// XmlHttp constructor can receive request settings:
// url - the server url
// contentType - request content type
// type - request type (default is GET)
// data - optional request parameters
// async - whether the request is asynchronous (default is true)
// showErrors - display errors
// complete - the callback function to call when the request
// completes
function XmlHttp(settings) {
	// store the settings object in a class property
	this.settings = settings;
	// override default settings with those received as parameter
	// the default url points to the current page
	var url = location.href;
	if (settings.url)
		url = settings.url;
	// the default content type is the content type for forms
	var contentType = "application/x-www-form-urlencoded";
	if (settings.contentType)
		contentType = settings.contentType;
	// by default the request is done through GET
	var type = "GET";
	if (settings.type)
		type = settings.type;
	// by default there are no parameters sent
	var data = null;
	if (settings.data) {
		data = settings.data;
		// if we go through GET we properly adjust the URL
		if (type == "GET")
			url = url + "?" + data;
	}
	// by default the postback is asynchronous
	var async = true;
	if (settings.async)
		async = settings.async;
	// by default we show all the infrastructure errors
	var showErrors = true;
	if (settings.showErrors)
		showErrors = settings.showErrors;
	// create the XmlHttpRequest object
	var xhr = XmlHttp.create();
	// set the postback properties
	xhr.open(type, url, async);
	xhr.onreadystatechange = onreadystatechange;
	xhr.setRequestHeader("Content-Type", contentType);
	xhr.send(data);
	// the function that displays errors
	function displayError(message) {
		// ignore errors if showErrors is false
		if (showErrors) {
			// display error message
			alert("Error encountered: \n" + message);
		}
	}
	// the function that reads the server response
	function readResponse() {
		try {
			// retrieve the response content type
			var contentType = xhr.getResponseHeader("Content-Type");
			// build the json object if the response has one
			if (contentType == "application/json") {
				response = JSON.parse(xhr.responseText);
			}
			// get the DOM element if the response is XML
			else if (contentType == "text/xml") {
				response = xhr.responseXml;
			}
			// by default get the response as text
			else {
				response = xhr.responseText;
			}
			// call the callback function if any
			if (settings.complete)
				settings.complete(xhr, response, xhr.status);
		} catch (e) {
			displayError(e.toString());
		}
	}
	// called when the request state changes
	function onreadystatechange() {
		// when readyState is 4, we read the server response
		if (xhr.readyState == 4) {
			// continue only if HTTP status is "OK"
			if (xhr.status == 200) {
				try {
					// read the response from the server
					readResponse();
				} catch (e) {
					// display error message
					displayError(e.toString());
				}
			} else {
				// display error message
				displayError(xhr.statusText);
			}
		}
	}
}
// static method that returns a new XMLHttpRequest object
XmlHttp.create = function() {
	// will store the reference to the XMLHttpRequest object
	var xmlHttp;
	// create the XMLHttpRequest object
	try {
		// assume IE7 or newer or other modern browsers
		xmlHttp = new XMLHttpRequest();
	} catch (e) {
		// assume IE6 or older
		try {
			xmlHttp = new ActiveXObject("Microsoft.XMLHttp");
		} catch (e) {
		}
	}
	// return the created object or display an error message
	if (!xmlHttp)
		alert("Error creating the XMLHttpRequest object.");
	else
		return xmlHttp;
}