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Source code with real example in c++
/*
* File:observer.cpp
* This file is described the Observer Design Pattern with help of example
* This Example will explain How observer pattern works
* How to register, deresigter and notify to registered client
* Author: Aplha
* Date: 27 Dec 2020
*/
///////////////////////////////////Header files////////////////////////////////////
#include <iostream>
#include <list>
#include <thread>
#include <chrono>
class Observer; // Forward Decleration
class Subject
{
int _m_time;
// List to observers, who will be notified when state change
std::list<Observer*> _m_list;
public:
Subject();
~Subject();
void Attach(Observer*);
void Detach(Observer*);
void SetState(int);
int GetState();
private:
Subject(const Subject &other) = delete; // Disallow copying
void operator=(const Subject &) = delete;// Disallow copying
void Notify();
};
class Observer
{
protected:
Subject* _m_sub;
public:
virtual void Update()=0;
};
//Constructor
Subject::Subject()
{
_m_time = 0;
}
//Destructor
Subject::~Subject()
{
_m_time = 0;
if(!_m_list.empty())
{
_m_list.clear();
}
}
//Attach Function is used for registeration and observer will use this fucntion
void Subject::Attach(Observer* ob)
{
_m_list.push_back(ob);
}
//Detach function is used for de-registeration
void Subject::Detach(Observer* ob)
{
_m_list.remove(ob);
}
//Notify Function is notify to registered observer when state change
void Subject::Notify()
{
std::list<Observer *>::iterator iterator = _m_list.begin();
while (iterator != _m_list.end()) {
(*iterator)->Update();
++iterator;
}
}
//Set state
void Subject::SetState(int t)
{
_m_time = t;
Notify();
}
int Subject::GetState()
{
return _m_time;
}
class AnalogObserver : public Observer
{
public:
AnalogObserver(Subject * sub)
{
_m_sub = sub;
_m_sub->Attach(this);
}
~AnalogObserver()
{ _m_sub->Detach(this);}
void Update()
{ PrintTime();}
void PrintTime()
{ std::cout<<"AnalogTimer got notification:"<<_m_sub->GetState()<<std::endl;}
};
class DigitalObserver : public Observer
{
public:
DigitalObserver(Subject * sub)
{
_m_sub = sub;
_m_sub->Attach(this);
}
~DigitalObserver()
{_m_sub->Detach(this);}
void Update()
{ PrintTime();}
void PrintTime()
{ std::cout<<"DigitalTimer got notification:"<<_m_sub->GetState()<<std::endl;}
};
int main()
{
std::cout<<"In Main"<<std::endl;
//Create the Subject
Subject sub;
//Create the Observers
AnalogObserver analogObj(&sub);
DigitalObserver digitalObj(&sub);
//Change the Subject, Observer should be notified
for(int i =1; i<5; i++)
{
sub.SetState(i);
std::this_thread::sleep_for(std::chrono::seconds(1));
}
return 0;
}
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