AsyncDown Demonstrates Asynchronous Data Download

ID: Q177244

The information in this article applies to:

The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++, 32-bit Editions, versions 4.2, 4.2b, 5.0, 6.0

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. Click the file names below to download the files:

Asyndown.exe

For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

http://www.microsoft.com/downloads/search.asp

and then click How to use the Microsoft Download Center.

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

Keywords : kbfile kbole kbsample kbMFC kbVC kbVC420 kbVC500 kbVC600
Version : winnt:4.2,4.2b,5.0,6.0
Platform : winnt
Issue type : kbprb