从Ajax的HelloWorld说起

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Ajax IE JavaScript Firefox Microsoft

从Ajax的HelloWorld说起收藏
就这个经典的HelloWorld示例，学习Ajax的交互模式。
还是从如何创建HelloWorld说起吧：
1、创建 XmlHttpRequest 对象
function createXmlHttpRequestObject() {
                if (window.XMLHttpRequest) {
                    return new XMLHttpRequest(); //Not IE
                }
                else if(window.ActiveXObject) {
                    return new ActiveXObject("Microsoft.XMLHTTP"); //IE
                }
                else {
                    //Display your error message here.
                    //and inform the user they might want to upgrade
                    //their browser.
                    alert("Your browser doesn't support the XmlHttpRequest object. Better upgrade to Firefox.");
                }
            }

var receiveReq = createXmlHttpRequestObject();
2、初始化异步请求，XmlHttpRequest对象如何做“中转站”的工作，就在这里体现：
先获取个客户端事件信号，再把它传给服务器；
服务器根据readyState属性的变化情况回馈给XmlHttpRequest对象；（建立对服务器的调用，open（））
XmlHttpRequest对象根据回馈，CallBack()函数就会在客户端上做些有意思的工作。

function sayHello() {
                //4表示请求完成，0表示未初始化;(点击按钮会初始化一个发到服务器的异步请求3333)
                if (receiveReq.readyState == 4 || receiveReq.readyState == 0) {
                    //建立对服务器的调用，call to SayHello.html(555)
                    receiveReq.open("GET", 'simpleResponse.xml', true);
                    //每个改变时都会触发这个事件处理器，通常会调用一0个javascript函数！(666) Set the function that will be called when the XmlHttpRequest objects state changes.
                    receiveReq.onreadystatechange = CallBack;
                    //向服务器发送请求
                    receiveReq.send(null);
                }
            }

2.2 CallBack()函数:

function CallBack() {
                //Check to see if the XmlHttpRequests state is finished.
                if (receiveReq.readyState == 4) {
                    //Set the contents of our span element to the result of the asyncronous call.
                    //document.getElementById('span_result').innerHTML = receiveReq.responseText;
                    alert("The"+receiveReq.responseText);

}
}
3、示意图：

4、完整代码（HelloWorld.HTML）：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>The Hello World of AJAX</title>

        <script language="JavaScript" type="text/javascript">
            //创建 XmlHttpRequest 对象
            function createXmlHttpRequestObject() {
                if (window.XMLHttpRequest) {
                    return new XMLHttpRequest(); //Not IE
                }
                else if(window.ActiveXObject) {
                    return new ActiveXObject("Microsoft.XMLHTTP"); //IE
                }
                else {
                    //Display your error message here.
                    //and inform the user they might want to upgrade
                    //their browser.
                    alert("Your browser doesn't support the XmlHttpRequest object. Better upgrade to Firefox.");
                }
            }

            var receiveReq = createXmlHttpRequestObject();



            //Initiate the asyncronous request.初始化异步请求。
            //点击按钮会初始化一个发到服务器的异步请求
            //服务器将发回一个简单的静态文本作为响应
            function sayHello() {
                //4表示请求完成，0表示未初始化;(点击按钮会初始化一个发到服务器的异步请求（示意图中第3步)
                if (receiveReq.readyState == 4 || receiveReq.readyState == 0) {
                    //建立对服务器的调用，call to SayHello.html    （示意图中第5步)
                    receiveReq.open("GET", 'simpleResponse.xml', true);
                    //每个改变时都会触发这个事件处理器，通常会调用一0个javascript函数！（示意图中第6步)
                        //Set the function that will be called when the XmlHttpRequest objects state changes.
                    receiveReq.onreadystatechange = CallBack;
                    //向服务器发送请求
                    receiveReq.send(null);
                }
            }
            //Called every time our XmlHttpRequest objects state changes.
            //handleStateChange回调函数，这个函数会检查XmlHttpRequest对象的readyState属性，
            //然后查看服务器返回的状态码，如果一切正常，handleStateChange就会在客户端上做些有意思的工作
            //CallBack()
            function CallBack() {
                //Check to see if the XmlHttpRequests state is finished.
                if (receiveReq.readyState == 4) {
                    //Set the contents of our span element to the result of the asyncronous call.
                    //document.getElementById('span_result').innerHTML = receiveReq.responseText;
                    alert("The"+receiveReq.responseText);

}
            }
            </script>

</head>
<body>
<a href="javascript:sayHello();">Say Hello</a><br />

</body>
</html>

本文来自CSDN博客，转载请标明出处：http://blog.csdn.net/chjk1/archive/2008/04/12/2285300.aspx

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