title: Firebase Realtime Database read all existing data
body: <p>It seems that Firebase Realtime Database notifies me if a new item is added or deleted, etc. That is great, but how to get all existing items of which I was previously notified?</p>
  
  <p>For example, suppose that my app is listing the items in the database. I started my app for the first time. And then "item 1" and "item 2" were created. Firebase notified me of "item 1" and "item 2", so I added them to the list of my app.</p>
  
  <p>Now, my app was closed and restarted. If a new item "item 3" is created, I will be notified of that, but how can I retrieve "item 1" and "item 2" again? In short, I want to display all items, again.</p>
  
  <p>I have searched for examples, but all examples I could find were getting events, not "reading all".</p>
  
  <p>PS: I think you two for spending your time for explanation. I misunderstood how Firebase Realtime Database works; I thought I would get notified for new items only once across sessions. It seems that I could be notified of only new items by <code>addChildEventListener</code>, and I can access the entire items with <code>addValueEventListener</code> and its dataSnapshot.</p>