title: Get number of items in a ReplaySubject with Array Element
body: <p>I have to use ReplaySubject in my project like that:</p>
  
  <pre><code>let myVariable = ReplaySubject&lt;[MyItems]&gt;.create(bufferSize: myBufferSize)
  </code></pre>
  
  <p>How can I get the number of items in the array of "MyItems"?</p>