OnWMMouseWheel - DfBaseDialog

Parameter	Description
wParam	Window wParam passed by WM_MOUSEWHEEL
lParam	Window lParam passed by WM_MOUSEWHEEL

Procedure OnWMMouseWheel Integer wParam Integer lParam

OnWMMouseWheel exposes the windows WM_MOUSEWHEEL event.

The parameters sent to the event wParam and lParam are the standard windows parameters for this event.

wParam

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.

MK_CONTROL	The CTRL key is down.
MK_LBUTTON	The left mouse button is down.
MK_MBUTTON	The middle mouse button is down.
MK_RBUTTON	The right mouse button is down.
MK_SHIFT	The SHIFT key is down.
MK_XBUTTON1	Windows 2000/XP: The first X button is down.
MK_XBUTTON2	Windows 2000/XP: 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.

This is sent to the container object of the focus object when the mouse wheel generates an event. If you wish to handle this event, you should set Windows_Override_State to True, which will inform windows that the event has been handled. Doing this will stop the delegation of this message.
By default this event does nothing, which causes the event to be sent to the parent container object, which will also do nothing and delegate.

If the focus object already handles the mouse wheel event, this event will not be sent to its parent container. cRichEdit, cTextEdit, List and Grid objects are examples of objects that handle this event.

This is a low level event and we do not expect that it will be used often. In advanced cases, it does provide the developer with a mechanism for responding to the mouse wheel.

    // low level event sent from windows.
    Procedure OnWmMouseWheel Integer wParam Integer lParam
        Integer iWheelDelta iKeys iDelta iClicks
        Move (low(abs(wParam))) to iKeys           // any keys down when pressed
        Move (hi(abs(wParam))) to iDelta           // number of click units
        If (wParam<0) Begin
            Move (-iDelta) to iDelta     // can be up or down
        End
        Get  piWheelDelta to iWheelDelta           // Current accumulated micro clicks
        Move (iWheelDelta+iDelta) to iWheelDelta
        // C_WHEELDATA is 120 as defined by MS as the delta to react to. Once click is usually 120
        Move (iWheelDelta/C_WHEELDELTA) to iClicks // Number of clicks to react to
        Set  piWheelDelta to (mod(iWheelDelta,C_WHEELDELTA)) // remainder unused microclicks
        // If we have enough Clicks send OnMouseWheel
        If (iClicks<>0) Begin
            Send OnMouseWheel iClicks iKeys
        End
        // tell windows that we've handled the event.    
        Set Windows_Override_State to True    
    End_Procedure

With the exceptions of the notifications for scrollbars and the mouse wheel evetns, all of the windows scrollbar interfaces can be accessed by creating external_functions. The events SetVScrollBox, SetHScrollBox and OnWMMouseWheel are provided to give you low level access to these events.