title: RxJS Observable: Emit when specific part of the object changes
body: <p>I have a complex object having child objects and arrays and so on.</p>
  
  <p>I can easily create a Subject/BehaviourSubject or Observable from this object so that I can "next" (emit) the new state to the subscribers.</p>
  
  <p>[ex: <code>let appEnv$ = new Rx.BehaviorSubject&lt;IWebSocketAppEnv&gt;(appEnv);</code>]</p>
  
  <p>However I don't want all my subscribers to be notified every time the object changes. For instance for one of my subscriber, I only want to be notified when a change occurs on an elements of an array of that object.</p>
  
  <p>Actually what I want is something redux is already doing. In redux, I can subscribe to the store but selecting only a child element.</p>
  
  <p>I want to implement the same infrastructure for my back-end websocket server application.</p>
  
  <p>How can I achieve that with RxJS?</p>