title: In Angular, how do you check if a form (or its elements) have changed in value?
body: <p>I'm aware of properties like <code>$pristine</code>, <code>$dirty</code> etc. However, <code>$dirty</code> simply indicates if that form element has been interacted with.</p>
  
  <p>How do I check if that form element has been actually <em>changed?</em></p>
  
  <p>I need something like a <code>$changed</code> property on the element that indicates if the value of that <code>input</code> (or in the case of a <code>form</code>, any child <code>input</code>) has been updated. If the value is set back to the original value, I want <code>$changed</code> to be set back to <code>false</code>.</p>
  
  <p>My question: does angular have something like this already? If not, how do I go about building such functionality?</p>