Listeners are classes dedicated to specific objects events handling. For example, a form class definition could be associated to a form listener class in order to give developpers and users the ability to intercept and manage special events such as maximization, close requests and many others. There is no message queue handling in those objects, the main purpose is just a user friendly front end API presentation.

    Listeners are not static functions, they are classes definitions and so inherits from the c++ polymorphic mechanism. The listeners functions will generally be declared as virtual ones to give the developpers the ability to handle all or part of the objects events with their own overloaded definitions.

    This concept is introduced in generics with two simple classes : CObject and CObjectListener. Both derive the CMetaModule definition in order to give developpers the possibility of defining such objects in dynamic linked modules and to deal with the CMetaModuleImporter API. Because they derive the CMetaModule definition, they inherits the serialization process too.

class CObjectListener : public CMetaModule {     // instanciation section     public :         CObjectListener                      ();         virtual ~CObjectListener             () =0;     // listener section     public :         // called when CObject constructor is executed on inSender (except when CObject is instanciated from serialization process         // because in such cases the cobject would already be allocated when the listener would be attached)         virtual void                         OnConstruct        (CObject *inSender)    { }         // called when CObject destructor is executed on inSender, don't forget that only CObject's memory stack is available at         //this point !         virtual void                         OnDestruct          (CObject *inSender)    { }     // private section     private :         // listener callers         CObjects                             m_Owners;         // friend object class         friend class                         CObject;         // generic metaclass association         SECTION_GENERIC_METACLASS; }; // metaclass and class tag declarations DECLARE_GENERIC_METACLASS ('_obj', CObjectListener, CMetaModule); class CObject : public CMetaModule {     // instanciation section     public :         CObject                            (const CObjectListener *inListener =NULL);         virtual ~CObject                   () =0;     // protected definition     protected :         // object expected listener type         virtual CMetaClass *               ListenerMustBe          () const;     // specific functions     public :         // listener affectation, reader and removal (do not force class type caller on AssignListener : to increase performance,         // types may be checked only while assignation in overwritten functions, not before calling specific listener functions;         // if you force, you could generate segmentation faults not on this call but later on the listener exploitation)         Bool                               AssignListener          (const CObjectListener *inListener);         CObjectListener *                  RemoveListener          ();         CObjectListener *                  GetListener             () const;     // serialization redefinition     public :         // object serialization, stores or instanciates associated listener if any         virtual void                       Serialize               (CXMLElementNode *&ioXMLElementNode, const int inMode);     // protected attributes     protected :                 // the object listener         CObjectListener *                  m_Listener;         // friend object listener class         friend class                       CObjectListener;         // generic metaclass specifications         SECTION_GENERIC_METACLASS; }; // generic metaclass and class tag declarations DECLARE_GENERIC_METACLASS ('objt', CObject, CMetaModule);