FILE: AsyncDown Demonstrates Asynchronous Data Download

 Last reviewed: November 24, 1997
Article ID: Q177244
The information in this article applies to:
  • The Microsoft Foundation Classes (MFC) included with: 

        - Microsoft Visual C++, 32-bit Editions, versions 4.2, 4.2b, 5.0

SUMMARY

The following sample, Asyndown.exe demonstrates asynchronous download of data in an MFC ActiveX control. MFC wraps this asynchronous download process in the CAsyncMonikerFile class.

The following file is available for download from the Microsoft Software Library: 

 ~ Asyndown.exe (size: 43128 bytes)
For more information about downloading files from the Microsoft Software Library, please see the following article in the Microsoft Knowledge Base: 

   ARTICLE-ID: Q119591
   TITLE     : How to Obtain Microsoft Support Files from Online Services

MORE INFORMATION

An Asynchronous moniker provides a way to download data asynchronously over a network. After a download is first requested, Asynchronous Monikers use a callback interface to indicate that data is available. This data can then be read with an Asynchronous Read operation.

A Read operation on an Asynchronous moniker returns immediately even if the operation has not been completed. If there are only 10 bytes available and Read is called with 1K, it does not block but returns with the 10 available bytes at that point in time. It is the caller's responsibility to return later and attempt to read more data when notified through the callback.

MFC wraps this asynchronous download process in the CAsyncMonikerFile class. ActiveX controls can use the CDataPathProperty class, derived from CAsyncMonikerFile, to help implement asynchronous control properties. This sample demonstrates how to set up an asynchronous loop using timers to read the data.

In the function CDataPathProperty::OnDataAvailable, a timer is set to indicate that data is available. When the timer message is received, the sample reads in blocks of 128 bytes and fills an edit control. If data is not available when the timer message is handled, the timer is turned off. OnDataAvailable turns on the timer if more data arrives later.

Please note that the timer duration is intentionally large and data blocks are intentionally small to demonstrate the behavior even with small files.

You can test the control either by using the HTML page with the sample or by inserting the control in a control container and setting the DataPath property. The control is not signed so you need to set your security settings appropriately if you want to test the control. Depending on your security setting, you may still be warned that the control is not safe for scripting.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Sridhar S. Madhugiri, Microsoft Corporation 

Keywords          : MfcOLE
Version           : WINNT:4.2,4.2b,5.0
Platform          : winnt
Issue type        : kbprb kbfile
Solution Type     : kbsample

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Last reviewed: November 24, 1997
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