Poco 观察者模式（Observer Pattern） 订阅和发布某个感兴趣的通知， Observer和Notification

flyfish

先写一个实例代码

#include "Poco/NotificationCenter.h"
#include "Poco/Notification.h"
#include "Poco/Observer.h"
#include "Poco/NObserver.h"
#include "Poco/AutoPtr.h"
#include <iostream>
using Poco::NotificationCenter;
using Poco::Notification;
using Poco::Observer;
using Poco::NObserver;
using Poco::AutoPtr;
class BaseNotification: public Notification
{
};

class FileReadableNotification: public BaseNotification
{};
class FileWritableNotification: public BaseNotification
{};
class FileTimeoutNotification: public BaseNotification
{};


class Target
{
public:
    void handleBase(BaseNotification* pNf)
    {
        std::cout << "handleBase: " << pNf->name() << std::endl;
        pNf->release(); 
    }
    void handleRead(const AutoPtr<FileReadableNotification>& pNf)
    {
        std::cout << "handleRead: " << pNf->name() << std::endl;
    }
    void handleWrite(const AutoPtr<FileWritableNotification>& pNf)
    {
        std::cout << "handleWrite: " << pNf->name() << std::endl;
    }
};


int main(int argc, char** argv)
{
    NotificationCenter nc;
    Target target;
    nc.addObserver(Observer<Target, BaseNotification>(target, &Target::handleBase));
    nc.addObserver(NObserver<Target, FileReadableNotification>(target, &Target::handleRead));
    nc.addObserver(NObserver<Target, FileWritableNotification>(target, &Target::handleWrite));

    nc.postNotification(new BaseNotification);
    nc.postNotification(new FileReadableNotification);
    nc.postNotification(new FileWritableNotification);


    nc.removeObserver(Observer<Target, BaseNotification>(target, &Target::handleBase));
    nc.removeObserver(NObserver<Target, FileReadableNotification>(target, &Target::handleRead));
    nc.removeObserver(NObserver<Target, FileWritableNotification>(target, &Target::handleWrite));
    return 0;
}

Observer是plain pointer
NObserver是 smart pointer

普通指针

typedef void (C::*Callback)(N*);

智能指针

typedef AutoPtr<N> NotificationPtr;
typedef void (C::*Callback)(const NotificationPtr&);

普通指针和智能指针按照各自的使用方式使用

Target通过NotificationCenter的addObserver()方法订阅感兴趣的notification
Target通过NotificationCenter的removeObserver()方法取消订阅感兴趣的notification

例如本实例对FileReadableNotification和FileWritableNotification感兴趣，
对FileTimeoutNotification这个notification不感兴趣，那就不订阅也不用取消订阅。
派送notification是由NotificationCenter的postNotification()方法进行派送
target只要订阅该notification，target就会收到notification

通知某个target发生了什么事。

依据Poco源码
C对应Target Class
N对应​Notification Class

NObserver

template <class C, class N>
class NObserver: public AbstractObserver
{
public:
	typedef AutoPtr<N> NotificationPtr;
	typedef void (C::*Callback)(const NotificationPtr&);

	NObserver(C& object, Callback method):
		_pObject(&object),
		_method(method)
	{
	}

	NObserver(const NObserver& observer):
		AbstractObserver(observer),
		_pObject(observer._pObject),
		_method(observer._method)
	{
	}

	~NObserver()
	{
	}

	......
private:
	NObserver();

	C*       _pObject;
	Callback _method;
	mutable Poco::Mutex _mutex;
};

Observer

template <class C, class N>
class Observer: public AbstractObserver
{
public:
	typedef void (C::*Callback)(N*);

	Observer(C& object, Callback method):
		_pObject(&object),
		_method(method)
	{
	}

	Observer(const Observer& observer):
		AbstractObserver(observer),
		_pObject(observer._pObject),
		_method(observer._method)
	{
	}

	~Observer()
	{
	}
......
private:
	Observer();

	C*       _pObject;
	Callback _method;
	mutable Poco::Mutex _mutex;
};

NotificationCenter

class Foundation_API NotificationCenter
	/// A NotificationCenter is essentially a notification dispatcher.
	/// It notifies all observers of notifications meeting specific criteria.
	/// This information is encapsulated in Notification objects.
	/// Client objects register themselves with the notification center as observers of
	/// specific notifications posted by other objects. When an event occurs, an object
	/// posts an appropriate notification to the notification center. The notification
	/// center invokes the registered method on each matching observer, passing the notification
	/// as argument.
	///
	/// The order in which observers receive notifications is undefined.
	/// It is possible for the posting object and the observing object to be the same.
	/// The NotificationCenter delivers notifications to observers synchronously.
	/// In other words the postNotification() method does not return until all observers have
	/// received and processed the notification.
	/// If an observer throws an exception while handling a notification, the NotificationCenter
	/// stops dispatching the notification and postNotification() rethrows the exception.
	///
	/// In a multithreaded scenario, notifications are always delivered in the thread in which the
	/// notification was posted, which may not be the same thread in which an observer registered itself.
	///
	/// The NotificationCenter class is basically a C++ implementation of the NSNotificationCenter class
	/// found in Apple's Cocoa (or OpenStep).
	///
	/// While handling a notification, an observer can unregister itself from the notification center,
	/// or it can register or unregister other observers. Observers added during a dispatch cycle
	/// will not receive the current notification.
	///
	/// The method receiving the notification must be implemented as
	///     void handleNotification(MyNotification* pNf);
	/// The handler method gets co-ownership of the Notification object
	/// and must release it when done. This is best done with an AutoPtr:
	///     void MyClass::handleNotification(MyNotification* pNf)
	///     {
	///         AutoPtr<MyNotification> nf(pNf);
	///         ...
	///     }
	///
	/// Alternatively, the NObserver class template can be used to register a callback
	/// method. In this case, the callback method receives the Notification in an
	/// AutoPtr and thus does not have to deal with object ownership issues:
	///     void MyClass::handleNotification(const AutoPtr<MyNotification>& pNf)
	///     {
	///         ...
	///     }
{
public:
	NotificationCenter();
		/// Creates the NotificationCenter.

	~NotificationCenter();
		/// Destroys the NotificationCenter.

	void addObserver(const AbstractObserver& observer);
		/// Registers an observer with the NotificationCenter.
		/// Usage:
		///     Observer<MyClass, MyNotification> obs(*this, &MyClass::handleNotification);
		///     notificationCenter.addObserver(obs);
		///
		/// Alternatively, the NObserver template class can be used instead of Observer.

	void removeObserver(const AbstractObserver& observer);
		/// Unregisters an observer with the NotificationCenter.

	bool hasObserver(const AbstractObserver& observer) const;
		/// Returns true if the observer is registered with this NotificationCenter.

	void postNotification(Notification::Ptr pNotification);
		/// Posts a notification to the NotificationCenter.
		/// The NotificationCenter then delivers the notification
		/// to all interested observers.
		/// If an observer throws an exception, dispatching terminates
		/// and the exception is rethrown to the caller.
		/// Ownership of the notification object is claimed and the
		/// notification is released before returning. Therefore,
		/// a call like
		///    notificationCenter.postNotification(new MyNotification);
		/// does not result in a memory leak.

	bool hasObservers() const;
		/// Returns true iff there is at least one registered observer.
		///
		/// Can be used to improve performance if an expensive notification
		/// shall only be created and posted if there are any observers.

	std::size_t countObservers() const;
		/// Returns the number of registered observers.

	static NotificationCenter& defaultCenter();
		/// Returns a reference to the default
		/// NotificationCenter.

private:
	typedef SharedPtr<AbstractObserver> AbstractObserverPtr;
	typedef std::vector<AbstractObserverPtr> ObserverList;

	ObserverList  _observers;
	mutable Mutex _mutex;
};