title: How to convert callback based API into one based on Observable?
body: <p>The library I'm using emits a series of <code>Message</code> objects using callback object.</p>
  
  <pre><code>interface MessageCallback {
      onMessage(Message message);
  }
  </code></pre>
  
  <p>The callback is added using some <code>libraryObject.setCallback(MessageCallback)</code> call and the process is started using non-blocking <code>libraryObject.start()</code> method call. </p>
  
  <p>What is the best way of creating an <code>Observable&lt;Message&gt;</code> that will emit those objects?</p>
  
  <p>What if the <code>libraryObject.start()</code> is blocking?</p>