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Drag List Boxes
A drag list box is a special type of list box that enables the user to drag items from one position to another. An application can use a drag list box to display strings in a particular sequence and allow the user to change the sequence by dragging the items into position.
Using Drag List Boxes
Drag list boxes have the same window styles and process the same messages as standard list boxes. To create a drag list box, first create a standard list box and then call the MakeDragList function. To convert a list box in a dialog box to a drag list box, you can call MakeDragList when the WM_INITDIALOG message is processed.
Drag List Box Messages
A drag list box notifies the parent window of drag events by sending it a drag list message. The parent window must process the drag list message.
The drag list box registers this message when the MakeDragList function is called. To get the message identifier (numeric value) of the drag list message, call the RegisterWindowMessage function and specify the DRAGLISTMSGSTRING value.
The wParam parameter of the drag list message is the control identifier for the drag list box. The lParam parameter is the address of a DRAGLISTINFO structure, which contains the notification code for the drag event and other information. The return value of the message depends on the notification.
Drag List Box Notification Messages
The drag list notification message, which is identified by the uNotification member of the DRAGLISTINFO structure included with the drag list message, can be DL_BEGINDRAG, DL_DRAGGING, DL_CANCELDRAG, or DL_DROPPED.
The DL_BEGINDRAG notification message is sent when the cursor is on a list item and the user clicks the left mouse button. The parent window can return TRUE to begin the drag operation or FALSE to disallow dragging. In this way, the parent window can enable dragging for some list items and disable it for others. You can determine which list item is at the specified location by using the LBItemFromPt function.
If dragging is in effect, the DL_DRAGGING notification message is sent whenever the mouse is moved, or at regular intervals if the mouse is not being moved. The parent window should first determine the list item under the cursor by using LBItemFromPt and then draw the insert icon by using the DrawInsert function. By specifying TRUE for the bAutoScroll parameter of LBItemFromPt, you can cause the list box to scroll by one line if the cursor is above or below its client area. The value you return for this notification specifies the type of mouse cursor that the drag list box should set.
The DL_CANCELDRAG notification message is sent if the user cancels a drag operation by clicking the right mouse button or pressing the ESC key. The DL_DROPPED notification message is sent if the user completes a drag operation by releasing the left mouse button, even if the cursor is not over a list item. The drag list box releases the mouse capture before sending either notification. The return value of these two notifications is ignored.
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Using Drag List Boxes