--- /dev/null
+<?php
+
+namespace Drupal\Core\Block;
+
+/**
+ * The interface for "main page content" blocks.
+ *
+ * A main page content block represents the content returned by the controller.
+ *
+ * @ingroup block_api
+ */
+interface MainContentBlockPluginInterface extends BlockPluginInterface {
+
+ /**
+ * Sets the main content render array.
+ *
+ * @param array $main_content
+ * The render array representing the main content.
+ */
+ public function setMainContent(array $main_content);
+
+}