Introduce extended schema plugin.

src/Plugin/GraphQL/Schema/SdlExtendedSchemaPluginBase.php

@@ -0,0 +1,54 @@
+<?php
+
+namespace Drupal\graphql\Plugin\GraphQL\Schema;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use GraphQL\Language\Parser;
+use GraphQL\Utils\SchemaExtender;
+
+/**
+ * Allows to extend the GraphQL schema.
+ *
+ * When GraphQL schema is distributed across multiple files it might be useful
+ * to allow extension of certain types. This schema plugin allows that. See an
+ * example of code to extend the schema with using webonyx/graphql-php library:
+ * https://github.com/webonyx/graphql-php/issues/180#issuecomment-444407411
+ */
+abstract class SdlExtendedSchemaPluginBase extends SdlSchemaPluginBase {
+
+  /**
+   * Retrieves the parsed AST of the extended schema definition.
+   *
+   * @return \GraphQL\Language\AST\DocumentNode
+   *   The parsed extended schema document.
+   */
+  protected function getExtendedSchemaDocument() {
+    // Only use caching of the parsed document if we aren't in development mode.
+    if (empty($this->inDevelopment) && $cache = $this->astCache->get($this->getPluginId())) {
+      return $cache->data;
+    }
+
+    $ast = Parser::parse($this->getExtendedSchemaDefinition());
+    if (!empty($this->inDevelopment)) {
+      $this->astCache->set($this->getPluginId(), $ast, CacheBackendInterface::CACHE_PERMANENT, ['graphql']);
+    }
+
+    return $ast;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSchema() {
+    return SchemaExtender::extend(parent::getSchema(), $this->getExtendedSchemaDocument());
+  }
+
+  /**
+   * Retrieves the raw extended schema definition string.
+   *
+   * @return string
+   *   The extended schema definition.
+   */
+  abstract protected function getExtendedSchemaDefinition();
+
+}

src/Plugin/GraphQL/Schema/SdlSchemaPluginBase.php

@@ -189,7 +189,7 @@ protected function getTypeResolver(TypeDefinitionNode $type) {
    *   The parsed schema document.
    */
   protected function getSchemaDocument() {
-    // Only use caching of the parsed document if aren't in development mode.
+    // Only use caching of the parsed document if we aren't in development mode.
     if (empty($this->inDevelopment) && $cache = $this->astCache->get($this->getPluginId())) {
       return $cache->data;
     }