title: Firebase: How to check query is finished? (swift)
body: <p>I'm trying to display spinner while querying data from firebase, and hide it when the query result is returned. 
  is there any way to know if the query has finished retrieving all data? ?</p>
  
  <pre><code>self.handle = self.ref.child("users").observeEventType(.Value, withBlock: { (snapshot) in
          if snapshot.exists() {
              print ("snapshot exist")
              print (snapshot.childrenCount)
          }
          else {
                  print ("snapshot doesn't exist")
              }
          })
  </code></pre>