poco/Foundation/include/Poco/AbstractEvent.h

//
// AbstractEvent.h
//
// $Id: //poco/svn/Foundation/include/Poco/AbstractEvent.h#2 $
//
// Library: Foundation
// Package: Events
// Module:  AbstractEvent
//
// Definition of the AbstractEvent class.
//
// Copyright (c) 2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
// this license (the "Software") to use, reproduce, display, distribute,
// execute, and transmit the Software, and to prepare derivative works of the
// Software, and to permit third-parties to whom the Software is furnished to
// do so, all subject to the following:
// 
// The copyright notices in the Software and this entire statement, including
// the above license grant, this restriction and the following disclaimer,
// must be included in all copies of the Software, in whole or in part, and
// all derivative works of the Software, unless such copies or derivative
// works are solely in the form of machine-executable object code generated by
// a source language processor.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
// SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
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// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.
//


#ifndef  Foundation_AbstractFoundation_INCLUDED
#define  Foundation_AbstractFoundation_INCLUDED


#include "Poco/Foundation.h"
#include "Poco/SingletonHolder.h"
#include "Poco/SharedPtr.h"
#include "Poco/ActiveResult.h"
#include "Poco/ActiveMethod.h"
#include "Poco/Mutex.h"


namespace Poco {


template <class TArgs, class TStrategy, class TDelegate, class TMutex = FastMutex> 
class AbstractEvent
	/// An AbstractEvent is the super-class of all events. 
	/// It works similar to the way C# handles notifications (aka events in C#).
	/// Events can be used to send information to a set of observers
	/// which are registered at the event. The type of the data is specified with
	/// the template parameter TArgs. The TStrategy parameter must be a subclass 
	/// of NotificationStrategy. The parameter TDelegate can either be a subclass of AbstractDelegate
	/// or of PriorityAbstractDelegate. 
	///
	/// Note that AbstractEvent should never be used directly. One ought to use
	/// one of its subclasses which set the TStrategy and TDelegate template parameters
	/// to fixed values. For most use-cases the BasicEvent template will be sufficient:
	///     #include "Poco/BasicEvent.h"
	///     #include "Poco/Delegate.h"
	///
	/// If one requires delegates to be called in the order they registered, use FIFOEvent:
	///     #include "Poco/FIFOEvent.h"
	///     #include "Poco/Delegate.h"
	///
	/// Both FIFOEvent and BasicEvent work with a standard delegate. They allow one object to register
	/// exactly one delegate at an event. In contrast, a PriorityDelegate comes with an attached priority value
	/// and allows one object to register for one priority value one delegate. Note that PriorityDelegates
	/// only work with PriorityEvents:
	///     #include "Poco/PriorityEvent.h"
	///     #include "Poco/PriorityDelegate.h"
	///
	/// Use events by adding them as public members to the object which is throwing notifications:
	///
	///     class MyData
	///     {
	///     public:
	///         Poco::BasicEvent<int> AgeChanged;
	///         
	///         MyData();
	///         ...
	///     };
	///
	/// Throwing the event can be done either by the events notify() or notifyAsync() method:
	///
	///
	/// Alternatively, instead of notify(), operator () can be used.
	///
	///     void MyData::setAge(int i)
	///     {
	///         this->_age = i;
	///         AgeChanged(this, this->_age);
	///     }
	///
	/// Note that notify and notifyAsync do not catch exceptions, i.e. in case a delegate 
	/// throws an exception, the notify is immediately aborted and the exception is thrown
	/// back to the caller.
	///
	/// Delegates can register methods at the event. In the case of a BasicEvent or FIFOEvent
	/// the Delegate template is used, in case of an PriorityEvent a PriorityDelegate is used.
	/// Mixing of observers, e.g. using a PriorityDelegate with a BasicEvent is not possible and 
	/// checked for during compile time.
	/// Events require the observers to follow one of the following method signature:
	///
	///     void onEvent(const void* pSender, TArgs& args);
	///     void onEvent(TArgs& args);
	///     static void onEvent(const void* pSender, TArgs& args);
	///     static void onEvent(void* pSender, TArgs& args);
	///     static void onEvent(TArgs& args);
	///
	/// For performance reasons arguments are always sent by reference. This also allows observers
	/// to modify the sent argument. To prevent that, use <const TArg> as template
	/// parameter. A non-conformant method signature leads to compile errors.
	///
	/// Assuming that the observer meets the method signature requirement, it can register
	/// this method with the += operator:
	///
	///     class MyController
	///     {
	///     protected:
	///         MyData _data;
	///         
	///         void onDataChanged(void* pSender, int& data);
	///         ...
	///     };
	///         
	///     MyController::MyController()
	///     {
	///         _data.AgeChanged += delegate(this, &MyController::onDataChanged);
	///     }
	///
	/// In some cases it might be desirable to work with automatically expiring registrations. Simply add
	/// to delegate as 3rd parameter a expireValue (in milliseconds):
	///
	///     _data.DataChanged += delegate(this, &MyController::onDataChanged, 1000);
	///
	/// This will add a delegate to the event which will automatically be removed in 1000 millisecs.
	///
	/// Unregistering happens via the -= operator. Forgetting to unregister a method will lead to
	/// segmentation faults later, when one tries to send a notify to a no longer existing object.
	///
	///     MyController::~MyController()
	///     {
	///         _data.DataChanged -= delegate(this, &MyController::onDataChanged);
	///     }
	///
	/// Working with PriorityDelegates as similar to working with BasicEvent/FIFOEvent.Instead of ''delegate''
	/// simply use ''priorityDelegate''.
	///
	/// For further examples refer to the event testsuites.
{
public:
	AbstractEvent(): 
		_executeAsync(this, &AbstractEvent::executeAsyncImpl),
		_enabled(true)
	{
	}

	AbstractEvent(const TStrategy& strat): 
		_executeAsync(this, &AbstractEvent::executeAsyncImpl),
		_strategy(strat),
		_enabled(true)
	{	
	}

	virtual ~AbstractEvent()
	{
	}

	void operator += (const TDelegate& aDelegate)
		/// Adds a delegate to the event. If the observer is equal to an
		/// already existing one (determined by the < operator),
		/// it will simply replace the existing observer.
		/// This behavior is determined by the TStrategy. Current implementations
		/// (DefaultStrategy, FIFOStrategy) follow that guideline but future ones
		/// can deviate.
	{
		typename TMutex::ScopedLock lock(_mutex);
		_strategy.add(aDelegate);
	}
	
	void operator -= (const TDelegate& aDelegate)
		/// Removes a delegate from the event. If the delegate is equal to an
		/// already existing one is determined by the < operator.
		/// If the observer is not found, the unregister will be ignored
	{
		typename TMutex::ScopedLock lock(_mutex);
		_strategy.remove(aDelegate);
	}
	
	void operator () (const void* pSender, TArgs& args)
	{
		notify(pSender, args);
	}

	void notify(const void* pSender, TArgs& args)
		/// Sends a notification to all registered delegates. The order is 
		/// determined by the TStrategy. This method is blocking. While executing,
		/// other objects can change the list of delegates. These changes don't
		/// influence the current active notifications but are activated with
		/// the next notify. If one of the delegates throws an exception, the notify
		/// method is immediately aborted and the exception is reported to the caller.
	{
		SharedPtr<TStrategy> ptrStrat;
		bool enabled = false;
		
		{
			typename TMutex::ScopedLock lock(_mutex);
			enabled = _enabled;
			if (_enabled)
			{
				// thread-safeness: 
				// copy should be faster and safer than blocking until
				// execution ends
				ptrStrat = new TStrategy(_strategy);
			}
		}

		if (enabled)
		{
			ptrStrat->notify(pSender, args);
		}
	}

	ActiveResult<TArgs> notifyAsync(const void* pSender, const TArgs& args)
		/// Sends a notification to all registered delegates. The order is 
		/// determined by the TStrategy. This method is not blocking and will
		/// immediately return. The delegates are invoked in a seperate thread.
		/// Call activeResult.wait() to wait until the notification has ended.
		/// While executing, other objects can change the delegate list. These changes don't
		/// influence the current active notifications but are activated with
		/// the next notify. If one of the delegates throws an exception, the execution
		/// is aborted and the exception is reported to the caller.
	{
		NotifyAsyncParams params(pSender, args);

		{
			typename TMutex::ScopedLock lock(_mutex);

			// thread-safeness: 
			// copy should be faster and safer than blocking until
			// execution ends
			// make a copy of the strategy here to guarantee that
			// between notifyAsync and the execution of the method no changes can occur
				
			params.ptrStrat = SharedPtr<TStrategy>(new TStrategy(_strategy));
			params.enabled  = _enabled;
		}

		ActiveResult<TArgs> result = _executeAsync(params);
		return result;
	}
	
	void enable()
		/// Enables the event
	{
		typename TMutex::ScopedLock lock(_mutex);
		_enabled = true;
	}

	void disable()
		/// Disables the event. notify and notifyAsnyc will be ignored,
		/// but adding/removing delegates is still allowed.
	{
		typename TMutex::ScopedLock lock(_mutex);
		_enabled = false;
	}

	bool isEnabled() const
	{
		typename TMutex::ScopedLock lock(_mutex);
		return _enabled;
	}

	void clear()
		/// Removes all delegates.
	{
		typename TMutex::ScopedLock lock(_mutex);
		_strategy.clear();
	}
	
	bool empty() const
		/// Checks if any delegates are registered at the delegate
	{
		typename TMutex::ScopedLock lock(_mutex);
		return _strategy.empty();
	}

protected:
	struct NotifyAsyncParams
	{
		SharedPtr<TStrategy> ptrStrat;
		const void* pSender;
		TArgs       args;
		bool        enabled;
		
		NotifyAsyncParams(const void* pSend, const TArgs& a):ptrStrat(), pSender(pSend), args(a), enabled(true)
			/// default constructor reduces the need for TArgs to have an empty constructor, only copy constructor is needed.
		{
		}
	};

	ActiveMethod<TArgs, NotifyAsyncParams, AbstractEvent> _executeAsync;

	TArgs executeAsyncImpl(const NotifyAsyncParams& par)
	{
		if (!par.enabled)
		{
			return par.args;
		}

		NotifyAsyncParams params = par;
		TArgs retArgs(params.args);
		params.ptrStrat->notify(params.pSender, retArgs);
		return retArgs;
	}

	TStrategy _strategy; /// The strategy used to notify observers.
	bool      _enabled;  /// Stores if an event is enabled. Notfies on disabled events have no effect
	                     /// but it is possible to change the observers.
	mutable TMutex _mutex;

private:
	AbstractEvent(const AbstractEvent& other);
	AbstractEvent& operator = (const AbstractEvent& other);
};


} // namespace Poco


#endif