title: Angular 9 listen to form pristine change
body: <p>I want to listen to the first change by the user in a form,
  what I need is something like this:</p>
  <pre><code>$rootScope.$watch(function() {
              return validationForm.$pristine;
          }, function(newVal, oldVal) {
  </code></pre>
  <p>but in angular 9.</p>
  <p>I tried using valueChanges</p>
  <pre><code>this.form.valueChanges.subscribe((result) =&gt; { })
  </code></pre>
  <p>but it emit on each change in the form build too.
  how can I subscribe only to the first user's change?</p>