Add a generic Boolean migrate process plugin that uses filter_var($va…

…lue, FILTER_VALIDATE_BOOLEAN).

modules/core/migrate/src/Plugin/migrate/process/Boolean.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\farm_migrate\Plugin\migrate\process;
+
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * This plugin converts values to a boolean.
+ *
+ * @codingStandardsIgnoreStart
+ *
+ * Example usage:
+ * @code
+ * destination:
+ *   plugin: 'entity:log'
+ * process:
+ *   is_movement:
+ *     plugin: boolean
+ *     source: is_movement
+ * @endcode
+
+ * @codingStandardsIgnoreEnd
+ *
+ * @MigrateProcessPlugin(
+ *   id = "boolean"
+ * )
+ */
+class Boolean extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+
+    // If the trimmed value is an empty string, return NULL so that the field's
+    // default value will be used.
+    if (trim($value) === '') {
+      return NULL;
+    }
+
+    // Use PHP's filter_var() with FILTER_VALIDATE_BOOLEAN constant.
+    return filter_var($value, FILTER_VALIDATE_BOOLEAN);
+  }
+
+}