DIY的 ajax 框架

(function(){
    var WT = {

        ID_SEL:'#',

        CLA_SEL:'.',

        newInstance:function(){

            var instance = {};

            var setAttrs = function (dom,attrs){
                for(var key in attrs){
                    dom.setAttribute(key,attrs[key]);
                }
            }

            var convertParam = function(data){
                var param = [];
                for (var key in data) {
                    param.push(encodeURIComponent(key) + '=' + encodeURIComponent(data[key]));
                }
                return param;
            }

            instance.select = function(selector){
                var h = selector.charAt(0);
                var o = null;
                var m = null;
                if(h==WT.ID_SEL){
                    m = selector.substr(1);
                    o = document.getElementById(m);
                }
                else if(h==WT.CLA_SEL){
                    //TODO
                    //m = selector.substr(1);
                    //o = document.getElementsByClassName(m);
                }else {
                    m = selector;
                    o = document.getElementsByTagName(m);
                }
                return o;
            }

            instance.load = function(tagName, attrs, callback){
                var dom = document.createElement(tagName);
                setAttrs(dom,attrs);
                dom.onload=function(){
                    typeof callback === 'function' && callback.call(this,dom);
                    dom = null; callback =null;
                }
                return dom;
            }


            instance.jsonp = function(jp){
                //---- attributes----//
                var callback = jp.callback;
                var url = jp.url;
                var data = jp.data;

                var cn = 'jsonp'+ Date.now();
                window[cn] = callback;
                var param = convertParam(data);
                param.push('callback=' + cn);
                url += (url.indexOf('?')==-1 ? '?' : '') + param.join('&');

                var script = document.createElement('script');
                script.type = 'text/javascript';
                script.onload = script.onreadystatechange = function() {
                    this.parentNode.removeChild(script);
                    delete window[cn];
                    script.onload = script.onreadystatechange = null;
                }
                script.src = url;
                this.select('body')[0].appendChild(script);
            }

            instance.ajax = function(jp){
                //---- attributes----//
                var type = jp.type;type ? type : 'GET';
                var url = jp.url;
                var data = jp.data;
                var success = jp.success;

                var xhr = XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
                xhr.open(type,url);
                if (type.toUpperCase() === 'POST') {
                    xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
                }
                xhr.onreadystatechange = function(){
                    if(xhr.readyState==4){
                        typeof success === 'function' && success(eval('(' + xhr.responseText + ')'), xhr.status);
                        xhr = null;
                    }
                }
                xhr.send(convertParam(data).join('&'));
            }

            instance.loadImg = function (url, callback) {
                /** img的onload在图片下载完成后,dom加载前发生 **/
                this.load('img',{src:url},callback);
            }

            instance.loadCss = function(url,callback){
                /** css的onload 在 dom加载后发生 **/
                var dom = this.load('link',{rel:'stylesheet',type:'text/css',href:url},callback);
                this.select('head')[0].appendChild(dom);
            }

            instance.loadJs = function(url,callback){
                /** js的onload 在 dom加载后发生 **/
                var js = this.load('script',{type:'text/javascript',src:url},callback);
                this.select('body')[0].appendChild(js);
            }

            return instance;
        }
    };
    window['$'] = WT.newInstance();
})();