Setting value of form element from $form_state not working

Question

I need to track previous values of a few form elements on a form to detect when they change to invalidate some other options on the page. Essentially, if the value of these fields change a field that controls pagination should reset.

I'm trying to accomplish this with this:

$values = $form_state->getValues();
$form['element1'] = [
      '#type' => 'radios',
      '#title' => $this->t('Element 1'),
      '#options' => [
        // Omitted for brevity
      ],
      '#weight' => '0',
      '#ajax' => [
        'callback' => [$this, 'my_callback'],
        'disable-refocus' => true
      ]
    ];

// dump($values['element1']);
$form['element1_previous'] = [
      '#type' => 'value',
      '#value' => (isset($values['element1'])) ? $values['element1'] : 'none',
    ];

No matter what I do the value of $values['element1_previous] is none. If I uncomment the dump() statement right before it I can see the value of $values['element1'] changing each time AJAX is submitted so the value is there.

If I do something like:

$form['element1_previous'] = [
          '#type' => 'value',
          '#value' => rand()
        ];

I can see $values['element1_previous'] being set to a different integer each AJAX submission. I just can't assign the value of $form['element1'] to that field.

What am I doing wrong? Is there a better way I could handle this?

Answer 1

This:

(isset($values['element1']))

will always return TRUE, since the form element exists, and therefore a value is always passed to the submit hander, even when the value is empty.

You likely want this:

!empty($values['element1'])