title: Is there any way to check if source is subscribed?
body: <p>Just as the title says, in Angular 2, is there any way to check if source is already subscribed? Because I have to check it before using </p>
  
  <pre><code>this.subscription.unsubscribe();
  </code></pre>
  
  <p>This is my code:</p>
  
  <pre><code>this.Source = Rx.Observable.timer(startTime, 60000).timeInterval().pluck('interval');
  
  this.Subscription = this.Source
    .subscribe(data =&gt; { something }
  </code></pre>
  
  <p>and then I want to be sure that it is subscribed before calling <code>unsubscribe()</code></p>