win32中WM_MOUSEWHEEL消息响应

wParam The low-order word indicates whether various virtual keys are down. This parameter can be one or more of the following values.

Value	Description
MK_CONTROL	The CTRL key is down.
MK_LBUTTON	The left mouse button is down.
MK_MBUTTON	The middle mouse button is down.
MK_RBUTTON	The right mouse button is down.
MK_SHIFT	The SHIFT key is down.
MK_XBUTTON1	Windows 2000: The first X button is down.
MK_XBUTTON2	Windows 2000: The second X button is down.

The high-order word indicates the distance the wheel is rotated, expressed in multiples or divisions of WHEEL_DELTA, which is 120. A positive value indicates that the wheel was rotated forward, away from the user; a negative value indicates that the wheel was rotated backward, toward the user.