To draw its own list items, an application must create a list that has the style LS_OWNERDRAW: the owner window of the list box must respond to the WM_MEASUREITEM and WM_DRAWITEM messages.

When the owner window receives a WM_MEASUREITEM message, it must return the height of the list item. All items in a list must have the same height (greater than or equal to 1). The WM_MEASUREITEM message is sent when the list box is created, and every time an item is added. You can change the item height by sending an LM_SETITEMHEIGHT message to the list-box window. The maximum width of a list box created with the LM_HORZSCROLL style can be set using an LM_SETITEMWIDTH message.

The owner window receives a WM_DRAWITEM message whenever an item in an owner-drawn list should be drawn or highlighted. Although it is quite common for an owner-drawn list to draw items, it is less common to override the system-default method of highlighting. (This method inverts the rectangle that contains the item.) Do not create your own highlighting unless, for some reason, the system-default method is unacceptable to you.

The WM_DRAWITEM message contains a pointer to an OWNERITEM data structure. The OWNERITEM structure contains the window identifier for the list box, a presentation-space handle, a bounding rectangle for the item, the position index for the item, and the application-defined item handle. This structure also contains two fields that determine whether a message draws, highlights, or removes the highlighting from an item. The OWNERITEM structure has the following form:

    typedef struct _OWNERITEM { /* oi */
        HWND    hwnd;
        HPS     hps;
        ULONG   fsState;
        ULONG   fsAttribute;
        ULONG   fsStateOld;
        ULONG   fsAttributeOld;
        RECTL   rclItem;
        LONG    idItem;
        ULONG   hItem;
    } OWNERITEM;

You can get the text of a list item by sending an LM_QUERYITEMTEXT message to the list-box window. You should draw the item using the hps and rclItem arguments provided in the OWNERITEM structure.

If the item being drawn is currently selected, the fsState and fsStateOld fields are both TRUE; they both will be FALSE if the item is not currently selected. The window receiving a WM_DRAWITEM message can use this information to highlight the selected item at the same time it draws the item. If the owner window highlights the item, it must leave the fsState and fsStateOld fields equal to each other. If the system provides default highlighting for the item (by inverting the item rectangle), the owner window must set the fsState field to 1 and the fsStateOld field to 0 before returning from the WM_DRAWITEM message.

The owner window also receives a WM_DRAWITEM message when the highlight state of a list item changes. For example, when a user clicks an item, the highlighting must be removed from the currently selected item, and the new selection must be highlighted. If these items are owner-drawn, the owner window receives one WM_DRAWITEM message for each unhighlighted item and one message for the newly highlighted item. To highlight an item, the fsState field must equal TRUE, and the fsStateOld field must equal FALSE. In this case, the application should highlight the item and return the fsState and fsStateOld fields equal to FALSE, which tells the system not to highlight the item. The application also can return the fsState and fsStateOld fields with two different (unequal) values and the list box will highlight the item (the default action).

To remove highlighting from an item, the fsState field must equal FALSE and the fsStateOld field must equal TRUE. In this case, the application removes the highlighting and returns both the fsState and the fsStateOld fields equal to FALSE. This tells the system not to attempt to remove the highlighting. The application also can return the fsState and fsStateOld fields with two different (unequal) values, and the list box will remove the highlighting (the default response). The following code fragment shows these selection processes:

    OWNERITEM *poi;

    case WM_DRAWITEM:

        /* Convert mp2 into an OWNERITEM structure pointer.                */
        poi = (POWNERITEM) PVOIDFROMMP(mp2);

        /* Test to see if this is drawing or highlighting/unhighlighting.  */
        if (poi->fsState != poi->fsStateOld) {

            /* This is either highlighting or unhighlighting.              */
            if (poi->fsState) {
                .
                . /* Highlight the item.                                   */
                .
            }
            else {
                .
                . /* Remove the highlighting.                              */
                .
            }

            /* Set fsState = fsStateOld to tell system you did it.         */
            poi->fsState = poi->fsStateOld = 0;

            return TRUE;  /* Tells list box you did the highlighting.      */

        }
        else {
            .
            . /* Draw the item.                                            */
            .
            if (poi->fsState) {   /* Checks to see if item is selected     */
                .
                . /* Highlight the item.                                   */
                .
                /* Set fsState = fsStateOld to tell system you did it.     */
           }
           return TRUE;  /* Tells list box you did the drawing.            */
        }