June 2001
Intermediate to advanced
706 pages
24h 10m
English
Content preview from Subclassing and Hooking with Visual Basic
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The Window Hooking Mechanism
The window hooking mechanism, or hooks, also deals with intercepting messages, but at a much broader scope than subclassing. Hooking allows us to intercept messages at various set points within the operating system. For example, we can intercept a message before and after a window has processed it.
There are several different kinds of hooks, each with their own special purpose and location within the operating system. They are:
WH_CALLWNDPROC
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WH_CALLWNDPROCRET
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WH_CBT
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WH_DEBUG
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WH_FOREGROUNDIDLE
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WH_GETMESSAGE
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WH_JOURNALPLAYBACK
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WH_JOURNALRECORD
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WH_KEYBOARD
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WH_KEYBOARD_LL
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WH_MOUSE
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WH_MOUSE_LL
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WH_MSGFILTER
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WH_SYSMSGFILTER
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WH_SHELL
|
Hooks, unlike subclassing, can have an application scope or a system-wide scope. By this, I mean a single hook can intercept specific messages within a single application, or it can be set up to intercept those same messages for all applications running in the system. Hooks give us control over the system, which cannot be achieved with subclassing. The following are just a few of the uses for hooks:
Modifying messages sent to dialog boxes, scroll bars, menus, or message boxes
Subclassing a window that resides in a separate process
Creating a macro recorder that can play back the recorded macro as well
Developing computer-based training (CBT) applications
Capturing and modifying mouse or keyboard messages at a system level
Providing a help function key for menu items and message boxes
Creating a utility ...
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