ClickHouse/base/poco/Foundation/include/Poco/NotificationCenter.h

//
// NotificationCenter.h
//
// Library: Foundation
// Package: Notifications
// Module:  NotificationCenter
//
// Definition of the NotificationCenter class.
//
// Copyright (c) 2004-2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// SPDX-License-Identifier:	BSL-1.0
//


#ifndef Foundation_NotificationCenter_INCLUDED
#define Foundation_NotificationCenter_INCLUDED


#include <cstddef>
#include <vector>
#include "Poco/Foundation.h"
#include "Poco/Mutex.h"
#include "Poco/Notification.h"
#include "Poco/SharedPtr.h"


namespace Poco
{


class AbstractObserver;


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;
};


} // namespace Poco


#endif // Foundation_NotificationCenter_INCLUDED