how to use firebase.auth().onauthstatechanged inside a function code example

Example 1: google auth.onstatechange

firebase.auth().onAuthStateChanged(function(user) {  if (user) { 
// User is signed in. 
} 
  else {   
    // No user is signed in.
  }});

Example 2: firebase auth update

In the newest version of firebase_auth:

FirebaseUser has been changed to User

AuthResult has been changed to UserCredential

GoogleAuthProvider.getCredential() has been changed to GoogleAuthProvider.credential()

onAuthStateChanged which notifies about changes to the user's sign-in state was replaced with authStateChanges()

currentUser() which is a method to retrieve the currently logged in user, was replaced with the property currentUser and it no longer returns a Future