Q177244: HOWTO: AsyncDown Demonstrates Asynchronous Data Download

Article: Q177244
Product(s): Microsoft C Compiler
Version(s): winnt:4.2,4.2b,5.0,6.0
Operating System(s): 
Keyword(s): kbfile kbole kbSample kbInternet kbMFC kbVC420 kbVC500 kbVC600 kbGrpDSInet
Last Modified: 17-MAY-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2b 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2b 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
- Microsoft Windows Internet Services (WinInet) 
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SUMMARY
=======

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

MORE INFORMATION
================

The following files are available for download from the Microsoft Download
Center:

Asyndown.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. After it is posted, the file is housed on
secure servers that prevent any unauthorized changes to the file.


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

Additional query words: softlib

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Keywords          : kbfile kbole kbSample kbInternet kbMFC kbVC420 kbVC500 kbVC600 kbGrpDSInet 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.2,4.2b,5.0,6.0
Issue type        : kbhowto

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