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diff --git a/lib/public/Activity/IEventMerger.php b/lib/public/Activity/IEventMerger.php
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+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2016 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+namespace OCP\Activity;
+
+/**
+ * Interface EventMerger
+ *
+ * @since 11.0
+ */
+interface IEventMerger {
+	/**
+	 * Combines two events when possible to have grouping:
+	 *
+	 * Example1: Two events with subject '{user} created {file}' and
+	 * $mergeParameter file with different file and same user will be merged
+	 * to '{user} created {file1} and {file2}' and the childEvent on the return
+	 * will be set, if the events have been merged.
+	 *
+	 * Example2: Two events with subject '{user} created {file}' and
+	 * $mergeParameter file with same file and same user will be merged to
+	 * '{user} created {file1}' and the childEvent on the return will be set, if
+	 * the events have been merged.
+	 *
+	 * The following requirements have to be met, in order to be merged:
+	 * - Both events need to have the same `getApp()`
+	 * - Both events must not have a message `getMessage()`
+	 * - Both events need to have the same subject `getSubject()`
+	 * - Both events need to have the same object type `getObjectType()`
+	 * - The time difference between both events must not be bigger then 3 hours
+	 * - Only up to 5 events can be merged.
+	 * - All parameters apart from such starting with $mergeParameter must be
+	 *   the same for both events.
+	 *
+	 * @param string $mergeParameter
+	 * @param IEvent $event
+	 * @param IEvent|null $previousEvent
+	 * @return IEvent
+	 * @since 11.0
+	 */
+	public function mergeEvents($mergeParameter, IEvent $event, ?IEvent $previousEvent = null);
+}