TVN_ASYNCDRAW notification code

Sent by a tree-view control to its parent when the drawing of a icon or overlay has failed. This notification code is sent in the form of a WM_NOTIFY message.

TVN_ASYNCDRAW
        
    pnmTVAsynchDraw =  (NMTVASYNCDRAW *) lParam; 

Parameters

lParam

Pointer to an NMTVASYNCDRAW structure. The NMTVASYNCDRAW structure contains the reason the draw failed.

Return value

No return value.

Remarks

The tree-view control must have the TVS_EX_DRAWIMAGEASYNC extended style. Note that this is equivalent to list-view's LVN_ASYNCDRAWN flag and its corresponding style.

This control does not draw asynchronously. Asynchronous is used in the context that the tree-view control does not synchronously extract an image if it is not available. (For instance, the image may not be available if the tree-view control uses a sparse image list, since the image may be unloaded.) Instead, when an image is not available, the control synchronously asks the parent what action to take by sending the parent an TVN_ASYNCDRAW notification with a NMTVASYNCDRAW structure. The hr member of this structure describes the reason the control's draw failed. An hr result of E_PENDING means the image is not present at all (the image needs to be extracted). Success indicates that the image is present but not at the required image quality.

The parent sets the dwRetFlags member of the structure to inform the control how to proceed. For instance, the parent may return another image, in the iRetImageIndex member, for the control to draw. In this case, the parent sets the dwRetFlags member to ADRF_DRAWIMAGE. If the control finds the returned image has not been extracted, yet another TVN_ASYNCDRAW notification may be sent by the control.

If an image is not available, the idea behind asynchronous is to allow the parent do the extraction in the background so that extraction does not block the UI thread, that is, the thread the control is on. The parent may return ADRF_DRAWNOTHING to the control, then launch a background thread to extract the icon. Once extracted, the parent may set the icon in the treeview control with macro TreeView_SetItem. This causes tree-view to invalidate the item and eventually repaint it with the extracted image in the image list.

The following code example, to be used as part of a larger program, shows how a parent may process two possible return codes in this notification by a control, and decide what action the control should take. Setting dwRetFlags is not shown.

case TVN_ASYNCDRAW:

   NMTVASYNCDRAW *pnm =  (NMTVASYNCDRAW *)lParam
   short dwDrawSuccessFlags = ShortFromResult(pnm->hr);

   if (dwDrawSuccessFlags & ILDRF_IMAGELOWQUALITY)
   {
        // Need to re-extract the icon
   }

   if (dwDrawSuccessFlags & ILDRF_OVERLAYLOWQUALITY)
   {
        // Need to re-extract the overlay
   }

Requirements