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Observing the world - how to build a reuseable implementation of a design pattern

, 11 Jun 2001
An article about the techniques to pour reusable design (a design pattern) into reusable code with an example of the observer pattern.
#ifndef __WEATHER_FORECAST_H__
#define __WEATHER_FORECAST_H__


#include "tool_observer.h"


////////////////////////////////////////////
// class Strategy
//
// Strategy is a very simple subject. 
//

class Strategy
{
public:
	// Define the type of our "subject". This defines also the interface of 
	// the observers.
	typedef tool::SimpleSubject<> StrategySource;

	// Export part of the "subjects" interface using delegation 
	bool AttachObserver( StrategySource::IObserver* pObserver )
	{
		return m_StrategySource.AttachObserver( pObserver );
	}
	bool DetachObserver( StrategySource::IObserver* pObserver )
	{
		return m_StrategySource.DetachObserver( pObserver );
	}

	void StrategyChanged()
	{
		std::cout << "Strategy::StrategyChanged(), notifying observers >>>" << std::endl;
		// Notify all observers
		m_StrategySource.NotifyObserver();
		std::cout << "<<< Strategy::StrategyChanged(), finished notifying observers" << std::endl;
	}

private:
	
	// Our subject implementation
	StrategySource m_StrategySource;
};


////////////////////////////////////////////
// class TempSens
//
// TempSens is a more sophisticated subject. It implements two subjects for different
// parts of the temperature sensor component: 
//   1: A subject for physical state events, like power failure (TechSource)
//   2: A subject for temperature change events (TempSource)
//
class TempSens
{
public:
	
	// The TechSource subject passes some event type information to the observers.
	// For this we adopt the ARG1 template argument of our SimpleSubject to techevent_t
	// Note that we don't need the OBSERVER_INFO and RETURN template arguments at all.
	// However, because you must pass a type for them, we simply use int.
	enum techevent_t { ePowerFailure, eOtherFailure };
	typedef tool::SimpleSubject< int, int, techevent_t > TechSource;
	
	// The TempSource subject is a more sophisticated subject that uses the OBSERVER_INFO
	// stored with each observer. We use this to implement a threshold value. An observer is
	// only notified if the new temperature value is equal or above the threshold. To implement
	// this we need to override SimpleSubject::NotifyObserver. However, this is not very much work:
	struct TempSource : public tool::SimpleSubject< double, int, double >
	{
		virtual void NotifyObserver( double fNewTemp )
		{
			for( clients_t::iterator i = m_aObservers.begin(); i != m_aObservers.end(); ++i )
				// Call Update only if new Temperature is equal or above the stored threshold
				if( i->Info <= fNewTemp )
					i->pObserver->Update( fNewTemp );
		}
	};

	bool AttachTechObserver( TechSource::IObserver* pObserver )
	{
		return m_TechSource.AttachObserver( pObserver );
	}
	bool DetachTechObserver( TechSource::IObserver* pObserver )
	{
		return m_TechSource.DetachObserver( pObserver );
	}
	bool AttachTempObserver( TempSource::IObserver* pObserver, double fThreshold )
	{
		return m_TempSource.AttachObserver( pObserver, fThreshold );
	}
	bool DetachTempObserver( TempSource::IObserver* pObserver )
	{
		return m_TempSource.DetachObserver( pObserver );
	}

	void SetTemp( double fTemp )
	{
		std::cout << "TempSens::SetTemp(), notifying observers >>>" << std::endl;
		// Notify all observer
		m_TempSource.NotifyObserver( fTemp );
		std::cout << "<<< TempSens::SetTemp(), finished notifying observers" << std::endl;
	}

	void TechEvent( techevent_t event )
	{
		std::cout << "TempSens::TechEvent(), notifying observers >>>" << std::endl;
		// Notify all observer
		m_TechSource.NotifyObserver( event );
		std::cout << "<<< TempSens::TechEvent(), finished notifying observers" << std::endl;
	}

private:
	TechSource	m_TechSource;
	TempSource	m_TempSource;

};

class Technician
{
public:
	Technician()
		: m_pTempSens( NULL )
	{}
	~Technician()
	{
		// Remove ourself from the list of observers
		if( m_pTempSens )
			m_pTempSens->DetachTechObserver( &m_Adapter );
	}

	void SetTempSensor( TempSens* pTempSens )
	{
		// Add ourself as observer to the passed sensor. To be an observer
		// of the passed sensor we have to implement the sensors IObserver interface.
		// However, we use an adapter that implements the interface and delegates calls
		// to our member fn SomethingHappened

		if( m_pTempSens )
			// Remove old observing
			m_pTempSens->DetachTechObserver( &m_Adapter );
		
		m_pTempSens = pTempSens;

		if( m_pTempSens ) {
			m_Adapter.SetUpdate( this, &Technician::SomethingHappened );
			m_pTempSens->AttachTechObserver( &m_Adapter );
		}
	}

private:

	// This is the member function used as callback
	int SomethingHappened( TempSens::techevent_t what )
	{
		std::cout << "Technician::SomethingHappened() :";
		switch( what )
		{
			case TempSens::ePowerFailure:
				std::cout << "Power Failure reported" << std::endl;
				break;
			case TempSens::eOtherFailure:
				std::cout << "Generic Failure reported" << std::endl;
				break;
			default:
				std::cout << "UNKNOWN EVENT!" << std::endl;
				break;
		}

		return 0;	// Return value is ignored
	}

	tool::ObserverAdapter< Technician, TempSens::TechSource > m_Adapter;
	TempSens* m_pTempSens;
};

class HurrDet
{
public:
	HurrDet()
		: m_pTempSens( NULL ), m_pStrategy( NULL )
	{}
	~HurrDet()
	{
		if( m_pTempSens )
			m_pTempSens->DetachTempObserver( &m_TempAdapter );
		if( m_pStrategy )
			m_pStrategy->DetachObserver( &m_StrategyAdapter );
	}
	
	void SetTempSensor( TempSens* pTempSens, double  fThreshold)
	{
		if( m_pTempSens )
			// Remove old observing
			m_pTempSens->DetachTempObserver( &m_TempAdapter );
		
		m_pTempSens = pTempSens;

		if( m_pTempSens ) {
			m_TempAdapter.SetUpdate( this, &HurrDet::OnNewTemp );
			m_pTempSens->AttachTempObserver( &m_TempAdapter, fThreshold );
		}
	}

	void SetStrategy( Strategy* pStrategy )
	{

		if( m_pStrategy )
			// Remove old observing
			m_pStrategy->DetachObserver( &m_StrategyAdapter );
		
		m_pStrategy = pStrategy;

		if( m_pStrategy ) {
			m_StrategyAdapter.SetUpdate( this, &HurrDet::OnNewStrategy );
			m_pStrategy->AttachObserver( &m_StrategyAdapter );
		}
	}

private:

	// Member function used as callback for "temperature changed" events.
	int OnNewTemp( double fNewTemp )
	{
		std::cout << "HurrDet::OnNewTemp( " << fNewTemp << " )" << std::endl;
		return 0;	// Return value is ignored
	}

	// Member function used as callback for "strategy changed" events. Note that
	// Strategy does not use the single parameter that can be passed to the observers.
	// However, because we pass the arguments type as template parameter to ISubject 
	// (and therefore to the derived SimpleSubject class) there must be an argument in
	// the notify fn signature. But we can ignore the value.
	int OnNewStrategy( int )
	{
		// Parameter is ignored
		std::cout << "HurrDet::OnNewStrategy()" << std::endl;
		return 0;	// Return value is ignored
	}

	TempSens* m_pTempSens;
	Strategy* m_pStrategy;
	tool::ObserverAdapter< HurrDet, TempSens::TempSource >		m_TempAdapter;
	tool::ObserverAdapter< HurrDet, Strategy::StrategySource >	m_StrategyAdapter;
};

#endif // __WEATHER_FORECAST_H__

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About the Author

Daniel Lohmann

Germany Germany
Daniel Lohmann (daniel@losoft.de) is Assistant Professor at the Distributed Systems and Operating Systems department at Friedrich-Alexander-University Erlangen-Nuremberg, Germany. His main research topic is the design of a highly customizable and scalable operating system product line for deeply embedded systems using static configuration and aspect-oriented techniques. Before joining Universität Erlangen he worked as a freelance trainer and consultant for NT system programming, advanced C++ programming and OOA/OOD. He is interested in upcoming programming techniques like aspect-oriented programming, generative programming and C++ meta coding and has written some nice and handy tools for Windows NT which you can download at his web site.

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