title: Observable from callback method
body: <p>Let say I have a class which iherits legacy API and overrides a virtual method which is called when something happens</p>
  
  <pre><code>type MyClass() as this =
      let somethingObservable: IObservable&lt;Something&gt; = ...
  
      override _.OnSomething(s: Something) = ...
  </code></pre>
  
  <p>How can I translate each invokation of OnSomething to a notification of somethingObservable?</p>
  
  <p>That's probably a simple question, but I could not find a way to do it properly (should I use not advised ISubject?). Appreciate your help.</p>