Question

I’m designing an observer pattern which should work this way: observer calls AddEventListener method of EventDispatcher and passes a string which is the name of the event, PointerToItself and a PointerToItsMemberMethod

After that event happens inside of the EventDispatcher; it looks through the list of subscriptions and if there are some, assigned to this event calls the action method of the observer.

I’ve come to this EventDispatcher.h. CAUTION contains bit of pseudo-code.

The are two questions:

1. How do I define the type of action in struct Subscription?
2. Am I moving the right way?

PS: No, I’m not gonna use boost or any other libraries .

#pragma once

#include <vector>
#include <string>

using namespace std;

struct Subscription
{
        void*                   observer;
        string                  event;
        /*  u_u  */             action;
};

class EventDispatcher
{
    private:
        vector<Subscription>    subscriptions;

    protected:
        void                    DispatchEvent ( string event );

    public:
        void                    AddEventListener ( Observer* observer , string event , /*  u_u  */ action );
        void                    RemoveEventListener ( Observer* observer , string event , /*  u_u  */ action );
};

This header implements like this in EventDispatcher.cpp

#include "EventDispatcher.h"

void    EventDispatcher::DispatchEvent ( string event )
{
    int key = 0;
    while ( key < this->subscriptions.size() )
    {
        Subscription subscription = this->subscriptions[key];
        if ( subscription.event == event )
        {
            subscription.observer->subscription.action;
        };
    };
};

void    EventDispatcher::AddEventListener ( Observer* observer , string event , /* */ action )
{
    Subscription subscription = { observer , event , action );
    this->subscriptions.push_back ( subscription );
};

void    EventDispatcher::RemoveEventListener ( Observer* observer , string event , /* */ action )
{
    int key = 0;
    while ( key < this->subscriptions.size() )
    {
        Subscription subscription = this->subscriptions[key];
        if ( subscription.observer == observer && subscription.event == event && subscription.action == action )
        {
            this->subscriptions.erase ( this->subscriptions.begin() + key );
        };
    };
};

Answer 1

Perhaps you should just create a class to be derived by “users”:

class Action {
   public:
      friend class EventDispatcher;

      virtual SomeResultType DoThis() = 0;

   private:
      /* Some common data */
};

Just pass some derived-from-class-Action typed variable to AddEventListener. When the corresponding event is triggered, just fill in the common data and call the DoThis() method.

void EventDispatcher::DispatchEvent ( string event )
{
    int key = 0;
    while ( key < this->subscriptions.size() )
    {
        Subscription subscription = this->subscriptions[key];
        if ( subscription.event == event )
        {
            subscription->action();
        };
    };
};

For AddEventListener:

void EventDispatcher::AddEventListener ( Observer* observer , string event , Action* action )
{
    Subscription subscription = { observer , event , action );
    this->subscriptions.push_back ( subscription );
};

An example of a Action derived class:

class myAction: public Action {
   public:
      // Implement the DoThis() method
      void SomeResultType DoThis() {
          cout << "Hello World!";
          return SomeValue;
      }
};

// To use the action,
myAction* act = new myAction;
myEventDispatcher.AddEventListener(someObserver, "HelloWorld", act);

This is one of the safest way to implement actions (and callbacks).