Taxonomy form autocomplete, Create taxonomy term field with FAPI

Simple taxonomy autocomplete field.

Example:

$form['tagtest'] = [

  '#type' => 'entity_autocomplete',

  '#target_type' => 'taxonomy_term',

  '#title' => 'Taxonomy Term',

];

Field with default value

Example : Default TID = 1

$form['tagtest'] = [

  '#type' => 'entity_autocomplete',

  '#target_type' => 'taxonomy_term',

  '#title' => 'Taxonomy Term',

  '#default_value' => \Drupal\taxonomy\Entity\Term::load(1),

];

With a selected vocabulary

Example : 'tags'

$form['tagtest'] = [

  '#type' => 'entity_autocomplete',

  '#target_type' => 'taxonomy_term',

  '#title' => 'Taxonomy Term',

  '#selection_settings' => [

    'target_bundles' => ['tags'],

  ],

];

Auto create a term if not exist

Example:

$form['tagtest'] = [

  '#type' => 'entity_autocomplete',

  '#target_type' => 'taxonomy_term',

  '#title' => 'Taxonomy Term',

  '#autocreate' => [

    'bundle' => 'tags', // Required. The bundle name for the new entity.

    'uid' => 1, // Optional. The user ID for the new entity

  ],

];

In this case the returned value is not only a string, it can be a Term.

If new term created, you must save it.

Example:

$tag = $form_state->getValue('tagtest');

if (empty($tag)) {

  drupal_set_message("Tag is empty, nothing to do");

}

elseif (is_string($tag)) {

  drupal_set_message("A term selected, tid = $tag");

}

elseif (isset($tag['entity']) && ($tag['entity'] instanceof \Drupal\taxonomy\Entity\Term)) {

  $entity = $tag['entity'];

  $entity->save();

  drupal_set_message("A new term : " . $entity->id() . " : " . $entity->label());

}