title: Close subscription on condition with takeUntil()
body: <p>I have a subscription to get providers from a NgRx reducer. I want to use <code>takeUntil()</code> to automatically close the subscription when is finally returns an array that has content:</p>
  
  <pre><code>// Fetch providers
  this.store.pipe(select(reducer.getProviders))
  //  .takeUntil(reducer.getProviders.length &gt; 0)
      .subscribe(providers =&gt; {
          if (providers) {
              this.providers = providers;
  //          takeUntil(/** something **/);
          }
      });
  </code></pre>
  
  <p>Can someone help me with this?</p>
  
  <p>I can't figure out how to make use of <code>takeUntil()</code></p>