title: How to combine multi Observable in RxSwift
body: <p>Let's say I create an Observable for each image download</p>
  
  <pre><code>for _ in 0...N {
      let s : Single&lt;UIImage?&gt; = fetchImage(from url)
  }
  </code></pre>
  
  <p>How can I:</p>
  
  <ul>
  <li>create a new Observable to emit an event when a download is completed</li>
  <li>create a new Observable to emit only completed event when all download is completed</li>
  </ul>
  
  <p>Thanks!</p>