Thursday 6 November 2014

Ajax

Ajax
·      AJAX = Asynchronous JavaScript and XML.
·      AJAX is not a new programming language, but a new way to use existing standards.
·      AJAX is the art of exchanging data with a server, and updating parts of a web page - without reloading the whole page.

AJAX is based on internet standards, and uses a combination of:
  • XMLHttpRequest object (to exchange data asynchronously with a server)
  • JavaScript/DOM (to display/interact with the information)
  • CSS (to style the data)
  • XML (often used as the format for transferring data)

The XMLHttpRequest object is used to exchange data with a server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

Create an XMLHttpRequest Object
All modern browsers (IE7+, Firefox, Chrome, Safari, and Opera) have a built-in XMLHttpRequest object.
Syntax for creating an XMLHttpRequest object:
variable=new XMLHttpRequest();
Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:
variable=new ActiveXObject("Microsoft.XMLHTTP");

 GET Requests
A simple GET request:
Example
xmlhttp.open("GET","demo_get.asp",true);
xmlhttp.send();


In the example above, you may get a cached result.
To avoid this, add a unique ID to the URL:
Example
xmlhttp.open("GET","demo_get.asp?t=" + Math.random(),true);
xmlhttp.send();
If you want to send information with the GET method, add the information to the URL:
Example
xmlhttp.open("GET","demo_get2.asp?fname=Henry&lname=Ford",true);
xmlhttp.send();


POST Requests
A simple POST request:
Example
xmlhttp.open("POST","demo_post.asp",true);
xmlhttp.send();
To POST data like an HTML form, add an HTTP header with setRequestHeader(). Specify the data you want to send in the send() method:
Example
xmlhttp.open("POST","ajax_test.asp",true);
xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
xmlhttp.send("fname=Henry&lname=Ford");
  
The onreadystatechange event
When a request to a server is sent, we want to perform some actions based on the response.
The onreadystatechange event is triggered every time the readyState changes.
The readyState property holds the status of the XMLHttpRequest.
Three important properties of the XMLHttpRequest object:
onreadystatechange
Stores a function (or the name of a function) to be called automatically each time the readyState property changes
readyState
Holds the status of the XMLHttpRequest. Changes from 0 to 4:
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready
status
200: "OK"
404: Page not found


Example:
<!DOCTYPE html>
<html>
<head>
<script>
function showHint(str)
{
var xmlhttp;
if (str.length==0)
  {
  document.getElementById("txtHint").innerHTML="";
  return;
  }
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("txtHint").innerHTML=xmlhttp.responseText;
    }
  }
xmlhttp.open("GET","gethint.asp?q="+str,true);
xmlhttp.send();
}
</script>
</head>
<body>

<h3>Start typing a name in the input field below:</h3>
<form action="">
First name: <input type="text" id="txt1" onkeyup="showHint(this.value)" />
</form>
<p>Suggestions: <span id="txtHint"></span></p>

</body>
</html>


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