title: Firebase: How to get User.uid?
body: <p>I'm using Firebaseauth to manage users and data for my iOS app. The users can log in, and their userinfo is stored correct in the database. 
  But when I try to get the users to write to the database in a different viewController, using this string:</p>
  
  <pre><code>self.ref.child("users").child(user.uid).setValue(["username": username])
  </code></pre>
  
  <p>The thrown error is</p>
  
  <blockquote>
    <p>Type user has no member .uid </p>
  </blockquote>
  
  <p>That makes sense I guess, since I haven't created the variable. But I can't figure out how to declare it?</p>