--- /dev/null
+<?php
+
+namespace Drupal\Core\TypedData;
+
+/**
+ * Provides common functionality for computed item lists.
+ *
+ * @see \Drupal\Core\TypedData\ListInterface
+ * @see \Drupal\Core\TypedData\Plugin\DataType\ItemList
+ * @see \Drupal\Core\Field\FieldItemListInterface
+ * @see \Drupal\Core\Field\FieldItemList
+ *
+ * @ingroup typed_data
+ *
+ * @internal
+ * This trait has been added in Drupal 8.4.3 as an internal helper and should
+ * not be used outside of core.
+ */
+trait ComputedItemListTrait {
+
+ /**
+ * Whether the values have already been computed or not.
+ *
+ * @var bool
+ */
+ protected $valueComputed = FALSE;
+
+ /**
+ * Computes the values for an item list.
+ */
+ abstract protected function computeValue();
+
+ /**
+ * Ensures that values are only computed once.
+ */
+ protected function ensureComputedValue() {
+ if ($this->valueComputed === FALSE) {
+ $this->computeValue();
+ $this->valueComputed = TRUE;
+ }
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getValue() {
+ $this->ensureComputedValue();
+ return parent::getValue();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function setValue($values, $notify = TRUE) {
+ parent::setValue($values, $notify);
+
+ // Make sure that subsequent getter calls do not try to compute the values
+ // again.
+ $this->valueComputed = TRUE;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getString() {
+ $this->ensureComputedValue();
+ return parent::getString();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function get($index) {
+ if (!is_numeric($index)) {
+ throw new \InvalidArgumentException('Unable to get a value with a non-numeric delta in a list.');
+ }
+
+ // Unlike the base implementation of
+ // \Drupal\Core\TypedData\ListInterface::get(), we do not add an empty item
+ // automatically because computed item lists need to behave like
+ // non-computed ones. For example, calling isEmpty() on a computed item list
+ // should return TRUE when the values were computed and the item list is
+ // truly empty.
+ // @see \Drupal\Core\TypedData\Plugin\DataType\ItemList::get().
+ $this->ensureComputedValue();
+
+ return isset($this->list[$index]) ? $this->list[$index] : NULL;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function set($index, $value) {
+ $this->ensureComputedValue();
+ return parent::set($index, $value);
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function appendItem($value = NULL) {
+ $this->ensureComputedValue();
+ return parent::appendItem($value);
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function removeItem($index) {
+ $this->ensureComputedValue();
+ return parent::removeItem($index);
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function isEmpty() {
+ $this->ensureComputedValue();
+ return parent::isEmpty();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function offsetExists($offset) {
+ $this->ensureComputedValue();
+ return parent::offsetExists($offset);
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getIterator() {
+ $this->ensureComputedValue();
+ return parent::getIterator();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function count() {
+ $this->ensureComputedValue();
+ return parent::count();
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function applyDefaultValue($notify = TRUE) {
+ // Default values do not make sense for computed item lists. However, this
+ // method can be overridden if needed.
+ return $this;
+ }
+
+}