/*
 * Ajax direacted by Zakas
 * 
 * Ajax.get("url?p=value", function (data) { // handle data }, false);
 *
 * Ajax.post("url",{
 *     data : "p=value&id=001",
 *    callback : function (data) { // handle data },
 *    async : false
 * });
 *
 */
 var Ajax = (function () {

     "use strict";

     var ajax = {

         // 惰性载入函数
         createXHR: (function () {

             if (window.XMLHttpRequest) {

                 // 不论new多少次XHR，if只需判断一次
                 return function () {
                     return new XMLHttpRequest();
                 };
             } else {

                 // only for ie 6 hack
                 return function () {
                     return new ActiveXObject("Microsoft.XMLHTTP");
                 };
             }
         }()),

         get: function (url, callback, async) {

             var XHR = this.createXHR();

             // 默认异步请求
             if (async !== false) {
                 async = true;
             }

             if (async) {

                 // 异步请求
                 XHR.onreadystatechange = function () {

                     if (XHR.readyState === 4 && XHR.status === 200) {
                         callback(XHR.responseText);

                         // 销毁不用的对象，因为每次ajax请求都会创建一个新的XHR
                         XHR = null;
                     }
                 }

                 XHR.open("get", url, true);
                 XHR.send(null);
             } else {

                 // 同步请求，返回结果前停止解析上下文
                 XHR.open("get", url, false);
                 XHR.send(null);
                 callback(XHR.responseText);
                 XHR = null;
             }
         },

         post: function (url, option) {

             var XHR = this.createXHR();
                 data = option.data,
                 callback = option.callback,
                 async = option.async;

             if (async !== false) {
                 async = true;
             }

             if (async) {

                 XHR.onreadystatechange = function () {

                     if (XHR.readyState === 4 && XHR.status === 200) {
                         callback(XHR.responseText);
                         XHR = null;
                     }
                 }

                 XHR.open("post", url, true);
                 XHR.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
                 XHR.send(data);
             } else {
                 XHR.open("post", url, false);
                 XHR.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
                 XHR.send(data);
                 callback(XHR.responseText);
                 XHR = null;
             }
         }
     };

     return ajax;
 }());

// jsonp 是一种可以通过约定进行自定义回调函数的跨域脚本

// 预先定义回调函数
function handleResponse (data) {
    // data is a json from remote-source-domain, now deal it in this.
}

var script = document.createElement("script");
script.src = "http://remote-source-domain?callback=handleResponse";  // 远程脚本中的数据：handleResponse({data: "json", time: "2014-06-11"});
document.getElementsByTagName("head")[0].appendChild(script);        // 当脚本被加载到文档中时，handleResponse函数立即执行

// 当img对象被赋予src属性时立即发生http请求
var img = new Image();
img.src = "http://remote-source-domain";  // 此时发生了http请求，远程资源被加载到本地

// 图片的预加载
var imgArr = ["0.jpg", "1.jpg", "2.jpg"],
    len = imgArr.length,
    i;

for (i = 0; i < len; i++) {
    img.src = imgArr[i];
}