About File Viewers

The shell allows the user to browse the information in the file system and on the network. The Quick View feature of the shell allows the user to quickly view the contents of a file without having to run the full application that created it and without even the presence of the application. To view the file contents, the user selects a file and chooses the Quick View command from the context menu of the selection (or from the File menu). The following illustration shows the context menu.

In response to the user choosing the Quick View command, the shell activates a file-specific viewer for the selected file. The shell uses the extension of the file to determine which viewer to activate. A file viewer associates itself with file classes and filename extensions in the system registry.

A file viewer is an OLE component object (not a compound document object) implemented inside a 32-bit in-process server dynamic-link library (DLL), which is associated, in turn, with the file viewer's class identifier. A file viewer provides the user interface for viewing a file. Menu items, a toolbar, and a status bar are standard parts of the file viewer interface. A file viewer can optionally add other functionality for further shell integration.

A file viewer object, which is separate from the class factory object in the in-process server, uses the standard OLE IPersistFile interface as well as the IFileViewer interface. The shell does not interact directly with file viewer objects. Instead, the shell starts an instance of a small program called Quick View (QUIKVIEW.EXE) for each file to be viewed. Each instance of Quick View defines a process for a file viewer, giving the viewer its own message queue. Although Quick View is a Windows executable file, it is not a complete application. It associates a path with a file viewer, creates an instance of the file viewer object, and instructs the file viewer to load and display the file.

Because a file viewer is an OLE component object, additional interfaces and functionality can be added in future versions of Windows to support new features. For example, a file viewer can act as an OLE container application and can perform in-place activation of embedded objects inside the file being viewed. A file viewer can let the user make a selection in a document and copy the selection to the clipboard or use the selection in a drag and drop operation. However, such functionality is entirely up to the developer of the file viewer. This overview describes the basic functionality that a file viewer must provide and discusses user interface guidelines that all developers of file viewers should follow.

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