title: ReactiveX C#: Do something when subscription starts
body: <p>I've created an <code>IObservable&lt;&gt;</code> combination using <code>CombineLatest()</code>:</p>
  
  <pre><code>IObservable&lt;IList&lt;List&lt;TileItem&gt;&gt;&gt; combination = Observable.CombineLatest(combinations);
  </code></pre>
  
  <p>I need:</p>
  
  <pre><code>1. to do something `doSomethingOnFirst()` when first element is reached.
  2. to do something `doSomethingOnComplete()` when subscription has completed.
  </code></pre>
  
  <p>Up to now, I've been able to do something when combination is completed. Nevertheless, I don't quite figure out what do I need to do in order to do something when subscription starts...</p>
  
  <pre><code>combination
      .Subscribe(
          _ =&gt; { },
          _ =&gt; { },
          () =&gt; this.doSomethingOnCompleted()
      );
  </code></pre>
  
  <p>Any ideas?</p>