Can Cloud Functions for Firebase execute on user login?

There's no event for login, because only the client side can define exactly when a login happens. Different clients may define this in different ways. If you need to trigger something on login, figure out when that point is in your app, then trigger it from the client via a database or HTTP function.

This worked in the controller:

firebase.auth().onAuthStateChanged(function(user) { // this runs on login
    if (user) { // user is signed in
      console.log("User signed in!");
      $scope.authData = user;
      firebase.database().ref('userLoginEvent').update({'user': user.uid}); // update Firebase database to trigger Cloud Function
    } // end if user is signed in
    else { // User is signed out
      console.log("User signed out.");
    }
  }); // end onAuthStateChanged

And this is the trigger in the Cloud Function:

exports.getWatsonToken = functions.database.ref('userLoginEvent').onUpdate(event => { // authentication trigger when user logs in

I made a location in Firebase Database called userLoginEvent.

One confusing bit is that in the functions console it's /userLoginEvent but in your code you must leave out the slash.