Folders are containers; therefore, they receive notification from the container controls when an object is added to a container. The Workplace Shell subclasses the owner of the container control. The Workplace Shell initiates the Drag operation, calls wpFormatDragItem to fill out the data structures, and calls DrgDrag or DrgLazyDrag to perform the actual dragging. A Workplace Shell object need not invoke DrgDrag or DrgLazyDrag, since this is done automatically by the Workplace Shell when the object completes the processing of wpFormatDragItem.

When the user begins to drag an object, the source object is notified by the system, which invokes the object's wpFormatDragItem. This method is used to build a DRAGITEM structure, which is passed to a potential target object. The DRAGITEM structure contains the source object's rendering information, which is used to determine whether a Drop operation is valid at that point. Default information for the DRAGITEM structure is inserted by the default processing provided by the parent class, but an object class may override the method and include its own class-specific processing. The DRAGITEM structure is nested within a DRAGINFO structure, which is passed to any object over which the current item is being dragged. In a situation where more than one object is being dragged, a separate DRAGITEM is produced for each object and the set of structures is combined into a single DRAGINFO structure. The DRAGINFO structure, as shown in the following code fragments contains information about the Drag operation:

typedef struct _DRAGINFO
{
  ULONG   cbDraginfo;             /* Size of DRAGINFO               */
  USHORT  cbDragitem;             /* Size of DRAGITEM               */
  USHORT  usOperation;            /* Current Drag operation         */
  HWND    hwndSource;             /* Window handle of source        */
  SHORT   xDrop;                  /* X coordinate of drop position  */
  SHORT   yDrop;                  /* Y coordinate of drop position  */
  USHORT  cditem;                 /* Count of DRAGITEM              */
  USHORT  usReserved;             /* Reserved for future use        */
} DRAGINFO;

The DRAGITEM data structure, as shown in the following code fragments is used to describe the items being dragged:

typedef struct _DRAGITEM
{
  HWND    hwndItem;               /* Conversation partner          */
  ULONG   ulItemID;               /* Identifies item being dragged */
  HSTR    hstrType;               /* Type of item                  */
  HSTR    hstrRMF;                /* Rendering mechanism and format*/
  HSTR    hstrContainerName;      /* Name of source container      */
  HSTR    hstrSourceName;         /* Name of item at source        */
  HSTR    hstrTargetName;         /* Suggested name of item at dest*/
  SHORT   cxOffset;               /* X offset of the origin of the */
                                  /* image from the mouse hotspot  */
  SHORT   cyOffset;               /* Y offset of the origin of the */
                                  /* image from the mouse hotspot  */
  USHORT  fsControl;              /* Source item control flags     */
  USHORT  fsSupportedOps;         /* Ops supported by source       */
} DRAGITEM;

The Workplace Shell itself performs the intialization of the DRAGINFO structure. If class-specific processing is required, the object may perform its own initialization during processing of wpFormatDragItem. A Workplace Shell object does not need to allocate the DRAGITEM structure, since it is allocated by the Workplace Shell, and a pointer is passed to wpFormatDragItem. The following sample code shows an example of how to use wpFormatDragItem:

/*
 *  Specify a unique RMF that will only be understood
 *  by my instances of class MYFOLDER.
 */

SOM_Scope BOOL SOMLINK myf_wpFormatDragItem(MYFILE *somSelf,
                 PDRAGITEM pdrgItem)

{
  /* MYFILEData *somThis = MYFILEGetData(somSelf); */
  MYFILEMethodDebug("MYFILE","myf_wpFormatDragItem");

  parent_wpFormatDragItem(somSelf,pdrgItem);

  /* We do NOT want to really let the workplace shell render
   * our object, so change the rendering mechanism and format
   * to be uniquely ours.
   */

  DrgDeleteStrHandle(pdrgItem->hstrRMF);
  pdrgItem->hstrRMF = DrgAddStrHandle("<DRM_OUROWNSPECIAL,DRF_OBJECT>");
  return(TRUE);
}