title: RxJs: Create observable from field value changing
body: <p>I'd like to create an <code>Observable</code> using as source a variables: <code>this.pending</code>.</p>
  
  <p>I want to create an Observable which generates a feed every time <code>this.pending</code> value changes.</p>
  
  <p>I mean, when I do:</p>
  
  <pre><code>this.pending = false;
  </code></pre>
  
  <p>in some place of my code, I want to receive a <code>false</code> feed on my subscription, and so on...</p>
  
  <p>Any ideas?</p>