How to Implement callback for file downloading? - scripting

Question

I wrote a script that downloads file from web using file URL. I have an ActiveXObject of following type.
var objHTTP = new ActiveXObject("MSXML2.XMLHTTP");
objHTTP.open("GET", strFileURL, false);
It works perfect for small size file says, file size less than 100MB. But when I try to download file with size greater than 100MB my script hanged. Then I tried,
objHTTP.open("GET", strFileURL, true);
but in this case we have to implement a callback function. I don't know how to implement callback and then use it. Can somebody help me. I am using TestComplete 7. Script that I wrote;
var objHTTP = new ActiveXObject("MSXML2.XMLHTTP");
objHTTP.open("GET", strFileURL, true);
objHTTP.onreadystatechange = Callback;
objHTTP.send();
while((objHTTP.readyState != 4) && (objHTTP.readyState != 'complete'))
{
Delay(100);
}
if(200 != objHTTP.Status)
{
Log.Error("The " + strFileURL + " file was not found." + " The returned status is " + objHTTP.Status);
return;
}
I don't know how to implement Callback function. Can somebody provide me implementation?
Thanks

Answer 1

Probably, the hanging is the result of the while loop waiting for a specific value of the readyState property. If the property never gets one of the expected values, the script will work forever.
I think the MSXML2.XMLHTTP object fails to load the large file, and never sets the readyState to one of the values your script expects. To understand what exactly is happening, I would check what value the property has after very long time, which is enough either for the file to load, or for the attempt to fail (say, 2 hours). If you know what value the readyState property has when the downloading fails, you can handle it in the script to avoid hanging.
That's it about the hanging itself. Now about the cause of the file downloading problem. I have found a page that tells about the problem and suggests setting higher timeouts - take a look:
http://edgylogic.com/blog/downloading-large-files-vbscript/
The example is in VBScript, but it should be easy to implement the same approach with JScript. Please note that the example uses a different COM object - ServerXMLHTTP. You can read about it (including differences from XMLHTTP) here:
http://msdn.microsoft.com/en-us/library/ms762278(v=VS.85).aspx
I hope this helps.