Subclassing and Hooking with Visual Basic

How Keyboard Messaging Operates

When the user presses and releases a key on the keyboard, several hidden things happen within the messaging system. When the user presses a key on the keyboard, the keyboard sends a scan code to the keyboard device driver indicating that a specific key was pressed. This device driver converts the scan code to a virtual key code that Windows defines. A message structure is filled with either a WM_KEYDOWN or WM_SYSKEYDOWN message, according to the type of key the user pressed.

System key-down and key-up messages are produced in one of three ways:

• By pressing the F10 key, which causes the menu bar to activate

• By holding down the ALT key while pressing another key such as ALT-F4, which closes a window

• When no window in the system has the keyboard input focus, such as when all windows are minimized

System key messages are used mainly by the system and provide a standard keyboard interface for manipulating windows. For example, pressing ALT-F4 closes a window -- you can find this key combination in a window’s system menu. Other examples of system key combinations are:

CTRL-ESC

Opens the Windows Start menu

CTRL-SHIFT-ESC

Opens the Windows Task Manager

ALT-ESC

Switches to the next running application in the Z-order

ALT-TAB

Switches to the next running application in the Z-order

ALT-DOWN ARROW

Opens a drop-down list box

ALT-SPACE

Displays the active window’s system menu

ALT-F6

Switches between windows within the same application

A nonsystem key is ...