Angular $watch

如果想在某个属性发生变化的时候执行某些操作，那么scope.$watch是最佳选择

https://docs.angularjs.org/api/ng/type/$rootScope.Scope#$watch

    //如果是中文...
    $rootScope.$watch('session', function() {
        if ($rootScope.session) {
            var re = /[\u4e00-\u9fa5]/;
            if (re.test($rootScope.session.last_name + $rootScope.session.first_name)) {
                //has Chinese in name then firstname+lastname
                $rootScope.session.display_name = $rootScope.session.last_name + $rootScope.session.first_name;
            } else {
                $rootScope.session.display_name = $rootScope.session.last_name + ' ' + $rootScope.session.first_name;
            }
        }
    });