libcamera: Add event notification infrastructure

Add three new classes, EventDispatcher, EventNotifier and Timer, that
define APIs for file descriptor event notification and timers. The
implementation of the EventDispatcher is meant to be provided to
libcamera by the application.

The event dispatcher is integrated twith the camera manager to implement
automatic registration of timers and events.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/timer.h

@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * timer.h - Generic timer
+ */
+#ifndef __LIBCAMERA_TIMER_H__
+#define __LIBCAMERA_TIMER_H__
+
+#include <cstdint>
+
+#include <libcamera/signal.h>
+
+namespace libcamera {
+
+class Timer
+{
+public:
+	Timer();
+
+	void start(unsigned int msec);
+	void stop();
+	bool isRunning() const;
+
+	unsigned int interval() const { return interval_; }
+	uint64_t deadline() const { return deadline_; }
+
+	Signal<Timer *> timeout;
+
+private:
+	unsigned int interval_;
+	uint64_t deadline_;
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_TIMER_H__ */