title: Angular(2 +) Detect if data in an Angular form (reactive) was changed
body: <p>I have some form and button to save it. The button must only be enabled when there are unsaved changes (inputs) on the form.</p>
  
  <pre><code>&lt;form&gt;
      &lt;div&gt;
      ... (inputs)
          &lt;span (click)="save()"&gt; Save &lt;/span&gt;
      &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>Is there some build-in mechanism for form dirty check in Angular 5? What is the easiest way to implement this scenario ?</p>