微信扫描二维码登录网页是什么原理，前后两个事件是如何联系的？

    function _poll(_asUUID) {
        var _self = arguments.callee,
            _nTime = 0;
        _sCurUUId = _asUUID;
 
        _logInPage("_poll Request Start, time: " + new Date().getTime());
        _nTime = new Date().getTime();
        $.ajax({
        type: "GET",
        url: "https://login." + _sBaseHost + "/cgi-bin/mmwebwx-bin/login?uuid=" + _asUUID + "&tip=" + show_tip,
        dataType: "script",
        cache: false,
        timeout: _nAjaxTimeout,
        success: function(data, textStatus, jqXHR) {
            _logInPage("_poll Request Success, code: " + window.code + ", time: " + (new Date().getTime() - _nTime) + "ms");
            switch (_aoWin.code) {
            case 200:
                _sSecondRequestTime = new Date().getTime() - _sSecondRequestTime;
                _logInPage("Second Request Success, time: " + _sSecondRequestTime + "ms");
                clearTimeout(_oResetTimeout);
 
                $.get(_aoWin.redirect_uri + "&fun=new", function(msg) {
                    _logInPage("new func reponse, reponseMsg: " + msg);
                    _reportNow("new func reponse, reponseMsg: " + msg);
                    var code = msg.match(/<script>(.*)<\/script>/);
                    if(code){
                        eval(code[1]);
                    }else{
                        $("#container").show();
                        $("#login_container").hide();
                    }
                });
 
                _reportNow("/cgi-bin/mmwebwx-bin/login, Second Request Success, uuid: " + _asUUID + ", time: " + _sSecondRequestTime + "ms");
                break;
 
            case 201:
                clearTimeout(_oResetTimeout);
                show_tip = 0;
                $('.errorMsg').hide();
                $('.normlDesc').hide();
                $('.successMsg').show();
                _logInPage("First Request Success");
                _reportNow("/cgi-bin/mmwebwx-bin/login, First Request Success, uuid: " + _asUUID);
//                setTimeout(function(){
                    _logInPage("Second Request Start");
                    _reportNow("/cgi-bin/mmwebwx-bin/login, Second Request Start, uuid: " + _asUUID);
 
                    _sSecondRequestTime = new Date().getTime();
 
                    _nAjaxTimeout = 5 * 1000;
                    _self(_asUUID);
//                }, 500);
                break;
 
            case 408:
                setTimeout(function(){
                    _self(_asUUID);
                }, 500);
                break;
 
            case 400:
            case 500:
                _reset();
                _afterLoadWebMMDo(function(){
                    _aoWin.Log.d("500, Login Poll Svr Exception");
                });
                break;
            }
        },
        error: function(jqXHR, textStatus, errorThrown) {
            if (textStatus == 'timeout') {
                setTimeout(function(){
                    _self(_asUUID);
                }, 500);
            } else {
                setTimeout(function(){
                    _self(_asUUID);
                }, 5000);
 
                _logInPage("_poll Request Error:" + textStatus);
                _afterLoadWebMMDo(function(){
                    _aoWin.Log.e("Login Poll Error:" + textStatus);
                });
            }
        }
        });
    }