The application may have a private data type that is difficult or impossible to display in a meaningful way.There are two reasons why you might want your application to control the display of information in Clipboard:
The application may have a private data type that is difficult or impossible to display in a meaningful way.
The application may have a private data type that requires special information to display.
You can use a display format to represent a private data format that would otherwise be difficult or impossible to display. The data associated with display formats are text, bitmaps, or metafile pictures that the clipboard viewer can display as substitutes for the corresponding private data. To use a display format, you copy both the private data and the display data to the clipboard. When the clipboard viewer chooses a format to display, it chooses the display format instead of the private data.
There are three display formats: CF_DSPTEXT, CF_DSPBITMAP, and CF_DSPMETAFILEPICT. The data associated with these formats is identical to the text, bitmap, and metafile-picture formats, respectively. Since text, bitmaps, and metafile pictures are also standard formats, the clipboard viewer can display them without help from the application.
The following description assumes that the application has already followed the steps described in Section 13.1.1, “Copying Text to the Clipboard,” to take ownership of the clipboard and set data handles.
To force the display of a private data type in a standard data format, the application must take the following steps:
1.Open the clipboard for alteration by calling the OpenClipboard function.
2.Create a global handle that contains text, a bitmap, or a metafile picture, specifying the information that should be displayed in the clipboard viewer.
3.Set the handle to the clipboard by calling the SetClipboardData function. The format code passed should be CF_DSPTEXT if the handle is for text, CF_DSPBITMAP if the handle is for a bitmap, and CF_DSPMETAFILEPICT if it is for a metafile picture.
4.Signal that the application has finished altering the clipboard by calling the CloseClipboard function.
An application can take complete control of the display and scrolling of information in the clipboard viewer. This control is useful when the application has a sophisticated private data type that only it can display. Microsoft Write uses this facility for displaying formatted text.
For the following description, assume that the application has already followed the steps described in Section 13.1.1, “Copying Text to the Clipboard,” to take ownership of the clipboard and set data handles.
To take full control of the clipboard-viewer display, follow these steps:
1.Open the clipboard for alteration by calling the OpenClipboard function.
2.Call the SetClipboardData function, using CF_OWNERDISPLAY as the data format, with a NULL handle.
3.Signal that the application has finished altering the clipboard by calling the CloseClipboard function.
The clipboard owner will then receive special messages associated with the display of information in the clipboard viewer:
| Message | Action |
| WM_PAINTCLIPBOARD | Paints the specified portion of the window. |
| WM_SIZECLIPBOARD | Notes the window size change. |
| WM_VSCROLLCLIPBOARD | Scrolls the window vertically. |
| WM_HSCROLLCLIPBOARD | Scrolls the window horizontally. |
| WM_ASKCBFORMATNAME | Supplies the name of the displayed format. |
For full descriptions of these messages, see the Microsoft Window's Programmer's Reference, Volume 3.
Chaining together clipboard-viewer windows provides a way for applications to be notified whenever a change is made to the clipboard. The notification, in the form of a WM_DRAWCLIPBOARD message, is passed down the viewer chain whenever the CloseClipboard function is called. The recipient of the WM_DRAWCLIPBOARD message must determine the nature of the change (Empty, Set, and so on) by calling EnumClipboardFormats, GetClipboardData, and other functions, as necessary.
Any window that has made itself a link in the viewer chain must be prepared to do the following:
1.Remove itself from the chain before it is destroyed.
2.Pass along WM_DRAWCLIPBOARD messages to the next link in the chain.
The code for this action looks like this:
case WM_DESTROY:
ChangeClipboardChain(hwnd, my_save_next);
/* rest of processing for WM_DESTROY */
break;
case WM_DRAWCLIPBOARD:
if (my_save_next != NULL)
SendMessage(my_save_next, WM_DRAWCLIPBOARD, wParam, lParam);
/* rest of processing for WM_DRAWCLIPBOARD */
break;The my_save_next variable is the value returned from the SetClipboardViewer function. These clipboard-viewer chain actions should be the first steps taken by the switch statement branches that process the WM_DESTROY and WM_DRAWCLIPBOARD messages.