簡(jiǎn)單ajax實(shí)例，最具兼容性，簡(jiǎn)單介紹ajax運行模式，根據此自認為最好的模式可以設計出很好的應用。
asycn.txt文檔
Hello client!
async.html文件
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "
http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html>
<head>
<title>AJAX Foundations: Using XMLHttpRequest</title>
<script type="text/javascript" src="async.js"></script>
</head>
<body onload="process()">
Hello, server!
<br/>
<div id="myDivElement" />
</body>
</html>
async.js文件
// holds an instance of XMLHttpRequest
//建立一個(gè)XmlHttpRequestObject對象實(shí)例
var xmlHttp = createXmlHttpRequestObject();
// creates an XMLHttpRequest instance
//建立XmlHttpRequestObject對象
function createXmlHttpRequestObject()
{
// will store the reference to the XMLHttpRequest object
//用于存儲XmlHttpRequest對象的引用
var xmlHttp;
// this should work for all browsers except IE6 and older
//創(chuàng )建除了ie6 或者其更早版本外的所有瀏覽器
//（用try catch結構是我見(jiàn)過(guò)最好的最具兼容性的創(chuàng )建XMLHttpRequest對象實(shí)例的方法）
try
{
// try to create XMLHttpRequest object
xmlHttp = new XMLHttpRequest();
}
catch(e)
{
// assume IE6 or older
//假設是ie6 或其更早版本
var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
"MSXML2.XMLHTTP.5.0",
"MSXML2.XMLHTTP.4.0",
"MSXML2.XMLHTTP.3.0",
"MSXML2.XMLHTTP",
"Microsoft.XMLHTTP");
// try every prog id until one works
//順序嘗試創(chuàng )建每一個(gè)對象，直到成功為止
for (var i=0; i<XmlHttpVersions.length && !xmlHttp; i++)
{
try
{
// try to create XMLHttpRequest object
//嘗試創(chuàng )建XMLHttpRequest對象
xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
}
catch (e) {}
}
}
// return the created object or display an error message
//返回已經(jīng)創(chuàng )建的對象，或顯示錯誤信息
//實(shí)際應用中這里最好不要把錯誤信息發(fā)送到客戶(hù)端
if (!xmlHttp)
alert("Error creating the XMLHttpRequest object.");
else
return xmlHttp;
}
// called to read a file from the server
// 創(chuàng )建process()函數，讀取服務(wù)器上的回顯文本
function process()
{
// only continue if xmlHttp isn't void
// 當 xmlHttp不為空時(shí)繼續
if (xmlHttp)
{
// try to connect to the server
//嘗試連接服務(wù)器
try
{
// initiate reading the async.txt file from the server
//開(kāi)始讀取服務(wù)器上的async.txt文件,也可以是
http://www.a.com/index.php?app=ajax&act=response xmlHttp.open("GET", "async.txt", true); //在這里只是設置，發(fā)送。設置get方法提交的request參數,像是在地址欄中輸入
http://async.txt，調用方法用異步(或者解釋成設置異步讀取的文件async.txt）
xmlHttp.onreadystatechange = handleRequestStateChange; //設置XMLHttpRequest處理狀態(tài)變化的函數。
xmlHttp.send(null); //把以上設置發(fā)送到服務(wù)器，get方法直接在以上的open方法設置提交參數，如果為post,設置send（app=ajax&act=response）
}
// display the error in case of failure
//如果出現異常，顯示錯誤信息
catch (e)
{
alert("Can't connect to server:\n" + e.toString());
}
}
}
// function that handles the HTTP response
//處理http響應的函數
function handleRequestStateChange()
{
// obtain a reference to the <div> element on the page
//獲取頁(yè)面上<div>元素的id
myDiv = document.getElementById("myDivElement");
// display the status of the request
//依次顯示請求狀態(tài)信息
if (xmlHttp.readyState == 1)
{
myDiv.innerHTML += "Request status: 1 (loading) <br/>";
}
else if (xmlHttp.readyState == 2)
{
myDiv.innerHTML += "Request status: 2 (loaded) <br/>";
}
else if (xmlHttp.readyState == 3)
{
myDiv.innerHTML += "Request status: 3 (interactive) <br/>";
}
// when readyState is 4, we also read the server response
//當轉換到狀態(tài)4時(shí)，讀取服務(wù)器的響應
else if (xmlHttp.readyState == 4)
{
// continue only if HTTP status is "OK"
// xmlHttp.status為200時(shí)表示處理成功
if (xmlHttp.status == 200)
{
try
{
// read the message from the server
//讀取服務(wù)器信息
response = xmlHttp.responseText;
// display the message
//顯示信息到指定id
myDiv.innerHTML +=
"Request status: 4 (complete). Server said said: <br/>";
myDiv.innerHTML += response;
}
catch(e)
{
// display error message
alert("Error reading the response: " + e.toString());
}
}
else
{
// display status message
alert("There was a problem retrieving the data:\n" +
xmlHttp.statusText);
}
}
}
