The framework calls this member function as a user rotates the mouse wheel and encounters the wheel's next notch.
Unless overridden, OnMouseWheel calls the default of WM_MOUSEWHEEL. Windows automatically routes the message to the control or child window that has the focus. The Win32 function DefWindowProc propagates the message up the parent chain to the window that processes it.
The zDelta parameter is a multiple of WHEEL_DELTA, which is set at 120. This value is the threshold for an action to be taken, and one such action (for example, scrolling forward one notch) should occur for each delta.
The delta was set to 120 to allow for future finer-resolution wheels, such as a freely-rotating wheel with no notches. Such a device might send more messages per rotation, but with a smaller value in each message. To support this possibility, either aggregate the incoming delta values until WHEEL_DELTA is reached (so you get the same response for a given delta-rotation), or scroll partial lines in response to the more frequent messages. You could also choose your scroll granularity and accumulate deltas until WHEEL_DELTA is reached.
Override this member function to provide your own mouse-wheel scrolling behavior.
OnMouseWheel handles messages for Windows NT 4.0. For Windows 95/98 or Windows NT 3.51 message handling, use OnRegisteredMouseWheel.