An application typically responds to a click in a scroll bar by scrolling the contents of the window. This operation has three parts. First, the application changes its internal data-representation state to show what portion of the image must now be in the window. Next, the application moves the current image in the window. Finally, the application draws in the area that has been uncovered by the scrolling operation.

For example, a simple text editor might display a small portion of several pages of text in a window. When the user clicks the Down arrow of the vertical scroll bar, the application moves all the text up one line and displays the next line at the bottom of the window.

This clicking also causes a message to be sent to the client window of the frame window that owns the scroll bar. The application responds to this message by changing its internal data-representation state to show which line of text is topmost in the window, scrolling the text in the window up one line, and drawing the new line at the bottom of the window. There normally is no need to completely redraw the entire window, because the scrolled portion of the image remains valid.

You can use the WinScrollWindow function to scroll the contents of your application windows. WinScrollWindow scrolls a specified rectangular area of the window by a specified x and y offset (in window coordinates). If you set the SW_INVALIDATERGN flag for this function, the areas you uncover by scrolling are added to the window's update region automatically, causing a WM_PAINT message for the areas to be sent to the window.

For example, as used in the simple text editor described previously, the following call scrolls the text up one line (assuming that the iVScrollInc parameter specifies the height of the current font) and adds the uncovered area at the bottom of the window to the update region.

    HWND hwnd;
    LONG iVScrollInc;

    /* Scroll, adding a new area to the update region.          */

    WinScrollWindow(hwnd,  /* Window handle                     */
        0,                 /* x displacement                    */
        -(iVScrollInc),    /* y displacement                    */
        (PRECTL) NULL,     /* Scroll rectangle is entire window */
        (PRECTL) NULL,     /* Clip rectangle is entire window   */
        (HRGN) NULL,       /* Update region                     */
        (PRECTL) NULL,     /* Update rectangle                  */
        SW_INVALIDATERGN); /* Scroll-window flag                */

When the uncovered area is added to the window's update region, a WM_PAINT message is sent to the window. Upon receiving the message, the window draws the line of text at the bottom of the window. If the window has the WS_SYNCPAINT style, the WM_PAINT message is sent to the window before WinScrollWindow returns.

To optimize scrolling speed for repeated scrolling operations, you can omit the SW_INVALIDATERGN flag from the call to WinScrollWindow, which prevents the function from adding the invalid region (uncovered by the scroll) to the window's update region. If you omit the SW_INVALIDATERGN flag, you must pass a region or rectangle to WinScrollWindow. The rectangle or region will contain the area that must be updated after scrolling.