Observer Pattern

[d.bignotti]

Hello everyone,

i'm tryng to implement the Observer Pattern as described here (https://www.tutorialspoint.com/design_pattern/observer_pattern.htm) but i'm failing really hard.

This is what i coded

#include <Object.mqh>
#include "Subject.mqh"
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
class CObserver : public CObject
  {
protected:
   Subject           subject;

public:

                     CObserver();
                    ~CObserver();

   void             ObserverUpdate() {};
  };
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CObserver::CObserver()
  {
  }
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CObserver::~CObserver()
  {
  }
//+------------------------------------------------------------------+

#include "Observer.mqh"
#include <Arrays/List.mqh>
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
class CSubject
  {
private:

   CList            *Observer;
   int               State;

public:
                     CSubject();
                    ~CSubject();

   int GetState() {return State;}
   void SetState(int state) {State=state; NotifyAllObserver();}

   void              NotifyAllObserver();

  };
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CSubject::CSubject()
  {
   Observer=new CList();
  }
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CSubject::~CSubject()
  {
  }
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CSubject::NotifyAllObserver(void)
  {
   for(int i=0; i<Observer.Total(); i++)
     {
      //Observer.ObserverUpdate();
     }
  }
//+------------------------------------------------------------------+

I get this error:

1- 'Subject' - declaration without type, Observer.mqh, 16 4

2- 'ObserverUpdate' - member function not defined, Subject.mqh, 51 16

First error i really don't understand why, the second i think i need a way to declare a list of observer type, what is the solution here? 

Thanks

[William Roeder]

d.bignotti:
1- 'Subject' - declaration without type, Observer.mqh, 16 4

2- 'ObserverUpdate' - member function not defined, Subject.mqh, 51 16

1. protected:
      Subject           subject;

   You never define a Subject. You defined a
   

   class CSubject{

2. for(int i=0; i<Observer.Total(); i++){
      //Observer.ObserverUpdate();
     }

   • Observer is a CList and that doesn't have any such method called ObserverUpdate.
   • You store subject pointers in the list as CObject pointers (code TBD.) You have to fetch them, one by one, cast them back to a subject and then tell the subject about the update.
   • Why did you make it a pointer? You forgot to delete it in the destructor. Why not just include it in the class?

[nicholish en]

#include <Arrays\ArrayObj.mqh>
//+------------------------------------------------------------------+
class Observer; //forward declarations
class Subject;
//+------------------------------------------------------------------+
class ObserverList : public CArrayObj
{
   public: Observer* operator[](int i){return At(i);}
};
//+------------------------------------------------------------------+
class Observer : public CObject
{
protected:
   Subject    *m_subject;
public:
   virtual void update()=0;
};
//+------------------------------------------------------------------+
class OctalObserver : public Observer
{
public: 
   OctalObserver(Subject *subject)
   {
      m_subject = subject;
      m_subject.attach(&this);
   }   
   virtual void update() override
   {
     printf( "Octal String: %d",m_subject.getState()); 
   }
};

//+------------------------------------------------------------------+
class Subject : public CObject
{
   int            m_state;
   ObserverList   m_observers;
public:
   Subject(){}
   int getState() {
      return m_state;
   }

   void setState(int state) {
      m_state = state;
      notifyAllObservers();
   }

   void attach(Observer *observer){
      m_observers.Add(observer);                
   }

   void notifyAllObservers(){
      for (int i=0;i<m_observers.Total();i++) {
         m_observers[i].update();
      }
   }    
};
//+------------------------------------------------------------------+

[d.bignotti]

whroeder1:

...............

nicholishen:

...........

Thanks whroeder1 for the fantastic insight.

Thanks nicholishen for the entire solution, i know the logic behind it now.

[d.bignotti]

nicholishen:

I'm applying nicholsen solution and it works well, but i see there's new data collection generic called ArrayList or LinkedList in include/Generic folder.

I'm trying to use it instead of CArrayObject, but i can't understand how to access list element throught index only.

Can anyone help here?

   CArrayList<CCar*>car;

   CCar *yama=new CCar("yamaha");

   car.Add(new CCar("honda"));
   car.Add(new CCar("zama"));
   car.Add(new CCar("ferrari"));
   car.Add(new CCar("mercedes"));
   car.Add(new CCar("audi"));
   car.Add(new CCar("pegueot"));
   car.Add(yama);

   for(int i=0; i<car.Count(); i++)
     {
     //how to iterate throught all the list and access the CCar methods?
     }

class CCar
  {
private:
   string            nome;
public:
                     CCar(string questonome);
                    ~CCar();
                    string getNome() {return nome;}
  };
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CCar::CCar(string questonome)
  {
   nome=questonome;
  }
//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
CCar::~CCar()
  {
  }
//+------------------------------------------------------------------+

[William Roeder]

d.bignotti: I'm trying to use it instead of CArrayObject, but i can't understand how to access list element throught index only.

1. There is no such thing as CArrayObject

2. You look at the Arrays\ArrayObj.mqh

   class CArrayObj : public CArray{
   protected:
      CObject          *m_data[];           // data array

   It only stores pointers to CObject, therefor your class must derive from that.
   

3. There is:

      //--- method of access to thre array
      CObject          *At(const int index) const;
   

   Thus get the object pointer, cast it to your class and use it.
   

      CArrayObj car;
      car.Add(new CCar("honda"));
      :
   
      for(int i=0; i<car.Count(); i++)
        {
        CCar* t = (CCar*)car.at(i); 
        Print(t.getNome() );
        }

[d.bignotti]

whroeder1:

1. There is no such thing as CArrayObject

2. You look at the Arrays\ArrayObj.mqhIt only stores pointers to CObject, therefor your class must derive from that.
   

3. There is:Thus get the object pointer, cast it to your class and use it.
   

You are right, the correct name is CArrayObj, i'm using it already and i understand how it works, i see there's a new colelction called CArrayList  in include/Generic folder.

Do you have any experience with that? 

I can't find a way to access CArrayList object throught index like in CArrayObj.

[William Roeder]

What part of these methods are unclear?

   //--- methods for navigating
   int               IndexOf(CObject *node);
   CObject          *GetNodeAtIndex(int index);
   CObject          *GetFirstNode(void);
   CObject          *GetPrevNode(void);
   CObject          *GetCurrentNode(void);
   CObject          *GetNextNode(void);
   CObject          *GetLastNode(void);

[nicholish en]

whroeder1:

What part of these methods are unclear?

What part of CArrayList is unclear?

I can't find a way to access CArrayList object throught index like in CArrayObj.

See the docs at the link I posted above.

Also note: The Generic collection classes don't implement CObject so you won't be able to use the CObject virtual methods such as Compare for sorting, Load, Save, etc. I would recommend sticking with a collection that implements CObject such as CArrayObj, CList, or CDictionary.

[Amir Yacoby]

This implimentation of observer is useless without multi inheritance.

[d.bignotti]

Amir Yacoby:
This implimentation of observer is useless without multi inheritance.

I'v posted a new topic on this argument in this section of the forum 30 min before your comment.