title: Stay subscribed to an array of Subjects
body: <p>I have a main file that creates and destroys instances of a class <code>Child</code>. The Child instances are kept in an array (<code>childArr</code>) within main and I want a way to listen for events from all of those Child instances.</p>
  <p>Each Child has a Subject exposed and I want to keep a subscription to all those Subjects that remains up to date as Child instances are added to or removed from the Array.</p>
  <p><a href="https://stackblitz.com/edit/typescript-ckb6qj?file=index.ts" rel="nofollow noreferrer">This stackblitz project is the simplest version I can come up with</a>. As it is it works while I manually add each child subject to the <code>merge()</code>  method (lines 22-24). The commented out lines at the bottom (lines 28-30) are what I had <em>hoped</em> to do but that don't seem to work.</p>
  <p>I would love someone to explain the difference between the merge on lines 22-24 and the one on line 28-30, why the first works and the second doesn't and hopefully how I can make the second one work.</p>
  <p>Any help appreciated.</p>