title: How to re-execute async observable on parameter change?
body: <p>I have a simple async observable which takes 2 parameters to perform some calculations:</p>
  <pre><code>  public preferredCurrency: string = '';
  
    wmData$ = this.http.get&lt;any&gt;(&quot;./api/wealthModeling/GetWMData&quot;);
    portfolioCalculation$ = this.wmData$.pipe(
      mergeMap((wmData) =&gt; {
        return this.portfolioCalculationService.getPortfolioValues(wmData, this.preferredCurrency);
      })
  </code></pre>
  <p>As you can see on initial load preferredCurrency is empty, However UI allows user to choose a different currency (via a dropdown) and hence the need to perform the calculations again.</p>
  <p>I am not able to figure out a way to execute the <code>portfolioCalculation$</code> again.</p>
  <p>Hopefully, this explains what I am trying to achieve.</p>
  <p>Thanks much.</p>