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/AbstractObserver.h

@@ -37,12 +37,42 @@ public:
 	AbstractObserver& operator = (const AbstractObserver& observer);
 
 	virtual void notify(Notification* pNf) const = 0;
+
 	virtual bool equals(const AbstractObserver& observer) const = 0;
-	virtual bool accepts(Notification* pNf, const char* pName = 0) const = 0;
+
+	[[deprecated("use `Poco::Any accepts(Notification*)` instead")]]
+	virtual bool accepts(Notification* pNf, const char* pName) const = 0;
+
+	virtual bool accepts(const Notification::Ptr& pNf) const = 0;
+
 	virtual AbstractObserver* clone() const = 0;
+
+	virtual void start();
+		/// No-op.
+		/// This method can be implemented by inheriting classes which require
+		/// explicit start in order to begin processing notifications.
+
 	virtual void disable() = 0;
+
+	virtual int backlog() const;
+		/// Returns number of queued messages that this Observer has.
+		/// For non-active (synchronous) observers, always returns zero.
 };
 
+//
+// inlines
+//
+
+inline void AbstractObserver::start()
+{
+}
+
+
+inline int AbstractObserver::backlog() const
+{
+	return 0;
+}
+
 
 } // namespace Poco