title: How do I trigger an Angular validation directive to update when a value it is compared to changes?
body: <p>I have built some custom form validation directives that compare form fields to other properties. They work great when you change the value of the form field, the validation fires and updates the validity of the form field just fine.</p>
  
  <p>The problem I have is when the property that I am comparing it to changes the form validation is not triggered and the form field being compared to the property doesn't change validity.</p>
  
  <p>I have created an example stackblitz here.</p>
  
  <p><a href="https://stackblitz.com/edit/angular-t7h2ok" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-t7h2ok</a></p>
  
  <p>When you change the value of the the first field it is invalid if it is the same as the other field and valid if not. The problem is when I change the other field I would like to make sure that the validity of the first form field is updated when the other property is changed.</p>