title: Swift: Check if username exists (Firebase V.3+)
body: <p>I am working with firebase version 3+ to check if a username exists in the database.  I am stuck as it seems the <code>block</code> is being skipped as the <code>print</code> lines are not making it to the console. I will eventually have this return a bool to pass to another function.</p>
  
  <pre><code> func FBaseEmailTakenVerification() {
  
      let ref: FIRDatabaseReference!
  
      ref = FIRDatabase.database().reference().child("users")
  
      print(ref) //Firebase location exists
  
      ref.observeSingleEventOfType(.Value, withBlock: { snapshot in
          //Nothing inside the block is running
          if snapshot.exists() {
  
              print("snapshot exists")
  
          } else {
  
             print("Snapshot does not exist")
  
          }
      })
  }
  </code></pre>
  
  <p>Any help getting this <code>block</code> to run will be greatly appreciated!</p>