observer design pattern

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Tip Anjali Sharma 1 GREPCC

Observer design pattern is useful when you are interested in the state of an object and want to get notified whenever there is any change.

In observer pattern, the object that watch on the state of another object are called Observer and the object that is being watched is called Subject.

public class MyTopic implements Subject {

	private List<Observer> observers;
	private String message;
	private boolean changed;
	private final Object MUTEX= new Object();
	
	public MyTopic(){
		this.observers=new ArrayList<>();
	}
	@Override
	public void register(Observer obj) {
		if(obj == null) throw new NullPointerException("Null Observer");
		synchronized (MUTEX) {
		if(!observers.contains(obj)) observers.add(obj);
		}
	}

	@Override
	public void unregister(Observer obj) {
		synchronized (MUTEX) {
		observers.remove(obj);
		}
	}

	@Override
	public void notifyObservers() {
		List<Observer> observersLocal = null;
		//synchronization is used to make sure any observer registered after message is received is not notified
		synchronized (MUTEX) {
			if (!changed)
				return;
			observersLocal = new ArrayList<>(this.observers);
			this.changed=false;
		}
		for (Observer obj : observersLocal) {
			obj.update();
		}

	}

	@Override
	public Object getUpdate(Observer obj) {
		return this.message;
	}
	
	//method to post message to the topic
	public void postMessage(String msg){
		System.out.println("Message Posted to Topic:"+msg);
		this.message=msg;
		this.changed=true;
		notifyObservers();
	}

}

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public class MyTopic implements Subject {

    private List<Observer> observers;

    private String message;

    private boolean changed;

    private final Object MUTEX= new Object();

    public MyTopic(){

        this.observers=new ArrayList<>();

    @Override

    public void register(Observer obj) {

        if(obj == null) throw new NullPointerException("Null Observer");

        synchronized (MUTEX) {

        if(!observers.contains(obj)) observers.add(obj);

    @Override

    public void unregister(Observer obj) {

        synchronized (MUTEX) {

        observers.remove(obj);

    @Override

    public void notifyObservers() {

        List<Observer> observersLocal = null;

        //synchronization is used to make sure any observer registered after message is received is not notified

        synchronized (MUTEX) {

            if (!changed)

                return;

            observersLocal = new ArrayList<>(this.observers);

            this.changed=false;

        for (Observer obj : observersLocal) {

            obj.update();

    @Override

    public Object getUpdate(Observer obj) {

        return this.message;

    //method to post message to the topic

    public void postMessage(String msg){

        System.out.println("Message Posted to Topic:"+msg);

        this.message=msg;

        this.changed=true;

        notifyObservers();

https://www.digitalocean.com/community/tutorials/observer-design-pattern-in-java

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Source: Grepper

Tags: java

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Contributed on Jan 23 2023

Anjali Sharma

0 Answers Avg Quality 2/10

observer pattern

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Tip R K 1 GREPCC

// Java program to demonstrate working of
// onserver pattern
import java.util.ArrayList;
import java.util.Iterator;

// Implemented by Cricket data to communicate
// with observers
interface Subject
{
	public void registerObserver(Observer o);
	public void unregisterObserver(Observer o);
	public void notifyObservers();
}

class CricketData implements Subject
{
	int runs;
	int wickets;
	float overs;
	ArrayList<Observer> observerList;

	public CricketData() {
		observerList = new ArrayList<Observer>();
	}

	@Override
	public void registerObserver(Observer o) {
		observerList.add(o);
	}

	@Override
	public void unregisterObserver(Observer o) {
		observerList.remove(observerList.indexOf(o));
	}

	@Override
	public void notifyObservers()
	{
		for (Iterator<Observer> it =
			observerList.iterator(); it.hasNext();)
		{
			Observer o = it.next();
			o.update(runs,wickets,overs);
		}
	}

	// get latest runs from stadium
	private int getLatestRuns()
	{
		// return 90 for simplicity
		return 90;
	}

	// get latest wickets from stadium
	private int getLatestWickets()
	{
		// return 2 for simplicity
		return 2;
	}

	// get latest overs from stadium
	private float getLatestOvers()
	{
		// return 90 for simplicity
		return (float)10.2;
	}

	// This method is used update displays
	// when data changes
	public void dataChanged()
	{
		//get latest data
		runs = getLatestRuns();
		wickets = getLatestWickets();
		overs = getLatestOvers();

		notifyObservers();
	}
}

// This interface is implemented by all those
// classes that are to be updated whenever there
// is an update from CricketData
interface Observer
{
	public void update(int runs, int wickets,
					float overs);
}

class AverageScoreDisplay implements Observer
{
	private float runRate;
	private int predictedScore;

	public void update(int runs, int wickets,
					float overs)
	{
		this.runRate =(float)runs/overs;
		this.predictedScore = (int)(this.runRate * 50);
		display();
	}

	public void display()
	{
		System.out.println("\nAverage Score Display: \n"
						+ "Run Rate: " + runRate +
						"\nPredictedScore: " +
						predictedScore);
	}
}

class CurrentScoreDisplay implements Observer
{
	private int runs, wickets;
	private float overs;

	public void update(int runs, int wickets,
					float overs)
	{
		this.runs = runs;
		this.wickets = wickets;
		this.overs = overs;
		display();
	}

	public void display()
	{
		System.out.println("\nCurrent Score Display:\n"
						+ "Runs: " + runs +
						"\nWickets:" + wickets +
						"\nOvers: " + overs );
	}
}

// Driver Class
class Main
{
	public static void main(String args[])
	{
		// create objects for testing
		AverageScoreDisplay averageScoreDisplay =
						new AverageScoreDisplay();
		CurrentScoreDisplay currentScoreDisplay =
						new CurrentScoreDisplay();

		// pass the displays to Cricket data
		CricketData cricketData = new CricketData();

		// register display elements
		cricketData.registerObserver(averageScoreDisplay);
		cricketData.registerObserver(currentScoreDisplay);

		// in real app you would have some logic to
		// call this function when data changes
		cricketData.dataChanged();

		//remove an observer
		cricketData.unregisterObserver(averageScoreDisplay);

		// now only currentScoreDisplay gets the
		// notification
		cricketData.dataChanged();
	}
}

xxxxxxxxxx

// Java program to demonstrate working of

// onserver pattern

import java.util.ArrayList;

import java.util.Iterator;

// Implemented by Cricket data to communicate

// with observers

interface Subject

    public void registerObserver(Observer o);

    public void unregisterObserver(Observer o);

    public void notifyObservers();

class CricketData implements Subject

    int runs;

    int wickets;

    float overs;

    ArrayList<Observer> observerList;

    public CricketData() {

        observerList = new ArrayList<Observer>();

    @Override

    public void registerObserver(Observer o) {

        observerList.add(o);

    @Override

    public void unregisterObserver(Observer o) {

        observerList.remove(observerList.indexOf(o));

    @Override

    public void notifyObservers()

        for (Iterator<Observer> it =

            observerList.iterator(); it.hasNext();)

            Observer o = it.next();

            o.update(runs,wickets,overs);

    // get latest runs from stadium

    private int getLatestRuns()

        // return 90 for simplicity

        return 90;

    // get latest wickets from stadium

    private int getLatestWickets()

        // return 2 for simplicity

        return 2;

    // get latest overs from stadium

    private float getLatestOvers()

        // return 90 for simplicity

        return (float)10.2;

    // This method is used update displays

    // when data changes

    public void dataChanged()

        //get latest data

        runs = getLatestRuns();

        wickets = getLatestWickets();

        overs = getLatestOvers();

        notifyObservers();

// This interface is implemented by all those

// classes that are to be updated whenever there

// is an update from CricketData

interface Observer

    public void update(int runs, int wickets,

                    float overs);

class AverageScoreDisplay implements Observer

    private float runRate;

    private int predictedScore;

    public void update(int runs, int wickets,

                    float overs)

        this.runRate =(float)runs/overs;

        this.predictedScore = (int)(this.runRate * 50);

        display();

    public void display()

        System.out.println("\nAverage Score Display: \n"

                        + "Run Rate: " + runRate +

                        "\nPredictedScore: " +

                        predictedScore);

class CurrentScoreDisplay implements Observer

    private int runs, wickets;

    private float overs;

    public void update(int runs, int wickets,

                    float overs)

        this.runs = runs;

        this.wickets = wickets;

        this.overs = overs;

        display();

    public void display()

        System.out.println("\nCurrent Score Display:\n"

                        + "Runs: " + runs +

                        "\nWickets:" + wickets +

                        "\nOvers: " + overs );

// Driver Class

class Main

    public static void main(String args[])

        // create objects for testing

        AverageScoreDisplay averageScoreDisplay =

                        new AverageScoreDisplay();

        CurrentScoreDisplay currentScoreDisplay =

                        new CurrentScoreDisplay();

        // pass the displays to Cricket data

        CricketData cricketData = new CricketData();

        // register display elements

        cricketData.registerObserver(averageScoreDisplay);

        cricketData.registerObserver(currentScoreDisplay);

        // in real app you would have some logic to

        // call this function when data changes

        cricketData.dataChanged();

        //remove an observer

        cricketData.unregisterObserver(averageScoreDisplay);

        // now only currentScoreDisplay gets the

        // notification

        cricketData.dataChanged();

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Source: www.geeksforgeeks.org

Tags: observer-pattern obse

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Contributed on Nov 03 2022

R K

0 Answers Avg Quality 2/10

observer design pattern

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Tip Shawmoon A 1 GREPCC

import java.util.List;
import java.util.ArrayList;
import java.util.Scanner;

class EventSource {
    public interface Observer {
        void update(String event);
    }
  
    private final List<Observer> observers = new ArrayList<>();
  
    private void notifyObservers(String event) {
        observers.forEach(observer -> observer.update(event));
    }
  
    public void addObserver(Observer observer) {
        observers.add(observer);
    }
  
    public void scanSystemIn() {
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            notifyObservers(line);
        }
    }
}

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import java.util.List;

import java.util.ArrayList;

import java.util.Scanner;

class EventSource {

    public interface Observer {

        void update(String event);

    private final List<Observer> observers = new ArrayList<>();

    private void notifyObservers(String event) {

        observers.forEach(observer -> observer.update(event));

    public void addObserver(Observer observer) {

        observers.add(observer);

    public void scanSystemIn() {

        Scanner scanner = new Scanner(System.in);

        while (scanner.hasNextLine()) {

            String line = scanner.nextLine();

            notifyObservers(line);

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Source: en.wikipedia.org

Tags: java

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Contributed on May 08 2022

Shawmoon A

0 Answers Avg Quality 2/10

What Is the Observer Pattern?

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Tip Deepika Sahu 1 GREPCC

Observer is a behavioral design pattern.
It specifies communication between objects: observable and observers. 
An observable is an object which notifies observers about the changes in its state.

For example, a news agency can notify channels when it receives news. 
Receiving news is what changes the state of the news agency, and it causes the channels to be notified.

xxxxxxxxxx

Observer is a behavioral design pattern.

It specifies communication between objects: observable and observers.

An observable is an object which notifies observers about the changes in its state.

For example, a news agency can notify channels when it receives news.

Receiving news is what changes the state of the news agency, and it causes the channels to be notified.

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Tags: whatever whateve

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Contributed on Dec 26 2022

Deepika Sahu

0 Answers Avg Quality 2/10

What is Observer design pattern

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Tip Pragya Keshap 1 GREPCC

Observer design pattern, there is a Subject that maintains the list

of Observers that are waiting for any update on the Subject. Once

there is an update in Subject it notifies all the observers for the

change.

E.g. In real life, students are waiting for the result of their test. Here

students are the observers and test is the subject. Once the result of

test is known, testing organization notifies all the students about

their result.

The most popular use of Observer pattern is in Model View

Controller (MVC) architectural pattern.

Main issue with Observer pattern is that it can cause memory leaks.

The subject holds a strong reference to observers. If observers are

not de-registered in time, it can lead to memory leak.

public class EventManager {
    Map<String, List<EventListener>> listeners = new HashMap<>();

    public EventManager(String... operations) {
        for (String operation : operations) {
            this.listeners.put(operation, new ArrayList<>());
        }
    }

    public void subscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        users.add(listener);
    }

    public void unsubscribe(String eventType, EventListener listener) {
        List<EventListener> users = listeners.get(eventType);
        users.remove(listener);
    }

    public void notify(String eventType, File file) {
        List<EventListener> users = listeners.get(eventType);
        for (EventListener listener : users) {
            listener.update(eventType, file);
        }
    }
}

xxxxxxxxxx

public class EventManager {

    Map<String, List<EventListener>> listeners = new HashMap<>();

    public EventManager(String... operations) {

        for (String operation : operations) {

            this.listeners.put(operation, new ArrayList<>());

    public void subscribe(String eventType, EventListener listener) {

        List<EventListener> users = listeners.get(eventType);

        users.add(listener);

    public void unsubscribe(String eventType, EventListener listener) {

        List<EventListener> users = listeners.get(eventType);

        users.remove(listener);

    public void notify(String eventType, File file) {

        List<EventListener> users = listeners.get(eventType);

        for (EventListener listener : users) {

            listener.update(eventType, file);

https://refactoring.guru/design-patterns/observer/java/example#:~:text=Observer%20is%20a%20behavioral%20design,that%20implements%20a%20subscriber%20interface.

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Contributed on Feb 10 2023

Pragya Keshap

0 Answers Avg Quality 2/10

Observer Pattern

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Tip Mahnaz foroutan 1 GREPCC

public interface IObserver
{
    void Update(string message);
}

public class ConcreteObserver : IObserver
{
    public string Name { get; }

    public ConcreteObserver(string name)
    {
        Name = name;
    }

    public void Update(string message)
    {
        Console.WriteLine($"{Name} received the message: {message}");
    }
}

public class Subject
{
    private List<IObserver> observers = new List<IObserver>();

    public void AddObserver(IObserver observer)
    {
        observers.Add(observer);
    }

    public void RemoveObserver(IObserver observer)
    {
        observers.Remove(observer);
    }

    public void NotifyObservers(string message)
    {
        foreach (var observer in observers)
        {
            observer.Update(message);
        }
    }
}

xxxxxxxxxx

public interface IObserver

    void Update(string message);

public class ConcreteObserver : IObserver

    public string Name { get; }

    public ConcreteObserver(string name)

        Name = name;

    public void Update(string message)

        Console.WriteLine($"{Name} received the message: {message}");

public class Subject

    private List<IObserver> observers = new List<IObserver>();

    public void AddObserver(IObserver observer)

        observers.Add(observer);

    public void RemoveObserver(IObserver observer)

        observers.Remove(observer);

    public void NotifyObservers(string message)

        foreach (var observer in observers)

            observer.Update(message);

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Tags: c# observer-pattern observe

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Contributed on Jul 30 2023

mahnaz foroutan

0 Answers Avg Quality 2/10

Observer pattern

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Tip Mahnaz foroutan 1 GREPCC

// Subject (Observable) interface
public interface IWeatherStation
{
    void AddObserver(IWeatherDisplay observer);
    void RemoveObserver(IWeatherDisplay observer);
    void NotifyObservers();
}

// Concrete subject (Observable) class that holds weather data
public class WeatherStation : IWeatherStation
{
    private List<IWeatherDisplay> observers = new List<IWeatherDisplay>();
    private float temperature;
    private float humidity;
    private float pressure;

    public void AddObserver(IWeatherDisplay observer)
    {
        observers.Add(observer);
    }

    public void RemoveObserver(IWeatherDisplay observer)
    {
        observers.Remove(observer);
    }

    public void NotifyObservers()
    {
        foreach (var observer in observers)
        {
            observer.Update(temperature, humidity, pressure);
        }
    }

    public void SetWeatherData(float temperature, float humidity, float pressure)
    {
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        NotifyObservers();
    }
}

// Observer interface
public interface IWeatherDisplay
{
    void Update(float temperature, float humidity, float pressure);
}

// Concrete observer classes representing different weather displays
public class CurrentConditionsDisplay : IWeatherDisplay
{
    public void Update(float temperature, float humidity, float pressure)
    {
        Console.WriteLine($"Current Conditions: Temperature {temperature}°C, Humidity {humidity}%, Pressure {pressure} hPa");
    }
}

public class StatisticsDisplay : IWeatherDisplay
{
    public void Update(float temperature, float humidity, float pressure)
    {
        Console.WriteLine($"Statistics: Max Temperature {temperature + 5}°C, Min Temperature {temperature - 5}°C");
    }
}

public class ForecastDisplay : IWeatherDisplay
{
    public void Update(float temperature, float humidity, float pressure)
    {
        if (pressure > 1000)
            Console.WriteLine("Forecast: Expect clear skies.");
        else
            Console.WriteLine("Forecast: Expect rain.");
    }
}

class Program
{
    static void Main()
    {
        // Client code
        WeatherStation weatherStation = new WeatherStation();

        CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay();
        StatisticsDisplay statisticsDisplay = new StatisticsDisplay();
        ForecastDisplay forecastDisplay = new ForecastDisplay();

        // Register display modules as observers
        weatherStation.AddObserver(currentDisplay);
        weatherStation.AddObserver(statisticsDisplay);
        weatherStation.AddObserver(forecastDisplay);

        // Simulate weather changes (Notify observers)
        weatherStation.SetWeatherData(25.5f, 60.0f, 1012.0f);
    }
}

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// Subject (Observable) interface

public interface IWeatherStation

    void AddObserver(IWeatherDisplay observer);

    void RemoveObserver(IWeatherDisplay observer);

    void NotifyObservers();

// Concrete subject (Observable) class that holds weather data

public class WeatherStation : IWeatherStation

    private List<IWeatherDisplay> observers = new List<IWeatherDisplay>();

    private float temperature;

    private float humidity;

    private float pressure;

    public void AddObserver(IWeatherDisplay observer)

        observers.Add(observer);

    public void RemoveObserver(IWeatherDisplay observer)

        observers.Remove(observer);

    public void NotifyObservers()

        foreach (var observer in observers)

            observer.Update(temperature, humidity, pressure);

    public void SetWeatherData(float temperature, float humidity, float pressure)

        this.temperature = temperature;

        this.humidity = humidity;

        this.pressure = pressure;

        NotifyObservers();

// Observer interface

public interface IWeatherDisplay

    void Update(float temperature, float humidity, float pressure);

// Concrete observer classes representing different weather displays

public class CurrentConditionsDisplay : IWeatherDisplay

    public void Update(float temperature, float humidity, float pressure)

        Console.WriteLine($"Current Conditions: Temperature {temperature}°C, Humidity {humidity}%, Pressure {pressure} hPa");

public class StatisticsDisplay : IWeatherDisplay

    public void Update(float temperature, float humidity, float pressure)

        Console.WriteLine($"Statistics: Max Temperature {temperature + 5}°C, Min Temperature {temperature - 5}°C");

public class ForecastDisplay : IWeatherDisplay

    public void Update(float temperature, float humidity, float pressure)

        if (pressure > 1000)

            Console.WriteLine("Forecast: Expect clear skies.");

        else

            Console.WriteLine("Forecast: Expect rain.");

class Program

    static void Main()

        // Client code

        WeatherStation weatherStation = new WeatherStation();

        CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay();

        StatisticsDisplay statisticsDisplay = new StatisticsDisplay();

        ForecastDisplay forecastDisplay = new ForecastDisplay();

        // Register display modules as observers

        weatherStation.AddObserver(currentDisplay);

        weatherStation.AddObserver(statisticsDisplay);

        weatherStation.AddObserver(forecastDisplay);

        // Simulate weather changes (Notify observers)

        weatherStation.SetWeatherData(25.5f, 60.0f, 1012.0f);

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Tags: c# observer-pattern observer-pat

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Contributed on Jul 30 2023

mahnaz foroutan

0 Answers Avg Quality 2/10

observer pattern design pattern

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Tip Amjad Saeed 1 GREPCC

7. Observer Pattern:
The Observer pattern defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically.

Example in React:
Suppose you want to create a simple notification system in your React app where multiple components can subscribe to updates.

import React, { useState } from 'react';

// Observer
const NotificationObserver = () => {
  const [subscribers, setSubscribers] = useState([]);

  const subscribe = (subscriber) => {
    setSubscribers([...subscribers, subscriber]);
  };

  const notify = (message) => {
    subscribers.forEach((subscriber) => {
      subscriber(message);
    });
  };

  return { subscribe, notify };
};

// Components
const SubscriberComponent = ({ observer }) => {
  const [message, setMessage] = useState('');

  const receiveNotification = (message) => {
    setMessage(message);
  };

  observer.subscribe(receiveNotification);

  return (
    <div>
      <p>Received Notification: {message}</p>
    </div>
  );
};

const ObserverPatternExample = () => {
  const notificationObserver = NotificationObserver();

  return (
    <div>
      <SubscriberComponent observer={notificationObserver} />
      <SubscriberComponent observer={notificationObserver} />
      <button onClick={() => notificationObserver.notify('New message')}>Notify</button>
    </div>
  );
};

export default ObserverPatternExample;

xxxxxxxxxx

7. Observer Pattern:

The Observer pattern defines a one-to-many dependency between objects, so that when one object changes state, all its dependents are notified and updated automatically.

Example in React:

Suppose you want to create a simple notification system in your React app where multiple components can subscribe to updates.

import React, { useState } from 'react';

// Observer

const NotificationObserver = () => {

  const [subscribers, setSubscribers] = useState([]);

  const subscribe = (subscriber) => {

    setSubscribers([...subscribers, subscriber]);

};

  const notify = (message) => {

    subscribers.forEach((subscriber) => {

      subscriber(message);

});

};

  return { subscribe, notify };

};

// Components

const SubscriberComponent = ({ observer }) => {

  const [message, setMessage] = useState('');

  const receiveNotification = (message) => {

    setMessage(message);

};

  observer.subscribe(receiveNotification);

  return (

    <div>

      <p>Received Notification: {message}</p>

    </div>

);

};

const ObserverPatternExample = () => {

  const notificationObserver = NotificationObserver();

  return (

    <div>

      <SubscriberComponent observer={notificationObserver} />

      <SubscriberComponent observer={notificationObserver} />

      <button onClick={() => notificationObserver.notify('New message')}>Notify</button>

    </div>

);

};

export default ObserverPatternExample;

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Contributed on Sep 17 2023

Amjad Saeed

0 Answers Avg Quality 2/10

observer design pattern

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