WM_MOUSEWHEEL Overview  Group

The WM_MOUSEWHEEL message is sent to the focus window when the mouse wheel is rotated. The DefWindowProc function propagates the message to the window’s parent. There should be no internal forwarding of the message, since DefWindowProc propagates it up the parent chain until it finds a window that processes it.

WM_MOUSEWHEEL

fwKeys = LOWORD(wParam);    // key flags

zDelta = (short) HIWORD(wParam);    // wheel rotation

xPos = (short) LOWORD(lParam);    // horizontal position of pointer

yPos = (short) HIWORD(lParam);    // vertical position of pointer

Parameters

fwKeys

Value of the low-order word of wParam. Indicates whether various virtual keys are down. This parameter can be any combination of the following values:

Value	Description
MK_CONTROL	Set if the CTRL key is down.
MK_LBUTTON	Set if the left mouse button is down.
MK_MBUTTON	Set if the middle mouse button is down.
MK_RBUTTON	Set if the right mouse button is down.
MK_SHIFT	Set if the SHIFT key is down.

zDelta

The value of the high-order word of wParam. Indicates the distance that 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.

xPos

Value of the low-order word of lParam. Specifies the x-coordinate of the pointer, relative to the upper-left corner of the screen.

yPos

Value of the high-order word of lParam. Specifies the y-coordinate of the pointer, relative to the upper-left corner of the screen.

Remarks

The zDelta parameter will be a multiple of WHEEL_DELTA, which is set at 120. This is the threshold for action to be taken, and one such action (for example, scrolling one increment) should occur for each delta.

The delta was set to 120 to allow Microsoft or other vendors to build finer-resolution wheels in the future, including perhaps a freely-rotating wheel with no notches. The expectation is that such a device would send more messages per rotation, but with a smaller value in each message. To support this possibility, you should either add the incoming delta values until WHEEL_DELTA is reached (so for a given delta-rotation you get the same response), or scroll partial lines in response to the more frequent messages. You could also choose your scroll granularity and accumulate deltas until it is reached.

See Also

GetSystemMetrics, mouse_event, SystemParametersInfo