Drupal 8: An example plugin for conditions API Commerce 2.x

OrderCurrency.php

<?php

namespace Drupal\commerce_price\Plugin\Commerce\Condition;

use Drupal\commerce\Plugin\Commerce\Condition\ConditionBase;
use Drupal\Core\Entity\EntityInterface;
use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides the currency condition for orders.
 *
 * @CommerceCondition(
 *   id = "order_currency",
 *   label = @Translation("Currency"),
 *   display_label = @Translation("Limit by currency"),
 *   category = @Translation("Order"),
 *   entity_type = "commerce_order",
 * )
 */
class OrderCurrency extends ConditionBase implements ContainerFactoryPluginInterface {

  /**
   * The currency storage.
   *
   * @var \Drupal\Core\Entity\EntityStorageInterface
   */
  protected $currencyStorage;

  /**
   * Constructs a new OrderCurrency object.
   *
   * @param array $configuration
   *   The plugin configuration, i.e. an array with configuration values keyed
   *   by configuration option name.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);

    $this->currencyStorage = $entity_type_manager->getStorage('commerce_currency');
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('entity_type.manager')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'currencies' => [],
    ] + parent::defaultConfiguration();
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildConfigurationForm($form, $form_state);

    $active_currencies = $this->currencyStorage
      ->loadByProperties([
        'status' => TRUE,
      ]);

    // If no currency is imported or active, return no form.
    if (empty($active_currencies)) {
      return $form;
    }

    foreach ($active_currencies as $currency_code => $currency) {
      /* @var \Drupal\commerce_price\Entity\Currency $currency */
      $currency_list[$currency_code] = $currency->getName();
    }

    $selected_currencies = $this->configuration['currencies'];

    $form['currencies'] = [
      '#type' => 'select',
      '#title' => $this->t('Currencies'),
      '#multiple' => TRUE,
      '#default_value' => $selected_currencies,
      '#options' => $currency_list,
      '#required' => TRUE,
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    parent::submitConfigurationForm($form, $form_state);

    $values = $form_state->getValue($form['#parents']);
    $this->configuration['currencies'] = $values['currencies'];
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate(EntityInterface $entity) {
    $this->assertEntity($entity);
    /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
    $order = $entity;
    /** @var \Drupal\commerce_product\Entity\ProductVariationInterface $purchasable_entity */
    $order_currency = $order->getTotalPrice()->getCurrencyCode();

    $selected_currencies = $this->configuration['currencies'];

    return in_array($order_currency, $selected_currencies) ? TRUE : FALSE;
  }

}