Issue #3214186: Scan for snippets of process plugins

src/Plugin/migrate/process/Snippet.php

+<?php
+
+namespace Drupal\migrate_scanner\Plugin\migrate\process;
+
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Plugin\MigratePluginManagerInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Yaml\Yaml;
+
+/**
+ * Applies process plugins described in YAML snippets.
+ *
+ * Available configuration keys:
+ * - module: the module providing the snippet
+ * - path: path of the YAML file, relative to module/migrations/process and
+ *   without the .yml extension.
+ *
+ * Example:
+ *
+ * Suppose the file mymodule/migrations/process/clean_whitespace.yml contains
+ * the following:
+ *
+ * @code
+ * -
+ *   plugin: callback
+ *   callable: htmlentities
+ * -
+ *   plugin: str_replace
+ *   search:
+ *     - '&#160;'
+ *     - '&nbsp;'
+ *   replace: ' '
+ * -
+ *   plugin: str_replace
+ *   regex: true
+ *   search: '@\s+@'
+ *   replace: ' '
+ * -
+ *   plugin: callback
+ *   callable: trim
+ * @endcode
+ *
+ * Then that process pipeline can be used as follows:
+ *
+ * @code
+ * process:
+ *   field_formatted_text:
+ *     plugin: snippet
+ *     module: mymodule
+ *     path: clean_whitespace
+ *     source: html_string
+ * @endcode
+ *
+ * @MigrateProcessPlugin(
+ *   id = "snippet"
+ * )
+ */
+class Snippet extends ProcessPluginBase implements ContainerFactoryPluginInterface {
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * The process pipeline.
+   *
+   * @var Drupal\migrate\Plugin\MigrateProcessInterface[]
+   */
+  protected $pipeline = [];
+
+  /**
+   * Constructs a download process plugin.
+   *
+   * @param array $configuration
+   *   The plugin configuration.
+   * @param string $plugin_id
+   *   The plugin ID.
+   * @param array $plugin_definition
+   *   The plugin definition.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   * @param \Drupal\migrate\Plugin\MigratePluginManagerInterface $plugin_manager
+   *   The process plugin manager.
+   */
+  public function __construct(array $configuration, $plugin_id, array $plugin_definition, ModuleHandlerInterface $module_handler, MigratePluginManagerInterface $plugin_manager) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+    $this->moduleHandler = $module_handler;
+
+    // @todo Add error checking: the module is enabled, the file exists, the
+    // YAML is valid.
+    $snippet_file = implode('/', [
+      $this->moduleHandler->getModule($this->configuration['module'])->getPath(),
+      'migrations/process',
+      $this->configuration['path'],
+    ]) . '.yml';
+    $steps = Yaml::parse(file_get_contents($snippet_file));
+    foreach ($steps as $step_configuration) {
+      $step_id = $step_configuration['plugin'];
+      unset($step_configuration['plugin']);
+      $this->pipeline[] = $plugin_manager->createInstance($step_id, $step_configuration);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('module_handler'),
+      $container->get('plugin.manager.migrate.process')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    foreach ($this->pipeline as $step) {
+      $value = $step->transform($value, $migrate_executable, $row, $destination_property);
+    }
+    return $value;
+  }
+
+}