Issue #3021648 by benjifisher, selwynpolit: Add a migrate plugin to debug one process step

src/Plugin/migrate/process/Debug.php

+<?php
+
+namespace Drupal\migrate_devel\Plugin\migrate\process;
+
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\MigrateException;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Row;
+use Drupal\migrate\MigrateSkipRowException;
+
+/**
+ * Debug the process pipeline.
+ *
+ * Prints the input value, assuming that you are running the migration from the
+ * command line, and sends it to the next step in the pipeline unaltered.
+ *
+ * Available configuration keys:
+ * - label: (optional) a string to print before the debug output. Include any
+ *   trailing punctuation or space characters.
+ * - multiple: (optional) set to TRUE to ask the next step in the process
+ *   pipeline to process array values individually, like the multiple_values
+ *   plugin from the Migrate Plus module.
+ *
+ * Examples:
+ *
+ * @code
+ * process:
+ *   field_tricky:
+ *     -
+ *       plugin: debug
+ *       source: whatever
+ *     -
+ *       plugin: next
+ * @endcode
+ *
+ * This will print the source before passing it to the next plugin.
+ *
+ * @code
+ * process:
+ *   field_tricky:
+ *     -
+ *       plugin: debug
+ *       source: whatever
+ *       label: 'Step 1: '
+ *       multiple: true
+ *     -
+ *       plugin: next
+ * @endcode
+ *
+ * This does the same thing, but ensures that the next plugin will be called
+ * once for each item in the source, if the source is an array.
+ * It will also print "Debug Step 1: " before printing the source.
+ *
+ * @see \Drupal\migrate\Plugin\MigrateProcessInterface
+ *
+ * @MigrateProcessPlugin(
+ *   id = "debug",
+ *   handle_multiples = TRUE
+ * )
+ */
+class Debug extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (isset($this->configuration['label'])) {
+      print_r($this->configuration['label']);
+    }
+    print_r($value);
+    if (!is_array($value)) {
+      print_r(PHP_EOL);
+    }
+
+    return $value;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function multiple() {
+    return !empty($this->configuration['multiple']);
+  }
+
+}