Sequence.php

<?php

namespace Drupal\Core\Config\Schema;


/**
 * Defines a configuration element of type Sequence.
 *
 * This object may contain any number and type of nested elements that share
 * a common definition in the 'sequence' property of the configuration schema.
 *
 * Read https://www.drupal.org/node/1905070 for more details about configuration
 * schema, types and type resolution.
 *
 * Note that sequences implement the typed data ComplexDataInterface (via the
 * parent ArrayElement) rather than the ListInterface. This is because sequences
 * may have named keys, which is not supported by ListInterface. From the typed
 * data API perspective sequences are handled as ordered mappings without
 * metadata about existing properties.
 */
class Sequence extends ArrayElement {
    
    /**
     * {@inheritdoc}
     */
    protected function getElementDefinition($key) {
        $value = $this->value[$key] ?? NULL;
        // @todo: Remove BC layer for sequence with hyphen in front. https://www.drupal.org/node/2444979
        $definition = [];
        if (isset($this->definition['sequence'][0])) {
            $definition = $this->definition['sequence'][0];
        }
        elseif ($this->definition['sequence']) {
            $definition = $this->definition['sequence'];
        }
        return $this->buildDataDefinition($definition, $value, $key);
    }

}