title: How to know if Observable changed value or final value was set
body: <p>I have a variable of type <code>returnDate: Observable&lt;string&gt;</code> which value is set from another function of type Observable, which looks something like:</p>
  
  <pre><code>getDates(): Observable&lt;string&gt; {
     return new Observable((observer) =&gt; { 
     observer.next(moment(this.data.Date).format('DD MMM'));}
     });
   }
  </code></pre>
  
  <p>Now how can I detect or know if returnDate value has been changed or if the observable has received another value.
  Basicly I am changing dates with this observables so i need to know for a specific object (this.data.Date) if the given value has been changed (the observer.next) was completed or maybe there is a helper function like final/finalaize for the observables.</p>