Answer / subeetha

The observer pattern (a subset of the asynchronous
publish/subscribe pattern) is a software design pattern in
which an object, called the subject, maintains a list of
its dependents, called observers, and notifies observers
automatically of any state changes, usually by calling one
of their methods. It is mainly used to implement
distributed event handling systems.


Sample Program:
#include <iostream>
#include <vector>
using namespace std;

class Subject {
// 1. "independent" functionality
vector < class Observer * > views; // 3. Coupled only
to "interface"
int value;
public:
void attach(Observer *obs) {
views.push_back(obs);
}
void setVal(int val) {
value = val;
notify();
}
int getVal() {
return value;
}
void notify();
};

class Observer {
// 2. "dependent" functionality
Subject *model;
int denom;
public:
Observer(Subject *mod, int div) {
model = mod;
denom = div;
// 4. Observers register themselves with the Subject
model->attach(this);
}
virtual void update() = 0;
protected:
Subject *getSubject() {
return model;
}
int getDivisor() {
return denom;
}
};

void Subject::notify() {
// 5. Publisher broadcasts
for (int i = 0; i < views.size(); i++)
views[i]->update();
}

class DivObserver: public Observer {
public:
DivObserver(Subject *mod, int div): Observer(mod, div){}
void update() {
// 6. "Pull" information of interest
int v = getSubject()->getVal(), d = getDivisor();
cout << v << " div " << d << " is " << v / d
<< '\n';
}
};

class ModObserver: public Observer {
public:
ModObserver(Subject *mod, int div): Observer(mod, div){}
void update() {
int v = getSubject()->getVal(), d = getDivisor();
cout << v << " mod " << d << " is " << v % d
<< '\n';
}
};

int main() {
Subject subj;
DivObserver divObs1(&subj, 4); // 7. Client configures
the number and
DivObserver divObs2(&subj, 3); // type of Observers
ModObserver modObs3(&subj, 3);
subj.setVal(14);
}
14 div 4 is 3
14 div 3 is 4
14 mod 3 is 2

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