A window can become a clipboard viewer and display the current contents of the clipboard. The clipboard viewer is informed whenever the clipboard contents change. Typically, the clipboard viewer is a window that can draw the standard clipboard formats. The clipboard viewer is a convenience for the user; it does not have any effect on the data-transaction functions of the clipboard.

To create a clipboard viewer, an application calls WinSetClipbrdViewer, specifying the window in which the clipboard data will be displayed. Usually this is the client window of an application. There can be only one clipboard viewer at any time in the system, so setting a clipboard viewer replaces any previous clipboard viewer. The WinQueryClipbrdViewer function receives the handle to the current clipboard viewer so that the application can reset it when finished with the clipboard viewer.

Once a window becomes the clipboard viewer, it receives WM_DRAWCLIPBOARD messages whenever the contents of the clipboard change. The window should respond to these messages by drawing the contents of the clipboard.

The clipboard viewer displays all the standard formats and should process CFI_OWNERDISPLAY items by sending the appropriate message to the clipboard owner.

The clipboard viewer cannot display private-format data. For this reason, an application that writes private-format data to the clipboard also must write the data in one of the three standard-display formats: CF_DSPTEXT, CF_DSPBITMAP, or CF_DSPMETAFILE.

If a standard format is not provided in addition to the private formats, the clipboard owner must draw the clipboard data in the clipboard-viewer window. An application uses the CFI_OWNERDRAW flag to identify clipboard data that the clipboard owner draws. When the clipboard viewer encounters data with the CFI_OWNERDRAW flag set, it sends WM_PAINTCLIPBOARD messages to the clipboard owner whenever the data must be drawn, scrolled, or sized.

The clipboard viewer determines the attributes of a particular clipboard format by calling the WinQueryClipbrdFmtInfo function. The identity of the current owner is found by calling the WinQueryClipbrdOwner function.