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diff --git a/src/lib/checker.h b/src/lib/checker.h
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+++ b/src/lib/checker.h
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+/*
+    Copyright (C) 2018 Carl Hetherington <cth@carlh.net>
+
+    This file is part of DCP-o-matic.
+
+    DCP-o-matic is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    DCP-o-matic is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with DCP-o-matic.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+/** @file  src/lib/checker.h
+ *  @brief Checker class.
+ */
+
+#ifndef DCPOMATIC_CHECKER_H
+#define DCPOMATIC_CHECKER_H
+
+#include "signaller.h"
+#include <boost/signals2.hpp>
+
+/** Parent for classes which check some condition every so often and signal
+ *  when the state of the condition changes.
+ */
+class Checker : public Signaller, public boost::noncopyable
+{
+public:
+	virtual ~Checker ();
+
+	void run ();
+
+	bool ok () const;
+
+	/** Emitted when the state of our condition changes */
+	boost::signals2::signal<void (void)> StateChanged;
+
+protected:
+
+	Checker (int period);
+
+	/** @return true if the condition is `ok', otherwise false */
+	virtual bool check () const = 0;
+
+private:
+
+	void thread ();
+
+	boost::thread* _thread;
+	mutable boost::mutex _mutex;
+	bool _terminate;
+	bool _ok;
+	/** check period in seconds */
+	int _period;
+};
+
+#endif