Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/migrate/src/Plugin/MigrateProcessInterface.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Plugin\MigrateProcessInterface.
+ */
+
+namespace Drupal\migrate\Plugin;
+
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Row;
+
+/**
+ * An interface for migrate process plugins.
+ *
+ * A process plugin can use any number of methods instead of (but not in
+ * addition to) transform with the same arguments and then the plugin
+ * configuration needs to provide the name of the method to be called via the
+ * "method" key. See \Drupal\migrate\Plugin\migrate\process\SkipOnEmpty and
+ * migrate.migration.d6_field_instance_widget_settings.yml for examples.
+ *
+ * @see \Drupal\migrate\Plugin\MigratePluginManager
+ * @see \Drupal\migrate\ProcessPluginBase
+ * @see \Drupal\migrate\Annotation\MigrateProcessPlugin
+ * @see plugin_api
+ *
+ * @ingroup migration
+ */
+interface MigrateProcessInterface extends PluginInspectionInterface {
+
+  /**
+   * Performs the associated process.
+   *
+   * @param mixed $value
+   *   The value to be transformed.
+   * @param \Drupal\migrate\MigrateExecutableInterface $migrate_executable
+   *   The migration in which this process is being executed.
+   * @param \Drupal\migrate\Row $row
+   *   The row from the source to process. Normally, just transforming the
+   *   value is adequate but very rarely you might need to change two columns
+   *   at the same time or something like that.
+   * @param string $destination_property
+   *   The destination property currently worked on. This is only used
+   *   together with the $row above.
+   *
+   * @return string|array
+   *   The newly transformed value.
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property);
+
+  /**
+   * Indicates whether the returned value requires multiple handling.
+   *
+   * @return bool
+   *   TRUE when the returned value contains a list of values to be processed.
+   *   For example, when the 'source' property is a string and the value found
+   *   is an array.
+   */
+  public function multiple();
+
+}