Add subscriber source

[MAILPOET-1377]

lib/Subscribers/Source.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace MailPoet\Subscribers;
+
+use MailPoet\Models\Subscriber;
+
+class Source {
+
+  const FORM = 'form';
+  const IMPORTED = 'imported';
+  const ADMINISTRATOR = 'administrator';
+  const API = 'api';
+  const WORDPRESS_USER = 'wordpress_user';
+  const UNKNOWN = 'unknown';
+
+  private static $allowed_sources = array(
+    Source::FORM,
+    Source::IMPORTED,
+    Source::ADMINISTRATOR,
+    Source::API,
+    Source::WORDPRESS_USER,
+    Source::UNKNOWN,
+  );
+
+  static function setSource(Subscriber $subscriber, $source) {
+    if((isset($subscriber->source)) && ($subscriber->source !== Source::UNKNOWN)) {
+      // we don't want to override source
+      return $subscriber;
+    }
+    if(!in_array($source, Source::$allowed_sources)) {
+      throw new \InvalidArgumentException('Invalid source "' . $source . '""');
+    }
+    $subscriber->set('source', $source);
+    return $subscriber;
+  }
+
+}