States between siblings

Children can use #states to react to other children. Write the selector as if the element had a single row. The module rewrites it for each row, so a row reacts only to its own inputs.

The selector uses the name of the element, then the name of the child, here links[type].

$form['links'] = [
  '#type' => 'element_multiple',
  '#title' => $this->t('Links'),
  '#header' => TRUE,
  '#element' => [
    'type' => [
      '#type' => 'select',
      '#title' => $this->t('Type'),
      '#title_display' => 'invisible',
      '#options' => [
        'a' => $this->t('Option A'),
        'b' => $this->t('Option B'),
        'other' => $this->t('Other'),
      ],
    ],
    'other' => [
      '#type' => 'textfield',
      '#title' => $this->t('Other'),
      '#title_display' => 'invisible',
      '#placeholder' => $this->t('Enter other'),
      '#states' => [
        'visible' => [
          ':input[name="links[type]"]' => ['value' => 'other'],
        ],
      ],
    ],
  ],
  '#default_value' => [
    ['type' => 'a', 'other' => ''],
    ['type' => 'a', 'other' => ''],
  ],
];

In row 3 the real name is links[items][2][type]. The module makes that change for you.

The text field is hidden until the select of the same row is set to Other.

States on the whole element

#states on the element itself works too. The module moves it to a wrapper around the list. A child with #required becomes required only while the list is visible.

$form['links']['#states'] = [
  'visible' => [':input[name="show_links"]' => ['checked' => TRUE]],
];

Notes

• A child can also be shown or hidden by a checkbox of the same row, with ['checked' => TRUE] in place of ['value' => 'other'].
• When the element sits inside another tree, use its full name, for example settings[links][has_link].
• The rewrite works for composite elements.
• Related property: #states.