AsyncObserver (#4444)

* feat(AsyncObserver): Improve NotificationCenter speed and usability #4414

* fix(Notification): add missing header

* feat(Any): add checkers for holding nullptr #4447

* feat(NotificationCenter): g++ build and refactoring #4414

* fix(Observer): compile errors on some compilers #4414

* fix(NotificationCenter): compile errors #4414

* chore(ParallelSocketAcceptor): remove unnecessary include and using from header

* feat(AsyncNotificationCenter): add #4414

* test(AsyncNotificationCenter): add mixed observer types to the test #4414

* fix(AsyncNotificationCenter): hangs on program exit #4414

* fix(dev): friend not honored, temporarily make private members public

* fix(AsyncNotificationCenter); remove default #4414

Foundation/include/Poco/AsyncNotificationCenter.h

@@ -0,0 +1,77 @@
+//
+// AsyncNotificationCenter.h
+//
+// Library: Foundation
+// Package: Notifications
+// Module:  AsyncNotificationCenter
+//
+// Definition of the AsyncNotificationCenter class.
+//
+// Copyright (c) 2006, Applied Informatics Software Engineering GmbH.
+// Aleph ONE Software Engineering d.o.o.,
+// and Contributors.
+//
+// SPDX-License-Identifier:	BSL-1.0
+//
+
+
+#ifndef Foundation_AsyncNotificationCenter_INCLUDED
+#define Foundation_AsyncNotificationCenter_INCLUDED
+
+
+#include "Poco/Foundation.h"
+#include "Poco/NotificationCenter.h"
+#include "Poco/Thread.h"
+#include "Poco/Stopwatch.h"
+#include "Poco/Debugger.h"
+#include "Poco/ErrorHandler.h"
+#include "Poco/Format.h"
+#include "Poco/RunnableAdapter.h"
+#include "Poco/NotificationQueue.h"
+
+
+namespace Poco {
+
+
+class Foundation_API AsyncNotificationCenter: public NotificationCenter
+	/// AsyncNotificationCenter decouples posting of notifications
+	/// from notifying subscribers by calling observers' notification
+	/// handler in a dedicated thread.
+{
+public:
+	AsyncNotificationCenter();
+		/// Creates the AsyncNotificationCenter and starts the notifying thread.
+
+	~AsyncNotificationCenter();
+		/// Stops the notifying thread and destroys the AsyncNotificationCenter.
+
+	AsyncNotificationCenter& operator = (const AsyncNotificationCenter&) = delete;
+	AsyncNotificationCenter(const AsyncNotificationCenter&) = delete;
+	AsyncNotificationCenter& operator = (AsyncNotificationCenter&&) = delete;
+	AsyncNotificationCenter(AsyncNotificationCenter&&) = delete;
+
+	virtual void postNotification(Notification::Ptr pNotification);
+		/// Enqueues notification into the notification queue.
+
+	virtual int backlog() const;
+		/// Returns the numbner of notifications in the notification queue.
+
+private:
+	void start();
+	void stop();
+	void dequeue();
+
+	using Adapter = RunnableAdapter<AsyncNotificationCenter>;
+
+	Thread _thread;
+	NotificationQueue _nq;
+	Adapter _ra;
+	std::atomic<bool> _started;
+	std::atomic<bool> _done;
+};
+
+
+} // namespace Poco
+
+
+#endif // Foundation_AsyncNotificationCenter_INCLUDED