How to Drag a Tree-View Item
This topic demonstrates code for handling dragging and dropping of tree-view items. The sample code consists of three functions. The first function begins the drag operation, the second function drags the image, and the third function ends the drag operation.
Note
Dragging a tree-view item typically involves processing the TVN_BEGINDRAG (or TVN_BEGINRDRAG) notification code, the WM_MOUSEMOVE message, and the WM_LBUTTONUP (or WM_RBUTTONUP) message. It also involves using the Image Lists functions to draw the item as it is being dragged.
What you need to know
Technologies
Prerequisites
- C/C++
- Windows User Interface Programming
Instructions
Step 1: Beginning the tree-view drag operation
A tree-view control sends the parent window a TVN_BEGINDRAG (or TVN_BEGINRDRAG) notification code whenever the user starts to drag an item. The parent window receives the notification in the form of a WM_NOTIFY message whose lParam parameter is the address of an NMTREEVIEW structure. The members of this structure include the screen coordinates of the mouse pointer and a TVITEM structure that contains information about the item to be dragged.
The following example shows how to process the WM_NOTIFY message to obtain TVN_BEGINDRAG.
case WM_NOTIFY:
switch (((LPNMHDR)lParam)->code)
{
case TVN_BEGINDRAG:
Main_OnBeginDrag(((LPNMHDR)lParam)->hwndFrom, (LPNMTREEVIEW)lParam);
break;
// Handle other cases here.
}
break;
Beginning the drag operation involves using the ImageList_BeginDrag function. The function's parameters include the handle to the image list that contains the image to use during the drag operation and the index of the image. You can either provide your own image list and image, or you can have the tree-view control create them for you by using the TVM_CREATEDRAGIMAGE message.
Because the drag image replaces the mouse pointer for the duration of the drag operation, ImageList_BeginDrag requires you to specify a hot spot within the image. The coordinates of the hot spot are relative to the upper left corner of the image. ImageList_BeginDrag also requires you to specify the initial location of the drag image. An application typically sets the initial location so that the hot spot of the drag image corresponds to that of the mouse pointer at the time the user began the drag operation.
The following function demonstrates how to begin dragging a tree-view item. It uses the drag image provided by the tree-view control and obtains the bounding rectangle of the item to determine the appropriate point for the hot spot. The dimensions of the bounding rectangle are the same as those of the image.
The function captures mouse input, causing mouse messages to be sent to the parent window. The parent window needs the subsequent WM_MOUSEMOVE messages to determine where to drag the image and the WM_LBUTTONUP message to determine when to end the drag operation.
// Begin dragging an item in a tree-view control.
// hwndTV - handle to the image list.
// lpnmtv - address of information about the item being dragged.
//
// g_fDragging -- global BOOL that specifies whether dragging is underway.
void Main_OnBeginDrag(HWND hwndTV, LPNMTREEVIEW lpnmtv)
{
HIMAGELIST himl; // handle to image list
RECT rcItem; // bounding rectangle of item
// Tell the tree-view control to create an image to use
// for dragging.
himl = TreeView_CreateDragImage(hwndTV, lpnmtv->itemNew.hItem);
// Get the bounding rectangle of the item being dragged.
TreeView_GetItemRect(hwndTV, lpnmtv->itemNew.hItem, &rcItem, TRUE);
// Start the drag operation.
ImageList_BeginDrag(himl, 0, 0, 0);
ImageList_DragEnter(hwndTV, lpnmtv->ptDrag.x, lpnmtv->ptDrag.x);
// Hide the mouse pointer, and direct mouse input to the
// parent window.
ShowCursor(FALSE);
SetCapture(GetParent(hwndTV));
g_fDragging = TRUE;
return;
}
Step 2: Dragging the tree-view item
You drag a tree-view item by calling the ImageList_DragMove function when the parent window receives a WM_MOUSEMOVE message, as the following example shows. The example also demonstrates how to perform hit testing during the drag operation to determine whether to highlight other items in the tree view as targets of a drag-and-drop operation.
// Drag an item in a tree-view control,
// highlighting the item that is the target.
// hwndParent - handle to the parent window.
// hwndTV - handle to the tree-view control.
// xCur and yCur - coordinates of the mouse pointer,
// relative to the parent window.
//
// g_fDragging - global BOOL that specifies whether dragging is underway.
void Main_OnMouseMove(HWND hwndParent, HWND hwndTV, LONG xCur, LONG yCur)
{
HTREEITEM htiTarget; // Handle to target item.
TVHITTESTINFO tvht; // Hit test information.
if (g_fDragging)
{
// Drag the item to the current position of the mouse pointer.
// First convert the dialog coordinates to control coordinates.
POINT point;
point.x = xCur;
point.y = yCur;
ClientToScreen(hwndParent, &point);
ScreenToClient(hwndTV, &point);
ImageList_DragMove(point.x, point.y);
// Turn off the dragged image so the background can be refreshed.
ImageList_DragShowNolock(FALSE);
// Find out if the pointer is on the item. If it is,
// highlight the item as a drop target.
tvht.pt.x = point.x;
tvht.pt.y = point.y;
if ((htiTarget = TreeView_HitTest(hwndTV, &tvht)) != NULL)
{
TreeView_SelectDropTarget(hwndTV, htiTarget);
}
ImageList_DragShowNolock(TRUE);
}
return;
}
Step 3: Ending the tree-view drag operation
The following example shows how to end a drag operation. The ImageList_EndDrag function is called when the parent window receives a WM_LBUTTONUP message. The handle of the tree-view control is passed to the function.
// Stops dragging a tree-view item, releases the
// mouse capture, and shows the mouse pointer.
//
// g_fDragging - global BOOL that specifies whether dragging is underway.
void Main_OnLButtonUp(HWND hwndTV)
{
if (g_fDragging)
{
// Get destination item.
HTREEITEM htiDest = TreeView_GetDropHilight(hwndTV);
if (htiDest != NULL)
{
// To do: handle the actual moving of the dragged node.
}
ImageList_EndDrag();
TreeView_SelectDropTarget(hwndTV, NULL);
ReleaseCapture();
ShowCursor(TRUE);
g_fDragging = FALSE;
}
return;
}
Related topics