how to subscribe to firebase user update? example

Example 1: update photoURL firebase

this.$fireAuth.currentUser.updateProfile({
    displayName: '',
    photoURL: '',
    email: ''
  })
  .then((r) => {
    console.log(r)
  })
  .catch((e) => {
  console.log(e)
})

Example 2: firebase auth update current user

var user = firebase.auth().currentUser;user.updateProfile({  displayName: "Jane Q. User",  photoURL: "https://example.com/jane-q-user/profile.jpg"}).then(function() {  // Update successful.}).catch(function(error) {  // An error happened.});