title: how to tell nav bar user just logged in or sign out in angular 2 and bootstrap
body: <p>I have a bootstrap navbar that has buttons on it like user profile and other "after login" components.
  now, after i sign in or log out i would like to tell the component which own the nav bar that the user is no longer valid or the opposite to the component that the user is just logged in so display the elements on the navbar</p>