MultiSelect DragImage in CTreeCtrl

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Introduction

This article describes how to create a multi select drag image in CTreeCtrl. I have created a custom tree control CDragDropTreeCtrl (derived from CTreeCtrl) for adding drag and drop support. CreateDragImageEx() is the core function, it creates the drag image list.

CDragDropTreeCtrl

This class implements multiple selection with mouse, with the Ctrl or Shift key down. The DragDetect() API is used to detect the start of drag. Once the dragging is detected, the OnDrag() function is invoked. The OnDrag() function invokes CreateImageEx() to create the drag image list. The rest is the same as single selection drag. BeginDrag() and DragEnter() functions are used to prepare the image list for dragging. In the mouse move event, the drag image is moved.

CreateDragImageEx

This is the core function, which creates the drag image list. Steps in creating drag image list are:

1. Get the image list associated with the tree control
2. Get the icon size with the help of the API ImageList_GetIconSize()
3. Calculate the bounding rect of the drag image
   1. Calculate the maximum width
   2. Calculate the height of single item and multiply it with list size to get the total height
4. Create a memory device context
5. Prepare the memory DC
6. Create and prepare bitmap object
7. Select Bitmap into the memory DC
8. For each element in the list, extract the icon and associated text and draw them to the memory DC
9. Create a new image list
10. Add the bitmap to the image list

// 1.Get the image list associated with the tree control
   CImageList *pImageList = GetImageList(TVSIL_NORMAL); 
// 2.Get the icon size with the help of the API ImageList_GetIconSize()
    int cx,cy;
    ImageList_GetIconSize(*pImageList, &cx, &cy); 
// 3.Calculate the bounding rect of the drag image
// a) Calculate the maximum width    
 
 for ( nIdx = 0; nIdx < nNumSelected; nIdx++) 
 {
   // Get the item's height and width one by one
   hItem = m_vSelItem[nIdx];
   strItemText = GetItemText(hItem);
   rectFirstItem.SetRectEmpty();
   pDragImageCalcDC->DrawText(strItemText, rectFirstItem, DT_CALCRECT);
   if(nMaxWidth < ( rectFirstItem.Width()+cx))
    nMaxWidth = rectFirstItem.Width()+cx;
 }

// b) Calculate the height of single item and multiply it
// with list size to get the total height
    hItem = m_vSelItem[0];
    strItemText = GetItemText(hItem);
    rectFirstItem.SetRectEmpty();
    pDragImageCalcDC->DrawText(strItemText, rectFirstItem, DT_CALCRECT);
    
    rectTextArea.SetRect(1, 1, nMaxWidth, rectFirstItem.Height());
    rectBounding.SetRect(0,0, nMaxWidth+2, 
                 (rectFirstItem.Height()+2)*nNumSelected);
    
// 4.Create a memory device context
   CDC MemoryDC;
// 5.Prepare the memory DC
  if(!MemoryDC.CreateCompatibleDC(&DraggedNodeDC))
    return NULL;
// 6.Create and prepare bitmap object
  CBitmap   DraggedNodeBmp;
  if(!DraggedNodeBmp.CreateCompatibleBitmap(&DraggedNodeDC, 
              rectBounding.Width(), rectBounding.Height()))
    return NULL;
// 7.Select Bitmap into the memory DC
  pBitmapOldMemDCBitmap = MemoryDC.SelectObject( &DraggedNodeBmp );
// 8.For each element in the list extract the icon
// and associated text and draw them to the memory DC

  for( nIdx = 0; nIdx < nNumSelected; nIdx++)
  {
    hItem = m_vSelItem[nIdx];
    int nImg = 0,nSelImg=0;
    GetItemImage(hItem,nImg,nSelImg);
    HICON hIcon = pImageList->ExtractIcon(nImg);
    //cdcMemory.DrawIcon(rectTextArea.left,rectTextArea.top,hIcon);
    MemoryDC.MoveTo(rectTextArea.left,rectTextArea.top);
    if( nIdx != nNumSelected-1 )
    {
      MemoryDC.LineTo(rectTextArea.left,rectTextArea.top+18);
    }
    else
    {
      MemoryDC.LineTo(rectTextArea.left,rectTextArea.top+8);
    }
    MemoryDC.MoveTo(rectTextArea.left,rectTextArea.top+8);
    MemoryDC.LineTo(rectTextArea.left+5,rectTextArea.top+8);

    int nLeft = rectTextArea.left;
    rectTextArea.left += 3;
    ::DrawIconEx(MemoryDC.m_hDC,rectTextArea.left,rectTextArea.top,hIcon,
                 16,16,0,NULL,DI_NORMAL);
    rectTextArea.left += cx;
    MemoryDC.Rectangle(rectTextArea);
    MemoryDC.DrawText(GetItemText(hItem), rectTextArea, 
                   DT_LEFT| DT_SINGLELINE|DT_NOPREFIX);
    rectTextArea.left = nLeft;
    rectTextArea.OffsetRect(0, rectFirstItem.Height()+2);
    DestroyIcon(hIcon);
  }
 
 // 9.Create a new image list
   pImageListDraggedNode = new CImageList;
   pImageListDraggedNode->Create(rectBounding.Width(), rectBounding.Height(), 
                                 ILC_COLOR | ILC_MASK, 0, 1);
 // 10.Add the bitmap to the image list
    pImageListDraggedNode->Add(&DraggedNodeBmp, RGB(255, 255,255));

Conclusion

This CDragDropTreeCtrl implements multiple selection with mouse only. The keyboard support will be updated later. With this method for creating drag image list, we can create drag image for list controls also.