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.
A window receives this message through its WindowProc function.
#define WM_MOUSEWHEEL 0x020A
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.
The low-order word indicates whether various virtual keys are down. This parameter can be one or more of the following values.
The CTRL key is down.
The left mouse button is down.
The middle mouse button is down.
The right mouse button is down.
The SHIFT key is down.
The first X button is down.
The second X button is down.
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 get 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.
Do not use the LOWORD or HIWORD macros to extract the x- and y- coordinates of the cursor position because these macros return incorrect results on systems with multiple monitors. Systems with multiple monitors can have negative x- and y- coordinates, and LOWORD and HIWORD treat the coordinates as unsigned quantities.
The wheel rotation 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 (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 the more frequent messages. You can also choose your scroll granularity and accumulate deltas until it is reached.
Note, there is no fwKeys for MSH_MOUSEWHEEL. Otherwise, the parameters are exactly the same as for WM_MOUSEWHEEL.
It is up to the application to forward MSH_MOUSEWHEEL to any embedded objects or controls. The application is required to send the message to an active embedded OLE application. It is optional that the application sends it to a wheel-enabled control with focus. If the application does send the message to a control, it can check the return value to see if the message was processed. Controls are required to return a value of TRUE if they process the message.
|Minimum supported client
|Windows 2000 Professional [desktop apps only]
|Minimum supported server
|Windows 2000 Server [desktop apps only]