Variable.php

<?php

namespace Drupal\migrate_drupal\Plugin\migrate\source;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\State\StateInterface;
use Drupal\migrate\Plugin\MigrationInterface;

/**
 * Drupal variable source from database.
 *
 * This source class always returns a single row and as such is not a good
 * example for any normal source class returning multiple rows.
 *
 * @MigrateSource(
 *   id = "variable",
 *   source_module = "system",
 * )
 */
class Variable extends DrupalSqlBase {
    
    /**
     * The variable names to fetch.
     *
     * @var array
     */
    protected $variables;
    
    /**
     * {@inheritdoc}
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, StateInterface $state, EntityTypeManagerInterface $entity_type_manager) {
        parent::__construct($configuration, $plugin_id, $plugin_definition, $migration, $state, $entity_type_manager);
        $this->variables = $this->configuration['variables'];
    }
    
    /**
     * {@inheritdoc}
     */
    protected function initializeIterator() {
        return new \ArrayIterator([
            $this->values(),
        ]);
    }
    
    /**
     * Return the values of the variables specified in the plugin configuration.
     *
     * @return array
     *   An associative array where the keys are the variables specified in the
     *   plugin configuration and the values are the values found in the source.
     *   Only those values are returned that are actually in the database.
     */
    protected function values() {
        // Create an ID field so we can record migration in the map table.
        // Arbitrarily, use the first variable name.
        $values['id'] = reset($this->variables);
        return $values + array_map('unserialize', $this->prepareQuery()
            ->execute()
            ->fetchAllKeyed());
    }
    
    /**
     * {@inheritdoc}
     */
    public function count($refresh = FALSE) {
        // Variable always returns a single row with at minimum an 'id' property.
        return 1;
    }
    
    /**
     * {@inheritdoc}
     */
    public function fields() {
        return array_combine($this->variables, $this->variables);
    }
    
    /**
     * {@inheritdoc}
     */
    public function query() {
        return $this->getDatabase()
            ->select('variable', 'v')
            ->fields('v', [
            'name',
            'value',
        ])
            ->condition('name', $this->variables, 'IN');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getIds() {
        $ids['id']['type'] = 'string';
        return $ids;
    }

}