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Observer

Sumit Rawal answered on December 26, 2021 Popularity 1/10 Helpfulness 1/10

answer Observer

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Observer

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This pattern is a one-to-many dependency between objects so that when one object changes state, all its dependents are notified. This is typically done by calling one of their methods.

For the sake of simplicity, think about what happens when you follow someone on Twitter. You are essentially asking Twitter to send you (the observer) tweet updates of the person (the subject) you followed. The pattern consists of two actors, the observer who is interested in the updates and the subject who generates the updates.

A subject can have many observers and is a one to many relationship. However, an observer is free to subscribe to updates from other subjects too. You can subscribe to news feed from a Facebook page, which would be the subject and whenever the page has a new post, the subscriber would see the new post.

Key consideration: In case of many subjects and few observers, if each subject stores its observers separately, it’ll increase the storage costs as some subjects will be storing the same observer multiple times.

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Contributed on May 14 2023

Sumit Rawal

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using System;
using System.Collections.Generic;
 
namespace ObserverPatternDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            Product Laptop = new Product("DELL", 1000);
            Console.WriteLine("--------Customer 1 and Customer 2 is subscribed to the Laptop Product----------");
            // attach or subscribe customer 1
            Customer Customer1 = new Customer("customer 1");
            Laptop.Subscribe(Customer1);
            // attach or subscribe customer 2
            Customer Customer2 = new Customer("customer 2");
            Laptop.Subscribe(Customer2);
            
            // publish notification to the subscribers
            Laptop.price = 800;
 
            Console.WriteLine("--------customer 2 is unsubscribed and customer 3 is subscribed to the Laptop Product----------");
            // customer 2 is unsubscribed and customer 3 is subscribed
            Laptop.Unsubscribe(Customer2);
            Customer Customer3 = new Customer("customer 3");
            Laptop.Subscribe(Customer3);
            Laptop.price = 900;
            Console.ReadLine();
        }
    }
    // ISubject Interface
    interface IProduct
    {
        // attach observer
        void Subscribe(Customer customer);
        // detach observer
        void Unsubscribe(Customer customer);
        // publish notification to the subscribers
        void Notify();
    }
    // ConcreteSubject class
    public class Product : IProduct
    {
        string name;
        float basePrice;
        float currentPrice;
        List<Customer> customers = new List<Customer>();
 
        public Product(string name, float basePrice)
        {
            this.name = name;
            this.basePrice = basePrice;
            this.currentPrice = basePrice;
        }
 
        public float price
        {
            get
            {
                return currentPrice;
            }
            set
            {
                // if price of the laptop will be less than the base price then 
                // notification will forwarded to the subscribers
                currentPrice = value;
                if (value <= basePrice)
                    Notify();
            }
        }
        public void Subscribe(Customer customer)
        {
            customers.Add(customer);
        }
 
        public void Unsubscribe(Customer customer)
        {
            customers.Remove(customer);
        }
 
        public void Notify()
        {
            foreach (Customer observer in customers)
            {
                observer.Update(this);
            }
        }
 
        public string Name
        {
            get { return name; }
        }
 
        public float discount
        {
            get { return (basePrice - currentPrice)*100/basePrice; }
        }
        public float CurrentPrice
        {
            get { return currentPrice; }
        }
    }
    // IObserver Interface
    interface ICustomer
    {
        void Update(Product product);
    }
    // ConcreteObserver Interface
    public class Customer : ICustomer
    {
        string name;
        public Customer(string name)
        {
            this.name = name;
        }
        public void Update(Product product)
        {
            Console.WriteLine("{0}: {1} Laptop is now available at {2}; Discount = {3}%",this.name, product.Name, product.CurrentPrice, product.discount);
        }
    }
 
    
}

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Source: gyanendushekhar.com

Tags: c#

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Contributed on Dec 26 2021

Misty Manatee

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