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.
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.
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.
If an application processes this message, it should return zero.
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.
Minimum supported client
|Windows Vista [desktop apps only]|
Minimum supported server
|Windows Server 2008 [desktop apps only]|
- Mouse Input
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