class TermForm

class TermForm extends ContentEntityForm {
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $term = $this->entity;
        $vocab_storage = $this->entityTypeManager
            ->getStorage('taxonomy_vocabulary');
        
        /** @var \Drupal\taxonomy\TermStorageInterface $taxonomy_storage */
        $taxonomy_storage = $this->entityTypeManager
            ->getStorage('taxonomy_term');
        $vocabulary = $vocab_storage->load($term->bundle());
        $parent = $this->getParentIds($term);
        $form_state->set([
            'taxonomy',
            'parent',
        ], $parent);
        $form_state->set([
            'taxonomy',
            'vocabulary',
        ], $vocabulary);
        $form['relations'] = [
            '#type' => 'details',
            '#title' => $this->t('Relations'),
            '#open' => $taxonomy_storage->getVocabularyHierarchyType($vocabulary->id()) == VocabularyInterface::HIERARCHY_MULTIPLE,
            '#weight' => 10,
        ];
        // \Drupal\taxonomy\TermStorageInterface::loadTree() and
        // \Drupal\taxonomy\TermStorageInterface::loadParents() may contain large
        // numbers of items so we check for taxonomy.settings:override_selector
        // before loading the full vocabulary. Contrib modules can then intercept
        // before hook_form_alter to provide scalable alternatives.
        if (!$this->config('taxonomy.settings')
            ->get('override_selector')) {
            $exclude = [];
            if (!$term->isNew()) {
                $children = $taxonomy_storage->loadTree($vocabulary->id(), $term->id());
                // A term can't be the child of itself, nor of its children.
                foreach ($children as $child) {
                    $exclude[] = $child->tid;
                }
                $exclude[] = $term->id();
            }
            $tree = $taxonomy_storage->loadTree($vocabulary->id());
            $options = [
                '<' . $this->t('root') . '>',
            ];
            if (empty($parent)) {
                $parent = [
                    0,
                ];
            }
            foreach ($tree as $item) {
                if (!in_array($item->tid, $exclude)) {
                    $options[$item->tid] = str_repeat('-', $item->depth) . $item->name;
                }
            }
        }
        else {
            $options = [
                '<' . $this->t('root') . '>',
            ];
            $parent = [
                0,
            ];
        }
        if ($this->getRequest()->query
            ->has('parent')) {
            $parent = array_values(array_intersect(array_keys($options), (array) $this->getRequest()->query
                ->all()['parent']));
        }
        $form['relations']['parent'] = [
            '#type' => 'select',
            '#title' => $this->t('Parent terms'),
            '#options' => $options,
            '#default_value' => $parent,
            '#multiple' => TRUE,
        ];
        $form['relations']['weight'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Weight'),
            '#size' => 6,
            '#default_value' => $term->getWeight(),
            '#description' => $this->t('Terms are displayed in ascending order by weight.'),
            '#required' => TRUE,
        ];
        $form['vid'] = [
            '#type' => 'value',
            '#value' => $vocabulary->id(),
        ];
        $form['tid'] = [
            '#type' => 'value',
            '#value' => $term->id(),
        ];
        return parent::form($form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function actions(array $form, FormStateInterface $form_state) {
        $element = parent::actions($form, $form_state);
        if (!$this->getRequest()->query
            ->has('destination')) {
            $element['overview'] = [
                '#type' => 'submit',
                '#value' => $this->t('Save and go to list'),
                '#weight' => 20,
                '#submit' => array_merge($element['submit']['#submit'], [
                    '::overview',
                ]),
                '#access' => $this->currentUser()
                    ->hasPermission('access taxonomy overview'),
            ];
        }
        return $element;
    }
    
    /**
     * Form submission handler for the 'overview' action.
     *
     * @param array[] $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function overview(array $form, FormStateInterface $form_state) : void {
        $vocabulary = $this->entityTypeManager
            ->getStorage('taxonomy_vocabulary')
            ->load($form_state->getValue('vid'));
        $form_state->setRedirectUrl($vocabulary->toUrl('overview-form'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function validateForm(array &$form, FormStateInterface $form_state) {
        parent::validateForm($form, $form_state);
        // Ensure numeric values.
        if ($form_state->hasValue('weight') && !is_numeric($form_state->getValue('weight'))) {
            $form_state->setErrorByName('weight', $this->t('Weight value must be numeric.'));
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildEntity(array $form, FormStateInterface $form_state) {
        $term = parent::buildEntity($form, $form_state);
        // Prevent leading and trailing spaces in term names.
        $term->setName(trim($term->getName()));
        // Assign parents with proper delta values starting from 0.
        $term->parent = array_values($form_state->getValue('parent'));
        return $term;
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditedFieldNames(FormStateInterface $form_state) {
        return array_merge([
            'parent',
            'weight',
        ], parent::getEditedFieldNames($form_state));
    }
    
    /**
     * {@inheritdoc}
     */
    protected function flagViolations(EntityConstraintViolationListInterface $violations, array $form, FormStateInterface $form_state) {
        // Manually flag violations of fields not handled by the form display. This
        // is necessary as entity form displays only flag violations for fields
        // contained in the display.
        // @see ::form()
        foreach ($violations->getByField('parent') as $violation) {
            $form_state->setErrorByName('parent', $violation->getMessage());
        }
        foreach ($violations->getByField('weight') as $violation) {
            $form_state->setErrorByName('weight', $violation->getMessage());
        }
        parent::flagViolations($violations, $form, $form_state);
    }
    
    /**
     * {@inheritdoc}
     */
    public function save(array $form, FormStateInterface $form_state) {
        $term = $this->entity;
        $result = $term->save();
        $edit_link = $term->toLink($this->t('Edit'), 'edit-form')
            ->toString();
        $view_link = $term->toLink()
            ->toString();
        switch ($result) {
            case SAVED_NEW:
                $this->messenger()
                    ->addStatus($this->t('Created new term %term.', [
                    '%term' => $view_link,
                ]));
                $this->logger('taxonomy')
                    ->info('Created new term %term.', [
                    '%term' => $term->getName(),
                    'link' => $edit_link,
                ]);
                break;
            case SAVED_UPDATED:
                $this->messenger()
                    ->addStatus($this->t('Updated term %term.', [
                    '%term' => $view_link,
                ]));
                $this->logger('taxonomy')
                    ->info('Updated term %term.', [
                    '%term' => $term->getName(),
                    'link' => $edit_link,
                ]);
                // Redirect to term view page if user has access, otherwise the form
                // will be displayed again.
                $canonicalUrl = $term->toUrl();
                if ($canonicalUrl->access()) {
                    $form_state->setRedirectUrl($canonicalUrl);
                }
                break;
        }
        $current_parent_count = count($form_state->getValue('parent'));
        // Root doesn't count if it's the only parent.
        if ($current_parent_count == 1 && $form_state->hasValue([
            'parent',
            0,
        ])) {
            $form_state->setValue('parent', []);
        }
        $form_state->setValue('tid', $term->id());
        $form_state->set('tid', $term->id());
    }
    
    /**
     * Returns term parent IDs, including the root.
     *
     * @param \Drupal\taxonomy\TermInterface $term
     *   The taxonomy term entity.
     *
     * @return array
     *   A list if parent term IDs.
     */
    protected function getParentIds(TermInterface $term) : array {
        $parent = [];
        // Get the parent directly from the term as
        // \Drupal\taxonomy\TermStorageInterface::loadParents() excludes the root.
        foreach ($term->get('parent') as $item) {
            $parent[] = (int) $item->target_id;
        }
        return $parent;
    }

}