Observer Pattern

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Keywords

Observable

Is the interface or abstract class which defines the operations for attaching and de-attaching observers to the client.

Concerete Observable

Maintains the state of the object and when a change occurs it notifies the attached Obersvers

Observer

Is the interface of abstract class defining the operation to be used to modify the obkect

Concrete Observer(s)

Implementations of Observers

Weather Station Example

Observer Header (Observer.h)

The Observer is the parent class of the concrete observers (cell phone clients in our case), it contains

#ifndef OBSERVER_PATTERN
#define OBSERVER_PATTERN

/**
 * Interface for the Observer
 */
class Observer {

public:

    virtual void update(float temp, float humidity, float pressure, float wind) = 0; // Virtual Function

};

#endif 

Subject Header (Subject.h)

#ifndef OBSERVER_PATTERN_SUBJECT_H
#define OBSERVER_PATTERN_SUBJECT_H


#include "Observer.h"

/**
 * Interface for the Subject
 */
class Subject {

public:

    virtual void registerObserver(Observer *observer) = 0;

    virtual void removeObserver(Observer *observer) = 0;

    virtual void notifyObservers() = 0;

};

#endif 

Weather Data Header (WeatherData.h)

#ifndef OBSERVER_PATTERN_WEATHERDATA_H
#define OBSERVER_PATTERN_WEATHERDATA_H

#include <vector>
#include <algorithm>
#include <iostream>

#include "Subject.h"
#include "Observer.h"

/**
 * A concrete implementation of the Subject interface
 */
class WeatherData : public Subject {

private:
    std::vector<Observer *> observers; // observers need a list of them so we can update

    float temp = 0.0f;
    float humidity = 0.0f;
    float pressure = 0.0f;
    float wind = 0.0f;

public:

    void registerObserver(Observer *observer) override;

    void removeObserver(Observer *observer) override;

    void notifyObservers() override;

    void setState(float temp, float humidity, float pressure, float wind);

};

#endif

Weather Data (WeatherData.cpp)

#include "WeatherData.h"

void WeatherData::registerObserver(Observer *observer) {
    // Adds new observer to vector
    observers.push_back(observer);
}

void WeatherData::removeObserver(Observer *observer) {
    // find the observer and remove it 
    auto iterator = std::find(observers.begin(), observers.end(), observer);

    if (iterator != observers.end()) { // observer found
        observers.erase(iterator); // remove the observer
    }
}

void WeatherData::notifyObservers() {
    for (Observer *observer : observers) { // notify all observers
        observer->update(temp, humidity, pressure, wind);
    }
}

void WeatherData::setState(float temp, float humidity, float pressure, float wind) {
    this->temp = temp;
    this->humidity = humidity;
    this->pressure = pressure;
    this->wind = wind;
    std::cout << std::endl;
    notifyObservers();
}

Client Header (Client.h)

#ifndef OBSERVER_PATTERN_CLIENT_1_H
#define OBSERVER_PATTERN_CLIENT_1_H

#include <iostream>

#include "Observer.h"

/**
 * a client that implements the Observer interface
 */
class Client : public Observer {

private:

    int id; // Label the Identification number of the Client

public:

    Client(int id);

    virtual void update(float temp, float humidity, float pressure, float wind) override;

};

#endif

Client (Client.cpp)

#include "Client.h"

void Client::update(float temp, float humidity, float pressure, float {
    // print the changed values
    std::cout << "---Client (" << id << ") Data---\tTemperature: " << temp
              << "\tHumidity: " << humidity
              << "\tPressure: " << pressure
              << "\tWind Speed (mph): " << wind 
              << std::endl;
}

Client::Client(int id) {
    // Set Client id
    this->id = id;
}

Main File (main.cpp)

#include <iostream>
#include "WeatherData.h"
#include "Client.h"


int main() {
    WeatherData weatherStation;
    Client one(1), two(2), three(3), four(4), five(5);

    float temp, humidity, pressure;

    weatherStation.registerObserver(&one);
    weatherStation.registerObserver(&two);
    weatherStation.registerObserver(&three);
    weatherStation.registerObserver(&four);
    weatherStation.registerObserver(&five);


    std::cout << "Enter Temperature, Humidity, Pressure (seperated by spaces) << ";
    std::cin >> temp >> humidity >> pressure;

    weatherStation.setState(temp,humidity,pressure);

    weatherStation.removeObserver(&two); // Remove from 

    std::cout << "\n\nEnter Temperature, Humidity, Pressure (seperated by spaces) << ";
    std::cin >> temp >> humidity >> pressure;

    weatherStation.setState(temp,humidity,pressure);

}

Output:

Enter Temperature, Humidity, Pressure (seperated by spaces) << 90 98.3 45

---Client (1) Data---   Temperature: 90 Humidity: 98.3  Pressure: 45
---Client (2) Data---   Temperature: 90 Humidity: 98.3  Pressure: 45
---Client (3) Data---   Temperature: 90 Humidity: 98.3  Pressure: 45
---Client (4) Data---   Temperature: 90 Humidity: 98.3  Pressure: 45
---Client (5) Data---   Temperature: 90 Humidity: 98.3  Pressure: 45


Enter Temperature, Humidity, Pressure (seperated by spaces) << 92 94.4 67

---Client (1) Data---   Temperature: 92 Humidity: 94.4  Pressure: 67
---Client (3) Data---   Temperature: 92 Humidity: 94.4  Pressure: 67
---Client (4) Data---   Temperature: 92 Humidity: 94.4  Pressure: 67