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Asynchronous Monikers on the Internet

The Internet requires new approaches to application design because of its slow network access. Applications should perform network access asynchronously to avoid stalling the user interface. The MFC class CAsyncMonikerFile provides asynchronous support for downloading files.

With asynchronous monikers, you can extend your COM application to download asynchronously across the Internet and to provide progressive rendering of large objects such as bitmaps and VRML objects. Asynchronous monikers enable an ActiveX control property or a file on the Internet to be downloaded without blocking the response of the user interface.

Advantages of Asynchronous Monikers

You can use asynchronous monikers to:

  • Download code and files without blocking.
  • Download properties in ActiveX controls without blocking.
  • Receive notifications of downloading progress.
  • Track progress and ready state information.
  • Provide status information to the user about progress.
  • Allow the user to cancel a download at any time.

MFC Classes for Asynchronous Monikers

CAsyncMonikerFile is derived from CMonikerFile, which in turn is derived from COleStreamFile. A COleStreamFile object represents a stream of data; a CMonikerFile object uses an IMoniker to obtain the data, and a CAsyncMonikerFile object does so asynchronously.

Asynchronous monikers are used primarily in Internet-enabled applications and ActiveX controls to provide a responsive user interface during file transfers. A prime example of this is the use of CDataPathProperty to provide asynchronous properties for ActiveX controls.

MFC Classes for Data Paths in ActiveX Controls

The MFC classes CDataPathProperty and CCachedDataPathProperty implement ActiveX control properties that can be loaded asynchronously. Asynchronous properties are loaded after synchronous initiation. Asynchronous ActiveX controls repeatedly invoke a callback to indicate availability of new data during a lengthy property exchange process.

CDataPathProperty is derived from CAsyncMonikerFile. CCachedDataPathProperty is derived from CDataPathProperty. To implement asynchronous properties in your ActiveX controls, derive a class from CDataPathProperty or CCachedDataPathProperty, and override OnDataAvailable and other notifications you wish to receive.

To download a file using asynchronous monikers

  1. Declare a class derived from CAsyncMonikerFile.
  2. Override OnDataAvailable to display the data.
  3. Override other member functions, including OnProgress, OnStartBinding, and OnStopBinding.
  4. Declare an instance of this class and use it to open URLs.

For information about downloading asynchronously in an ActiveX control, see ActiveX Controls on the Internet.

See Also

MFC Internet Programming Tasks | MFC Internet Programming Basics

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