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diff --git a/src/undo-stack-observer.h b/src/undo-stack-observer.h
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+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Authors:
+ * David Yip <yipdw@rose-hulman.edu>
+ *
+ * Copyright (c) 2005 Authors
+ *
+ * Released under GNU GPL v2+, read the file 'COPYING' for more information.
+ */
+
+#ifndef SEEN_UNDO_COMMIT_OBSERVER_H
+#define SEEN_UNDO_COMMIT_OBSERVER_H
+
+#include "inkgc/gc-managed.h"
+
+namespace Inkscape {
+
+struct Event;
+
+/**
+ * Observes changes made to the undo and redo stacks.
+ *
+ * More specifically, an UndoStackObserver is a class that receives notifications when
+ * any of the following events occur:
+ * <ul>
+ * 	<li>A change is committed to the undo stack.</li>
+ * 	<li>An undo action is made.</li>
+ * 	<li>A redo action is made.</li>
+ * </ul>
+ *
+ * UndoStackObservers should not be used on their own.  Instead, they should be registered
+ * with a CompositeUndoStackObserver.
+ */
+class UndoStackObserver : public GC::Managed<> {
+public:
+	UndoStackObserver() = default;
+	virtual ~UndoStackObserver() = default;
+
+	/**
+	 * Triggered when the user issues an undo command.
+	 *
+	 * \param log Pointer to an Event describing the undone event.
+	 */
+	virtual void notifyUndoEvent(Event* log) = 0;
+
+	/**
+	 * Triggered when the user issues a redo command.
+	 *
+	 * \param log Pointer to an Event describing the redone event.
+	 */
+	virtual void notifyRedoEvent(Event* log) = 0;
+
+	/**
+	 * Triggered when a set of transactions is committed to the undo log.
+	 *
+	 * \param log Pointer to an Event describing the committed events.
+	 */
+	virtual void notifyUndoCommitEvent(Event* log) = 0;
+
+	/**
+	 * Triggered when the undo log is cleared.
+	 */
+	virtual void notifyClearUndoEvent() = 0;
+
+	/**
+	 * Triggered when the redo log is cleared.
+	 */
+	virtual void notifyClearRedoEvent() = 0;
+
+};
+
+}
+
+#endif // SEEN_UNDO_COMMIT_OBSERVER_H