title: How can I add action Items to a subscription after it has been activated?
body: <p>I'm trying to build a service in angular that manages the database the problem is that I am trying to store the subscriptions on the service class and not the component class so they are all in the same place however as I understand it, the callback for a subscription needs to be defined when the subscription is activated is there any way to add to a subscription after it has been activated? I looked at the .add() operator but that seems to work only when the subscription is canceled.</p>
  
  <p><strong>EDIT</strong><br>
  below is the best function I could come up with; it works but it doesn't seem elegant or even best practice</p>
  
  <pre><code>  test(col, doc, callback) {
      (this[col][doc] as Observable&lt;unknown&gt;) = this.afs
        .collection(col)
        .doc(doc)
        .valueChanges().subscribe(val =&gt; {callback(val)})
    }
  </code></pre>