title: Firebase email authentication not opening the iOS app upon link click
body: <p>I'm using Firebase authentication in my app (email, passwordless), and trying to follow <a href="https://firebase.google.com/docs/auth/ios/email-link-auth" rel="nofollow noreferrer">these directions</a>.</p>
  
  <p>There are several parts that are particularly confusing:</p>
  
  <ol>
  <li>Part of the code is <code>actionCodeSettings.url = URL(string: "https://www.example.com")</code> -- What exactly should this URL be? Why does it matter in the case of an iOS app where I want the user to tap on the link on their iPhone and for it to launch the app?</li>
  <li>It mentions that <code>the App Store ID and the Apple Developer Team ID also need to be specified</code> -- what if my app is not on the app store and only distributed via TestFlight?</li>
  <li>It says to <code>You will also need to configure your email action handler domain as an Associated Domain in your application capabilities. By default, the email action handler is hosted on a domain like the following example: APP_ID.firebaseapp.com</code> -- What is the APP_ID, is it the same as the Apple ID from the previous step? What is the email action handler domain, and how do I add one?</li>
  <li>I tried adding the <code>application:openURL:options:</code> method as <a href="https://github.com/firebase/quickstart-ios/blob/3a8875720b55785d662bbd37b45ea4ce919cc9c4/authentication/AuthenticationExampleSwift/AppDelegate.swift#L67" rel="nofollow noreferrer">seen in the example</a>, but it's never actually called.</li>
  <li>What exactly will the flow look like? E.g. in the perfect implementation, will Safari ever need to be launched? Which links should my app be able to handle?</li>
  <li>How can I verify that I implemented everything correctly and that it's not just redirecting to my app/authenticating incorrectly?</li>
  <li>When I use GoogleSignIn, I must call the signInSilently method upon app start to know which user is already signed in. This is an asynchronous method, meaning I probably need to show a spinner. Is there a similar requirement for this authentication method? Or how do I know who is logged in?</li>
  </ol>