//封裝XMLHTTP的MyAjaxObj類
var MyAjaxObj = new Object();
var maxXmlHttpCount = 5; //最多5個xmlhttp對象存在

MyAjaxObj.reqList = []; //可以清空里面的項

MyAjaxObj.getFreeObj = function() {
var req = null;
var len = this.reqList.length;
//先從當前的池里取
for (var i = 0; i < len; i++) {
if (this.reqList[i]) {
if (this.reqList[i].readyState == 4 || this.reqList[i].readyState == 0) {
req = this.reqList[i];
break;
}
}
}
//如果沒有閑置的對象，自己獨立創(chuàng)建
if (req == null) {
if (this.reqList.length < maxXmlHttpCount) {
req = getXmlHttp();
this.reqList.push(req);
}
}
return req;
}


//創(chuàng)建一個XMLHTTP對象，兼容不同的瀏覽器
function getXmlHttp() {
var xmlHttp = false;
var arrSignatures = ["MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0",
"MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP",
"Microsoft.XMLHTTP"];
for (var i = 0; i < arrSignatures.length; i++) {
try {
xmlHttp = new ActiveXObject(arrSignatures[i]);
return xmlHttp;
}
catch (oError) {
xmlHttp = false; //ignore
}
}
// throw new Error("MSXML is not installed on your system.");
if (!xmlHttp && typeof XMLHttpRequest != 'undefined') {
xmlHttp = new XMLHttpRequest();
}
return xmlHttp;
}

/*封裝XMLHTTP向服務器發(fā)送請求的操作
url:向服務器請求的路徑；method：請求的方法，即是get/post；***callback：當服務器成功返回結(jié)果時，調(diào)用的函數(shù)(類似c#回調(diào)函數(shù))***
data：向服務器請求時附帶的數(shù)據(jù)；urlencoded：url是否編碼；cached:是否使用緩存； callBackError;當服務器返回錯誤時調(diào)用的函數(shù)
*/
MyAjaxObj.send = function(url, method, callback, data, urlencoded, cached, callBackError) {
var req = this.getFreeObj(); //從池里或者直接實例化一個XMLHTTP的實例

//當XMLHTTP的請求狀態(tài)發(fā)生改變時調(diào)用 (核心處理函數(shù))
req.onreadystatechange = function() {
// 當請求已經(jīng)加載

if (req.readyState == 4) {
// 當請求返回成功
if (req.status == 200) { //或者 req.status < 400
// 當定義了成功回調(diào)函數(shù)時，執(zhí)行成功回調(diào)函數(shù)
if (callback)
callback(req, data);
}
// 當請求返回錯誤

else {
//當定義了失敗回調(diào)函數(shù)時，執(zhí)行失敗回調(diào)函數(shù)
if (callBackError)
callBackError(req, data);
}

// 有池的管理，我們可以省卻釋放資源的方法
// try {
// delete req;
// req = null;
// }
// catch (e) {
// alert(e.message);
// }
}
}

//如果以POST方式回發(fā)服務器
if (method.toUpperCase() == "POST") {
req.open("POST", url, true);
//請求是否需要緩存(只有在req.open之后才可以設置此項)
if (cached)
req.setRequestHeader("If-Modified-Since", "0");
//請求需要編碼
if (urlencoded)
req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
req.send(data);
MyAjaxObj.reqList.push(req);
}
//以GET方式請求
else {
req.open("GET", url, true);
//請求是否需要緩存
if (cached)
req.setRequestHeader("If-Modified-Since", "0");
req.send(null);
MyAjaxObj.reqList.push(req);
}
return req;
}

//全部清除XMLHTTP數(shù)組元素，釋放資源
MyAjaxObj.clearReqList = function() {
var len = MyAjaxObj.reqList.length;
for (var i = 0; i < len; i++) {
var req = MyAjaxObj.reqList[i];
if (req) {
try {
delete req;
} catch (e) { }
}
}
MyAjaxObj.reqList = [];
}

//進一步封裝XMLHTTP以POST方式發(fā)送請求時的代碼
//isClear：是否清除XMLHTTP數(shù)組的所有元素；其他參數(shù)的意義見 MyAjaxObj.send
MyAjaxObj.sendPost = function(url, data, callback, isClear, isCached, callBackError) {
if (isClear) {
MyAjaxObj.clearReqList();
}
MyAjaxObj.send(url, "POST", callback, data, true, isCached, callBackError); //post方法需要編碼
}
//進一步封裝XMLHTTP以GET方式發(fā)送請求時的代碼
MyAjaxObj.sendGet = function(url, args, callback, isClear, isCached, callBackError) {
if (isClear)
MyAjaxObj.clearReqList();
return MyAjaxObj.send(url, "GET", callback, args, false, isCached, callBackError);
}