Macros for Observers

Posted on Nov 2, 2009

Here are two little macros too make implementing the Observer design pattern less repetitive. This is C++

File: observermacros.h




#ifndef DDM_OBSERVERMACRO_H
#define DDM_OBSERVERMACRO_H

#define MAKE_OBSERVABLE( obscls ) \
    private:\
        std::list< obscls *> m_observers;\
    public:\
        void addObserver( obscls *obs ) {\
            m_observers.push_back( obs );\
        }\
        void delObserver( obscls *obs ) {\
            m_observers.remove( obs );\
        }

#define NOTIFY( obscls, obsmethod, argument ) \
        for( std::list::const_iterator it = m_observers.begin();\
                it != m_observers.end(); it++ )\
            (*it)->obsmethod( argument )
    
#endif

Now if you want a class to be observable do



#include "observermacros.h"

class NeedsObserving
{
    MAKE_OBSERVABLE( Observes );
};

Observes is the name of the class which will observe NeedsObserving.



// add/del observers
NeedsObserving no;
Observes o = new Observes;
no.addObserver( o );
no.delObserver( o );

// To notify observers of changes

void NeedsObserving::didSomething()
{
    // CoolThing *coolThing is modified/used here
    // ...
    NOTIFY( Observes, coolThingDone, coolThing );
    // Calls Observes::coolThingDone( CoolThing * );
}

The code is pretty naive, but it might help. Cheers