Sebastopol, CA--Windows is essentially a messaging system: every user
action creates one or more messages to carry out the action. Clicking a
button control is a good example, as author Stephen Teilhet explains in
his new book, Subclassing & Hooking with
Visual Basic (O'Reilly, US
$49.95). In addition to generating the message for the mouse button
click, the action produces a wide array of other messages, including
messages to repaint the button in its depressed state, messages to
determine the state of the mouse cursor, and others. The way it works
is not unlike the way a simple action of your hand sends sensory
information, or messages, from your motor nerves to your brain. Neurons
in your brain receive and analyze the information, then pass other
messages on to trigger appropriate actions. In just this way, a simple
action in Windows can produce an astonishing flurry of messages to
carry out the actions of the operating system.
Subclassing and the Windows hooking mechanism are two very powerful
techniques that operate on messages within the Windows messaging
system. As Teilhet explains, "Simply put, subclassing and hooking
involve the interception of messages. Being able to examine and
manipulate messages allows developers to have more control over their
applications as well as the operating system. For example, when a
window is being resized, several different types of messages are being
sent to that window. With subclassing, the developer can control how
and if the user can resize or move the window by intercepting and
modifying a few of these messages." According to Teilhet, the Windows
operating system has hundreds of messages it can send to a window.
Using subclassing and hooking, developers can control or manipulate any
of these messages to achieve their desired results.
Teilhet also explains that responsibility accompanies the power that
subclassing and hooking give developers. He cautions, "It's up to the
developer to make sure that he or she is using these techniques
correctly. Windows is very unforgiving if these techniques are used
incorrectly."
Teilhet guides developers through the basics of these techniques and
well beyond, beginning with a thorough introduction to the Windows
messaging system. He proceeds with a detailed discussion of the methods
of intercepting and handling Windows messages, including the
differences between subclassing and hooking. Subclassing techniques
deal with intercepting messages bound for one or more windows or
controls. The messages are intercepted before they reach their
destination window. Hooking also deals with intercepting messages, but
with a much broader scope than subclassing. Hooking allows a developer
to intercept messages at various set points within the operating
system.
Teilhet has written this book primarily for VB and VB.NET developers,
but adds that developers using other languages such as C/C++/Managed
C++, C# or even Borland's Delphi will be able to get a better
understanding of how subclassing and hooking operate at a lower level.
"This is a very advanced book," Teilhet says, "but I have written and
organized it so that anyone from the beginner to the more advanced
developer who wants a deeper understanding of VB, VB.NET, the Windows
messaging system, subclassing, and hooking can obtain that information
without becoming confused or having to buy other books."
Teilhet adds that many of the rules outlined in his book for writing
good subclassing and hooking code carry over to VB.NET from VB5 and
VB6. The author also discusses WinForm and delegates as they are used
in VB.NET. C# developers will find that this information is easily
ported from VB.NET to C#.
Online Resources:
Subclassing & Hooking
with Visual Basic
By Stephen Teilhet
June 2001
ISBN 0-596-00118-5, 685 pages, $49.95 (US)
order@oreilly.com
1-800-998-9938
