class ItemList

class ItemList extends TypedData implements \IteratorAggregate, ListInterface {
    
    /**
     * Numerically indexed array of items.
     *
     * @var \Drupal\Core\TypedData\TypedDataInterface[]
     */
    protected $list = [];
    
    /**
     * {@inheritdoc}
     */
    public function getValue() {
        $values = [];
        foreach ($this->list as $delta => $item) {
            $values[$delta] = $item->getValue();
        }
        return $values;
    }
    
    /**
     * Overrides \Drupal\Core\TypedData\TypedData::setValue().
     *
     * @param array|null $values
     *   An array of values of the field items, or NULL to unset the field.
     * @param bool $notify
     *   (optional) Whether to notify the parent object of the change. Defaults to
     *   TRUE.
     */
    public function setValue($values, $notify = TRUE) {
        if (!isset($values) || $values === []) {
            $this->list = [];
        }
        else {
            // Only arrays with numeric keys are supported.
            if (!is_array($values)) {
                throw new \InvalidArgumentException('Cannot set a list with a non-array value.');
            }
            // Assign incoming values. Keys are renumbered to ensure 0-based
            // sequential deltas. If possible, reuse existing items rather than
            // creating new ones.
            foreach (array_values($values) as $delta => $value) {
                if (!isset($this->list[$delta])) {
                    $this->list[$delta] = $this->createItem($delta, $value);
                }
                else {
                    $this->list[$delta]
                        ->setValue($value, FALSE);
                }
            }
            // Truncate extraneous pre-existing values.
            $this->list = array_slice($this->list, 0, count($values));
        }
        // Notify the parent of any changes.
        if ($notify && isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function getString() {
        $strings = [];
        foreach ($this->list as $item) {
            $strings[] = $item->getString();
        }
        // Remove any empty strings resulting from empty items.
        return implode(', ', array_filter($strings, 'mb_strlen'));
    }
    
    /**
     * {@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.');
        }
        return $this->list[$index] ?? NULL;
    }
    
    /**
     * {@inheritdoc}
     */
    public function set($index, $value) {
        if (!is_numeric($index)) {
            throw new \InvalidArgumentException('Unable to set a value with a non-numeric delta in a list.');
        }
        // Ensure indexes stay sequential. We allow assigning an item at an existing
        // index, or at the next index available.
        if ($index < 0 || $index > count($this->list)) {
            throw new \InvalidArgumentException('Unable to set a value to a non-subsequent delta in a list.');
        }
        // Support setting values via typed data objects.
        if ($value instanceof TypedDataInterface) {
            $value = $value->getValue();
        }
        // If needed, create the item at the next position.
        $item = $this->list[$index] ?? $this->appendItem();
        $item->setValue($value);
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function removeItem($index) {
        if (isset($this->list) && array_key_exists($index, $this->list)) {
            // Remove the item, and reassign deltas.
            unset($this->list[$index]);
            $this->rekey($index);
        }
        else {
            throw new \InvalidArgumentException('Unable to remove item at non-existing index.');
        }
        return $this;
    }
    
    /**
     * Renumbers the items in the list.
     *
     * @param int $from_index
     *   Optionally, the index at which to start the renumbering, if it is known
     *   that items before that can safely be skipped (for example, when removing
     *   an item at a given index).
     */
    protected function rekey($from_index = 0) {
        // Re-key the list to maintain consecutive indexes.
        $this->list = array_values($this->list);
        // Each item holds its own index as a "name", it needs to be updated
        // according to the new list indexes.
        for ($i = $from_index; $i < count($this->list); $i++) {
            $this->list[$i]
                ->setContext($i, $this);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function first() {
        return $this->get(0);
    }
    
    /**
     * {@inheritdoc}
     */
    public function offsetExists($offset) {
        // We do not want to throw exceptions here, so we do not use get().
        return isset($this->list[$offset]);
    }
    
    /**
     * {@inheritdoc}
     */
    public function offsetUnset($offset) {
        $this->removeItem($offset);
    }
    
    /**
     * {@inheritdoc}
     */
    public function offsetGet($offset) {
        return $this->get($offset);
    }
    
    /**
     * {@inheritdoc}
     */
    public function offsetSet($offset, $value) {
        if (!isset($offset)) {
            // The [] operator has been used.
            $this->appendItem($value);
        }
        else {
            $this->set($offset, $value);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function appendItem($value = NULL) {
        $offset = count($this->list);
        $item = $this->createItem($offset, $value);
        $this->list[$offset] = $item;
        return $item;
    }
    
    /**
     * Helper for creating a list item object.
     *
     * @return \Drupal\Core\TypedData\TypedDataInterface
     */
    protected function createItem($offset = 0, $value = NULL) {
        return $this->getTypedDataManager()
            ->getPropertyInstance($this, $offset, $value);
    }
    
    /**
     * {@inheritdoc}
     */
    public function getItemDefinition() {
        return $this->definition
            ->getItemDefinition();
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIterator() {
        return new \ArrayIterator($this->list);
    }
    
    /**
     * {@inheritdoc}
     */
    public function count() {
        return count($this->list);
    }
    
    /**
     * {@inheritdoc}
     */
    public function isEmpty() {
        foreach ($this->list as $item) {
            if ($item instanceof ComplexDataInterface || $item instanceof ListInterface) {
                if (!$item->isEmpty()) {
                    return FALSE;
                }
            }
            elseif ($item->getValue() !== NULL) {
                return FALSE;
            }
        }
        return TRUE;
    }
    
    /**
     * {@inheritdoc}
     */
    public function filter($callback) {
        if (isset($this->list)) {
            $removed = FALSE;
            // Apply the filter, detecting if some items were actually removed.
            $this->list = array_filter($this->list, function ($item) use ($callback, &$removed) {
                if (call_user_func($callback, $item)) {
                    return TRUE;
                }
                else {
                    $removed = TRUE;
                }
            });
            if ($removed) {
                $this->rekey();
            }
        }
        return $this;
    }
    
    /**
     * {@inheritdoc}
     */
    public function onChange($delta) {
        // Notify the parent of changes.
        if (isset($this->parent)) {
            $this->parent
                ->onChange($this->name);
        }
    }
    
    /**
     * Magic method: Implements a deep clone.
     */
    public function __clone() {
        foreach ($this->list as $delta => $item) {
            $this->list[$delta] = clone $item;
            $this->list[$delta]
                ->setContext($delta, $this);
        }
    }

}