WM_MOUSEHWHEEL message

Sent to the active window when the mouse's horizontal scroll wheel is tilted or 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.

A window receives this message through its WindowProc function.

#define WM_MOUSEHWHEEL                  0x020E

Parameters

wParam

The high-order word indicates the distance the wheel is rotated, expressed in multiples or factors of WHEEL_DELTA, which is set to 120. A positive value indicates that the wheel was rotated to the right; a negative value indicates that the wheel was rotated to the left.

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

Value	Meaning
MK_CONTROL 0x0008	The CTRL key is down.
MK_LBUTTON 0x0001	The left mouse button is down.
MK_MBUTTON 0x0010	The middle mouse button is down.
MK_RBUTTON 0x0002	The right mouse button is down.
MK_SHIFT 0x0004	The SHIFT key is down.
MK_XBUTTON1 0x0020	The first X button is down.
MK_XBUTTON2 0x0040	The second X button is down.

lParam

The low-order word specifies the x-coordinate of the pointer, relative to the upper-left corner of the screen.

The high-order word specifies the y-coordinate of the pointer, relative to the upper-left corner of the screen.

Return value

If an application processes this message, it should return zero.

Remarks

Use the following code to obtain the information in the wParam parameter.

fwKeys = GET_KEYSTATE_WPARAM(wParam);
zDelta = GET_WHEEL_DELTA_WPARAM(wParam);

Use the following code to obtain the horizontal and vertical position.

xPos = GET_X_LPARAM(lParam); 
yPos = GET_Y_LPARAM(lParam);

As noted above, the x-coordinate is in the low-order short of the return value; the y-coordinate is in the high-order short (both represent signed values because they can take negative values on systems with multiple monitors). If the return value is assigned to a variable, you can use the MAKEPOINTS macro to obtain a POINTS structure from the return value. You can also use the GET_X_LPARAM or GET_Y_LPARAM macro to extract the x- or y-coordinate.

The wheel rotation is a multiple of WHEEL_DELTA, which is set to 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 (for example, a freely-rotating wheel with no notches) to send more messages per rotation, but with a smaller value in each message. To use this feature, you can either add the incoming delta values until WHEEL_DELTA is reached (so for a delta-rotation you get the same response), or scroll partial lines in response to more frequent messages. You can also choose your scroll granularity and accumulate deltas until it is reached.

Requirements

See also

Reference

GET_KEYSTATE_WPARAM

GET_X_LPARAM

GET_Y_LPARAM

GET_WHEEL_DELTA_WPARAM

HIWORD

LOWORD

mouse_event

Conceptual

Mouse Input

Other Resources

GetSystemMetrics

MAKEPOINTS

POINTS

SystemParametersInfo