libcamera: Add signal/slot communication mechanism

Introduce a Signal class that allows connecting event sources (signals)
to event listeners (slots) without adding any boilerplate code usually
associated with the observer or listener design patterns.

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/meson.build

@@ -2,6 +2,7 @@ libcamera_api = files([
     'camera.h',
     'camera_manager.h',
     'libcamera.h',
+    'signal.h',
 ])
 
 install_headers(libcamera_api,

include/libcamera/signal.h

@@ -0,0 +1,154 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * signal.h - Signal & slot implementation
+ */
+#ifndef __LIBCAMERA_SIGNAL_H__
+#define __LIBCAMERA_SIGNAL_H__
+
+#include <list>
+#include <vector>
+
+namespace libcamera {
+
+template<typename... Args>
+class Signal;
+
+template<typename... Args>
+class SlotBase
+{
+public:
+	SlotBase(void *obj)
+		: obj_(obj) { }
+	virtual ~SlotBase() { }
+
+	virtual void invoke(Args... args) = 0;
+
+protected:
+	friend class Signal<Args...>;
+	void *obj_;
+};
+
+template<typename T, typename... Args>
+class SlotMember : public SlotBase<Args...>
+{
+public:
+	SlotMember(T *obj, void(T::*func)(Args...))
+		: SlotBase<Args...>(obj), func_(func) { }
+
+	void invoke(Args... args) { (reinterpret_cast<T *>(this->obj_)->*func_)(args...); }
+
+private:
+	friend class Signal<Args...>;
+	void(T::*func_)(Args...);
+};
+
+template<typename... Args>
+class SlotStatic : public SlotBase<Args...>
+{
+public:
+	SlotStatic(void(*func)(Args...))
+		: SlotBase<Args...>(nullptr), func_(func) { }
+
+	void invoke(Args... args) { (*func_)(args...); }
+
+private:
+	friend class Signal<Args...>;
+	void(*func_)(Args...);
+};
+
+template<typename... Args>
+class Signal
+{
+public:
+	Signal() { }
+	~Signal()
+	{
+		for (SlotBase<Args...> *slot : slots_)
+			delete slot;
+	}
+
+	template<typename T>
+	void connect(T *object, void(T::*func)(Args...))
+	{
+		slots_.push_back(new SlotMember<T, Args...>(object, func));
+	}
+
+	void connect(void(*func)(Args...))
+	{
+		slots_.push_back(new SlotStatic<Args...>(func));
+	}
+
+	void disconnect()
+	{
+		for (SlotBase<Args...> *slot : slots_)
+			delete slot;
+		slots_.clear();
+	}
+
+	template<typename T>
+	void disconnect(T *object)
+	{
+		for (auto iter = slots_.begin(); iter != slots_.end(); ) {
+			SlotBase<Args...> *slot = *iter;
+			if (slot->obj_ == object) {
+				iter = slots_.erase(iter);
+				delete slot;
+			} else {
+				++iter;
+			}
+		}
+	}
+
+	template<typename T>
+	void disconnect(T *object, void(T::*func)(Args...))
+	{
+		for (auto iter = slots_.begin(); iter != slots_.end(); ) {
+			SlotBase<Args...> *slot = *iter;
+			/*
+			 * If the obj_ pointer matches the object types must
+			 * match, so we can safely cast to SlotMember<T, Args>.
+			 */
+			if (slot->obj_ == object &&
+			    reinterpret_cast<SlotMember<T, Args...> *>(slot)->func_ == func) {
+				iter = slots_.erase(iter);
+				delete slot;
+			} else {
+				++iter;
+			}
+		}
+	}
+
+	void disconnect(void(*func)(Args...))
+	{
+		for (auto iter = slots_.begin(); iter != slots_.end(); ) {
+			SlotBase<Args...> *slot = *iter;
+			if (slot->obj_ == nullptr &&
+			    reinterpret_cast<SlotStatic<Args...> *>(slot)->func_ == func) {
+				iter = slots_.erase(iter);
+				delete slot;
+			} else {
+				++iter;
+			}
+		}
+	}
+
+	void emit(Args... args)
+	{
+		/*
+		 * Make a copy of the slots list as the slot could call the
+		 * disconnect operation, invalidating the iterator.
+		 */
+		std::vector<SlotBase<Args...> *> slots{ slots_.begin(), slots_.end() };
+		for (SlotBase<Args...> *slot : slots)
+			slot->invoke(args...);
+	}
+
+private:
+	std::list<SlotBase<Args...> *> slots_;
+};
+
+} /* namespace libcamera */
+
+#endif /* __LIBCAMERA_SIGNAL_H__ */