Finally, I have make a function in mainController($scope)
:
$scope.login2= function(){
$scope.logeado = (Parse.User.current()!=null);
if (Parse.User.current() == null)
{
$scope.showuser=""
}
else
{
$scope.showuser=Parse.User.current().get('username')
}
};
and call it since loginController($scope)
because $scope.login2 is inherited in this.