class FieldItemList

class FieldItemList extends ItemList implements FieldItemListInterface {
    
    /**
     * Numerically indexed array of field items.
     *
     * @var \Drupal\Core\Field\FieldItemInterface[]
     */
    protected $list = [];
    
    /**
     * The langcode of the field values held in the object.
     *
     * @var string
     */
    protected $langcode = LanguageInterface::LANGCODE_NOT_SPECIFIED;
    
    /**
     * {@inheritdoc}
     */
    protected function createItem($offset = 0, $value = NULL) {
        return \Drupal::service('plugin.manager.field.field_type')->createFieldItem($this, $offset, $value);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getEntity() {
        // The "parent" is the TypedData object for the entity, we need to unwrap
        // the actual entity.
        return $this->getParent()
            ->getValue();
    }
    
    /**
     * {@inheritdoc}
     */
    public function setLangcode($langcode) {
        $this->langcode = $langcode;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getLangcode() {
        return $this->langcode;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getFieldDefinition() {
        return $this->definition;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSettings() {
        return $this->definition
            ->getSettings();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getSetting($setting_name) {
        return $this->definition
            ->getSetting($setting_name);
    }
    
    /**
     * {@inheritdoc}
     */
    public function filterEmptyItems() {
        $this->filter(function ($item) {
            return !$item->isEmpty();
        });
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function setValue($values, $notify = TRUE) {
        // Support passing in only the value of the first item, either as a literal
        // (value of the first property) or as an array of properties.
        if (isset($values) && (!is_array($values) || !empty($values) && !is_numeric(current(array_keys($values))))) {
            $values = [
                0 => $values,
            ];
        }
        parent::setValue($values, $notify);
    }
    
    /**
     * {@inheritdoc}
     */
    public function __get($property_name) {
        // For empty fields, $entity->field->property is NULL.
        if ($item = $this->first()) {
            return $item->__get($property_name);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function __set($property_name, $value) {
        // For empty fields, $entity->field->property = $value automatically
        // creates the item before assigning the value.
        $item = $this->first() ?: $this->appendItem();
        $item->__set($property_name, $value);
    }
    
    /**
     * {@inheritdoc}
     */
    public function __isset($property_name) {
        if ($item = $this->first()) {
            return $item->__isset($property_name);
        }
        return FALSE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function __unset($property_name) {
        if ($item = $this->first()) {
            $item->__unset($property_name);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function access($operation = 'view', ?AccountInterface $account = NULL, $return_as_object = FALSE) {
        $access_control_handler = \Drupal::entityTypeManager()->getAccessControlHandler($this->getEntity()
            ->getEntityTypeId());
        return $access_control_handler->fieldAccess($operation, $this->getFieldDefinition(), $account, $this, $return_as_object);
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultAccess($operation = 'view', ?AccountInterface $account = NULL) {
        // Grant access per default.
        return AccessResult::allowed();
    }
    
    /**
     * {@inheritdoc}
     */
    public function applyDefaultValue($notify = TRUE) {
        if ($value = $this->getFieldDefinition()
            ->getDefaultValue($this->getEntity())) {
            $this->setValue($value, $notify);
        }
        else {
            // Create one field item and give it a chance to apply its defaults.
            // Remove it if this ended up doing nothing.
            // @todo Having to create an item in case it wants to set a value is
            // absurd. Remove that in https://www.drupal.org/node/2356623.
            $item = $this->first() ?: $this->appendItem();
            $item->applyDefaultValue(FALSE);
            $this->filterEmptyItems();
        }
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function preSave() {
        // Filter out empty items.
        $this->filterEmptyItems();
        $this->delegateMethod('preSave');
    }
    
    /**
     * {@inheritdoc}
     */
    public function postSave($update) {
        $result = $this->delegateMethod('postSave', $update);
        return (bool) array_filter($result);
    }
    
    /**
     * {@inheritdoc}
     */
    public function delete() {
        $this->delegateMethod('delete');
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteRevision() {
        $this->delegateMethod('deleteRevision');
    }
    
    /**
     * Calls a method on each FieldItem.
     *
     * Any argument passed will be forwarded to the invoked method.
     *
     * @param string $method
     *   The name of the method to be invoked.
     *
     * @return array
     *   An array of results keyed by delta.
     */
    protected function delegateMethod($method) {
        $result = [];
        $args = array_slice(func_get_args(), 1);
        foreach ($this->list as $delta => $item) {
            // call_user_func_array() is way slower than a direct call so we avoid
            // using it if have no parameters.
            $result[$delta] = $args ? call_user_func_array([
                $item,
                $method,
            ], $args) : $item->{$method}();
        }
        return $result;
    }
    
    /**
     * {@inheritdoc}
     */
    public function view($display_options = []) {
        $view_builder = \Drupal::entityTypeManager()->getViewBuilder($this->getEntity()
            ->getEntityTypeId());
        return $view_builder->viewField($this, $display_options);
    }
    
    /**
     * {@inheritdoc}
     */
    public function generateSampleItems($count = 1) {
        $field_definition = $this->getFieldDefinition();
        $field_type_class = $field_definition->getItemDefinition()
            ->getClass();
        for ($delta = 0; $delta < $count; $delta++) {
            $values[$delta] = $field_type_class::generateSampleValue($field_definition);
        }
        $this->setValue($values);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getConstraints() {
        $constraints = parent::getConstraints();
        // Check that the number of values doesn't exceed the field cardinality. For
        // form submitted values, this can only happen with 'multiple value'
        // widgets.
        $cardinality = $this->getFieldDefinition()
            ->getFieldStorageDefinition()
            ->getCardinality();
        if ($cardinality != FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED) {
            $options['max'] = $cardinality;
            if ($label = $this->getFieldDefinition()
                ->getLabel()) {
                $options['maxMessage'] = $this->t('%name: this field cannot hold more than @count values.', [
                    '%name' => $label,
                    '@count' => $cardinality,
                ]);
            }
            $constraints[] = $this->getTypedDataManager()
                ->getValidationConstraintManager()
                ->create('Count', $options);
        }
        return $constraints;
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesForm(array &$form, FormStateInterface $form_state) {
        if (empty($this->getFieldDefinition()
            ->getDefaultValueCallback())) {
            if ($widget = $this->defaultValueWidget($form_state)) {
                // Place the input in a separate place in the submitted values tree.
                $element = [
                    '#parents' => [
                        'default_value_input',
                    ],
                ];
                $element += $widget->form($this, $element, $form_state);
                return $element;
            }
            else {
                return [
                    '#markup' => $this->t('No widget available for: %type.', [
                        '%type' => $this->getFieldDefinition()
                            ->getType(),
                    ]),
                ];
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesFormValidate(array $element, array &$form, FormStateInterface $form_state) {
        // Extract the submitted value, and validate it.
        if ($widget = $this->defaultValueWidget($form_state)) {
            $widget->extractFormValues($this, $element, $form_state);
            // Force a non-required field definition.
            // @see self::defaultValueWidget().
            $this->getFieldDefinition()
                ->setRequired(FALSE);
            $violations = $this->validate();
            // Assign reported errors to the correct form element.
            if (count($violations)) {
                $widget->flagErrors($this, $violations, $element, $form_state);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function defaultValuesFormSubmit(array $element, array &$form, FormStateInterface $form_state) {
        // Extract the submitted value, and return it as an array.
        if ($widget = $this->defaultValueWidget($form_state)) {
            $widget->extractFormValues($this, $element, $form_state);
            return $this->getValue();
        }
        return [];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function processDefaultValue($default_value, FieldableEntityInterface $entity, FieldDefinitionInterface $definition) {
        return $default_value;
    }
    
    /**
     * Returns the widget object used in default value form.
     *
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state of the (entire) configuration form.
     *
     * @return \Drupal\Core\Field\WidgetInterface|null
     *   A Widget object or NULL if no widget is available.
     */
    protected function defaultValueWidget(FormStateInterface $form_state) {
        if (!$form_state->has('default_value_widget')) {
            $entity = $this->getEntity();
            // Force a non-required widget.
            $definition = $this->getFieldDefinition();
            $definition->setRequired(FALSE);
            $definition->setDescription('');
            
            /** @var \Drupal\Core\Entity\Display\EntityFormDisplayInterface $entity_form_display */
            $entity_form_display = \Drupal::service('entity_display.repository')->getFormDisplay($entity->getEntityTypeId(), $entity->bundle());
            
            /** @var \Drupal\Core\Field\WidgetPluginManager $field_widget_plugin_manager */
            $field_widget_plugin_manager = \Drupal::service('plugin.manager.field.widget');
            // Use the widget currently configured for the 'default' form mode, or
            // fallback to the default widget for the field type.
            if (($configuration = $entity_form_display->getComponent($definition->getName())) && isset($configuration['type'])) {
                // Get the plugin instance manually to ensure an up-to-date field
                // definition is used.
                // @see \Drupal\Core\Entity\Entity\EntityFormDisplay::getRenderer
                $widget = $field_widget_plugin_manager->getInstance([
                    'field_definition' => $definition,
                    'form_mode' => $entity_form_display->getOriginalMode(),
                    'prepare' => FALSE,
                    'configuration' => $configuration,
                ]);
            }
            else {
                $options = [
                    'field_definition' => $this->getFieldDefinition(),
                ];
                // If the field does not have a widget configured in the 'default' form
                // mode, check if there are default entity form display options defined
                // for the 'default' form mode in the form state.
                // @see \Drupal\field_ui\Controller\FieldConfigAddController::fieldConfigAddConfigureForm
                if (($default_options = $form_state->get('default_options')) && isset($default_options['entity_form_display']['default'])) {
                    $options['configuration'] = $default_options['entity_form_display']['default'];
                }
                $widget = $field_widget_plugin_manager->getInstance($options);
            }
            $form_state->set('default_value_widget', $widget);
        }
        return $form_state->get('default_value_widget');
    }
    
    /**
     * {@inheritdoc}
     */
    public function equals(FieldItemListInterface $list_to_compare) {
        $count1 = count($this);
        $count2 = count($list_to_compare);
        if ($count1 === 0 && $count2 === 0) {
            // Both are empty we can safely assume that it did not change.
            return TRUE;
        }
        if ($count1 !== $count2) {
            // One of them is empty but not the other one so the value changed.
            return FALSE;
        }
        $value1 = $this->getValue();
        $value2 = $list_to_compare->getValue();
        if ($value1 === $value2) {
            return TRUE;
        }
        // If the values are not equal ensure a consistent order of field item
        // properties and remove properties which will not be saved.
        $property_definitions = $this->getFieldDefinition()
            ->getFieldStorageDefinition()
            ->getPropertyDefinitions();
        $non_computed_properties = array_filter($property_definitions, function (DataDefinitionInterface $property) {
            return !$property->isComputed();
        });
        $callback = function (&$value) use ($non_computed_properties) {
            if (is_array($value)) {
                $value = array_intersect_key($value, $non_computed_properties);
                // Also filter out properties with a NULL value as they might exist in
                // one field item and not in the other, depending on how the values are
                // set. Do not filter out empty strings or other false-y values as e.g.
                // a NULL or FALSE in a boolean field is not the same.
                $value = array_filter($value, function ($property) {
                    return $property !== NULL;
                });
                ksort($value);
            }
        };
        array_walk($value1, $callback);
        array_walk($value2, $callback);
        return $value1 == $value2;
    }
    
    /**
     * {@inheritdoc}
     */
    public function hasAffectingChanges(FieldItemListInterface $original_items, $langcode) {
        return !$this->equals($original_items);
    }

}