trait ComputedItemListTrait

trait ComputedItemListTrait {
    
    /**
     * Whether the values have already been computed or not.
     *
     * @var bool
     */
    protected $valueComputed = FALSE;
    
    /**
     * Computes the values for an item list.
     */
    protected abstract 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;
    }

}