActiveX.
For a Windows 95 installation, the files that may
need to be made available to the user are discussed in the sidebar titled "ActiveX
File Dependencies." If you use any other controls, you may want to keep a similar
chart as a reference for what other files those controls may require.
There is no one file or one group of files that you
can use to make your system a complete ActiveX or DCOM system. Each feature has its own
requirements. Windows NT 4.0 is based on DCOM, and others will certainly follow, but other
operating systems must use an add-in (such as the Internet Explorer version 3.x for
Windows 95) to use ActiveX features.
ActiveX File Dependencies
Microsoft Foundation Class–based custom controls (OCXs)require MFC40.dll,
MSVCRT.dll, and OLEPro32.dll.
Trust Verification services require WinTrust.dll and digsig.dll.
ICP Controls
All of the controls in the Internet Control Pack require the NMSCKN.dll in addition to
their .OCX control, and all but the WinSck.ocx requires NMORENU.dll and NMOCOD.dll.
The FTPct.ocx requires the additional NMFTPSN.dll. The HTML.ocx requires the additional
NMW3VWN.dll.
Many ActiveX packages may also require the installation of DirectX to operate. DirectX was
originally developed as Microsoft's Game SDK (Software Developer's Kit). DirectX gives the
programmer a powerful tool for accessing the more complex features of 3-D video and sound.
Other ActiveX Controls
Included with the ActiveX SDK are two redistributable, packaged, self-installing files
that will install the controls most important for ActiveX on Windows 32-bit machines.
WintDist.exe (WinINet) installs the most basic Windows Internet files—WinInet.dll and
InLoader.dll.
AXDist.exe (ActiveX) will install the WinINet files and the additional ActiveX
controls—WinINet.dll, InLoader.dll, URLMon.dll, HLink.dll, HLinkPrx.dll,
OLEAut32.dll, and STDOLE2.tlb.
Summary
In this chapter, you have been introduced to ActiveX
controls and the concept of COM (Component Object Modeling) and the emerging DCOM
(Distributed Component Object Model).
ActiveX controls are the first wide implementation
of DCOM, and provide the programmer with a simplified interface to the highly complex
features of OLE in an internetworked environment. These controls are the product of the
evolution of custom controls and OLE and the market demand for a system that combines the
two.
ActiveX controls can be embedded within an HTML
document or in an application developed in an OLE Container IDE (Integrated Development
Environment) such as Visual Basic. The programmer can then develop a user interface to
those controls using VBScript or another scripting language. The programmer can also use
those same ActiveX controls, programmatically, to develop standalone client or server
applications.
ActiveX controls can be acquired quite easily, and
almost by accident. An individual who is browsing the Web and accesses a page with ActiveX
controls embedded within it may receive a prompt asking if he wants to install the custom
control. Also, when a user installs an application that uses ActiveX controls, those
controls are installed on and made available to their system.
Although the ActiveX controls themselves will not
necessarily work on every machine, a user's Windows 95 system can be easily upgraded and
modified to support them. This is done by installing a few freely redistributable files on
the user's system.
Q&A
- Q How complicated are OLE and the COM model?
- A Very! One could go as far as to say it is
the most complex to understand, yet the most basic to use, concept in object-oriented
programming. This is because it is the basis of OOP. Most of your ActiveX programming will
be in a scripting language like VBScript or JavaScript, or in an interpreted programming
environment like Visual Basic or Access. The most important features of OLE are exposed to
these programming systems through the properties, methods, and events of ActiveX controls.
To access the lower-level features of OLE, a program must support the interaction with MFC
(Microsoft Foundation Classes). If you do not use MFC, you will have to make sure your
code implement the many classes required of an OLE server. For this you would need a much
lower level of language, such as Visual C++.
- Q Is a Java applet the same as an ActiveX control?
- A No. Java applets could best be defined as
Internet-aware custom controls. Because these applets are not OLE enabled, they are not
true ActiveX controls. From a scripting or interpreted language standpoint, however,
working with Java applets is very similar to working with ActiveX controls. ActiveX will
support the Java Virtual Machine.
- Q Is it possible to receive a control that will
perform maliciously on my machine?
- A Yes—if you are not careful. Different
Web browsers have varying degrees of security that the user can enable or disable as he
sees fit. There are a number of methods an ActiveX control developer can use to assure the
users of his product that they are receiving a genuine product. It remains up to the user,
for the most part, to let those features work and not disable them.
Workshop
Retrieve copies of, or create shortcuts to, the following ActiveX-related
specifications (Note: The URLs mentioned only show the specification's sponsor's home
page.):
Quiz
- Explain the significance of each of the following:
- OLE
- VBX
- OCX
- When was object-oriented programming developed?
- What are the two methods for interacting with ActiveX
controls?
- What is the name of the new technology behind ActiveX
controls?
- If you develop an HTML document with embedded ActiveX
controls, what scripting language, besides VBScript, can you use to program the user
interface to those controls?
- If you develop an ActiveX application for a Web
document, what would be the easiest programming language to use to make it a standalone
application?
- What is the most common file that must be installed
on a user's machine to enable Windows Internet functionality?
- What do COM and DCOM mean?
- Name one operating system that has built-in support
for DCOM.
- Within what two types of items can an ActiveX object
be incorporated?
- What copany developed the JAVA tecnhologies?
- An ActiveX control must support what two features?
- What basic concept is behind the shift from COM to
DCOM?
- What operating systems are able to implement DCOM?
- What operating systems are able to run ActiveX?
- What features are used in the most basic level of
security?
- What filetype tells an ActiveX browser how to
register a control?
- What must a user obtain, after he installs the
control, in order to redistribute his applications?
- What is the difference between JavaScript and a Java
applet.
- Of what programming language is
VBScript a subset?
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