ajax gotcha for new players

Being fairly new to the "Brave New World" of ajax, this week I fell into a fairly common trap that gets new players. I studiously looked around for the best practice implementation of making an http request using javascript and came up with the following javascript function which allows the user to make the http request, specify a callback function, and even goes the extra mile of implimenting timeouts (I guess the only other thing I really should do is implement an OnErrorCallback).

function MakeRequest(url, timeout, 
    onCompleteCallback, onTimeoutCallback)
{
    var completeCallback = onCompleteCallback;
    var timeoutCallback = onTimeoutCallback;        
         
    var xmlHttp=createXMLHttpRequest(); 
    
    if (xmlHttp)
    {          
        xmlHttp.onreadystatechange = function()
        {                
            if (xmlHttp.readyState == 4)
            {            
                window.clearTimeout(timeoutId);
                if (xmlHttp.status == 200 || 
                    xmlHttp.status == 304)
                {                
                    completeCallback(
                        xmlHttp.responseText);
                }
            }
        };
                                 
        xmlHttp.open("GET",url,true);
        
        var timeoutId = 
            window.setTimeout(function()
        {        
            if (callInProgress(xmlHttp))
            {
                xmlHttp.abort();
                timeoutCallback('timeout');
            }
        }         , timeout);
        
        xmlHttp.send(null);        
    }
}

function callInProgress(xmlHttp) 
{
    switch ( xmlHttp.readyState ) 
    {
        case 1, 2, 3:
        return true;
        break;

        // Case 4 and 0
        default:
        return false;
        break;
    }
}


function createXMLHttpRequest() {
  if(window.XMLHttpRequest) {
    try { 
      xmlHttpRequest = new XMLHttpRequest();
    } catch(e) { return null; }
  } else if(window.ActiveXObject) {
    try {
      xmlHttpRequest = 
        new ActiveXObject("Msxml2.XMLHTTP");
    } catch(e) {
      try {
        xmlHttpRequest = 
          new ActiveXObject("Microsoft.XMLHTTP");
      } catch (e) { return null; }
    }
  } else return null;
  return xmlHttpRequest;
}

I then wanted to call a .Net HttpHandler, and lets just say for arguments sake that the htp handler looks like this

public class Handler : IHttpHandler
{
    
    public void ProcessRequest (HttpContext context) 
    {
        System.Threading.Thread.Sleep(6000);
        context.Response.ContentType = "text/plain";
        context.Response.Write(Guid.NewGuid().ToString());
    }
 
    public bool IsReusable 
    {
        get 
        {
            return false;
        }
    }       
}

so the call from javascript would simply look something like this

        MakeRequest(
            'Handler.ashx?cookie=abcd&another=xyz', 
            20000, onRequestComplete, 
            onRequestTimeout);       
        
        function onRequestTimeout(response)
        {
            alert('timeout');
        }
        
        function onRequestComplete(response)
        {
            alert(response);
        }

The problem as I found out through good old wikipedia relates to the internet explorers caching of the "GET" http command as described in this article on HMLHttpRequest.
As discussed in the article, there are a number of ways around this, switch to using POST or ensure that you switch the caching off in the http headers. I chose the former.

Automatic updates + Automatic Reboots

Is it only me or does anyone else find this really frustrating.



I got this in the middle of reading my email this morning, and I really didn't want to reboot in 5 minutes. I would have quite hapoily rebooted in half an hour or so when I'd finished what I was doing. The fact that the restart later button was there but disabled just added to the insult.

I think from a layer 8 perspective, Microsoft need to be really careful when forcing reboots. If this happens too frequently, it may be frustrating enough that people actually turn Automatic Updates off, making their systems progressively less secure and vulnerable.

Web Directions

On thursday night I went along to a Web Directions preview night where 2 of the speakers from the upcomming Web Directions conference Ben Barren from gnoos and John Allsop from westciv were giving previews of their presentatoins at the conference. It was a good night, I particularly enjoyed Johns talk on microformats. This is a topic that I have been interested in for a while now, and may have more to say about it later.

Unlike other events I attend, where I know a fair few people, this event I only found one person I'd met before was Nigel Watson, an Architect Advisor from Microsoft. So it was good to make new contacts and expand my circle beyond the sphere of the Melbourne .Net developer and SQL Server sphere. I got to meet Cameron Adams whose book "The Javascript Anthology" I am currently half way through, and also met the people from westciv who write the best damn web standards online training courses in existance.

When I first heard about the web directions conference, I thought that as much as I'd love to go along, I probably wouldn't be able to attend given that I was already attending Tech Ed at the end of August, and I'd be pushing it a little if I asked my employer to pay for another conference and let me have the time off so close after Tech Ed. As the saying goes, "the best layed plans of mice and men gang aft aglay", there was a lucky door prize drawn at the end of the night, which was a free ticket to attend the Web Directions conference. Guess who won........ So now all I have to do is convince my employer to let me have a couple of days off to attend..... that shouldn't be too hard.