title: Extbase: Check if a form has been submitted in a controller action
body: <p>I have got a form in which I want to show html links. To avoid that a user looses form inputs in case clicking the link, I want to inform the user before he leaves the form.</p>
  
  <p>To achieve that, I tried</p>
  
  <pre><code>public function registerAction(Registration $registration = null) {
      $formHasBeenSubmitted = !is_null($registration);
      $this-&gt;view-&gt;assign('formHasBeenSubmitted', $formHasBeenSubmitted);
  }
  </code></pre>
  
  <p>But it is always false.</p>
  
  <p>What is the correct way to find out if the form for a specific action has been submitted?</p>
  
  <p>Update after <a href="https://stackoverflow.com/a/43515178/7173655">undkos answer</a>:</p>
  
  <ul>
  <li>I would like to show the message in case the form has been submitted but validation errors have been showing up and of course also as soon as the user started entering data.</li>
  <li>Behind a link there is a form which edits data (postal addresses) which are shown on the main form and should be updated there after editing.</li>
  </ul>