Take a look at CanActivate and CanDeactivate. The official angular docs give an example of creating an admin guard which I found quite useful when creating a general login guard.

CanActivate is used to check if the router can navigate to the new route, whereas CanDeactivate is used to check if the router can navigate away from the current route. If the guard is present on a route it will check with the guard each time a navigation occurs.

Here is a link directly to the example.