/*分支技术*/
/* SimpleXhrFactory singleton */

var SimpleXhrFactory = (function() {
    //The three branches.
    var standard = {
        createXhrObject: function() {
            return new XMLHttpRequest();
        }
    };
   
    var activeXNew= {
        createXhrObject: function() {
            return new ActiveXObject('Msxml2.XMLHTTP');
        }
    };
   
    var activeXOld = {
        createXhrObject: function() {
            return new ActiveXObject('Microsoft.XMLHTTP');
        }
    }
   
    // To assign the branch, try each method; return whatever doesn't fail.
    var testObject;
   
    try {
        testObject = standard.createXhrObject();
        return standard;
    }catch(e){
        try{
            testObject = activeXNew.createXhrObject();
            return activeXNew;
        }catch(e){
            try{
                testObject = activeXOld.createXhrObject();
                return activeXOld;
            }catch(e){
                throw new Error('No XHR object found in this environment.');
            }
        }
    }
   
})();