How can we implement the observer pattern in .NET?

Question

There are many design patterns in the .NET architecture. I want to know what is meant with the observer pattern and how it is implemented.

Answer 1

The Observer pattern is pretty well characterized in C # as events.

Answer 2

Wikipedia summarizes:

( /) , , , , , , . .

( publish-subscribe pattern) , (, , gui, , ). EventListeners ( Observer).

:

• MSDN: http://msdn.microsoft.com/en-us/library/ms998543.aspx
• : http://en.wikipedia.org/wiki/Observer_pattern

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Answer 3

MSDN.

:

http://msdn.microsoft.com/en-us/library/ms998543.aspx

Answer 4

DOFactory.

:

• UML-
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• .

Answer 5

#, , , , , :)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SimpleObserver
{
    class Program
    {
        static void Main(string[] args)
        {
            Newspaper newspaper = new Newspaper(); //create a Newspaper, which is a realization of Subject
            Grandma grandma = new Grandma(newspaper); //create a new Grandma, which is a realization of Observer
            newspaper.ChangeNews("no news today..."); //when the news get changed, grandma will automatically get it
            newspaper.ChangeNews("still no news..."); //and again
        }
    }

    //the subject which the observers are 'observing'
    //whenever something changes in the subject, the 
    //observers that are registered should be updated
    public interface Subject 
    {
        void RegisterObserver(Observer o); 
        void RemoveObserver(Observer o);
        void NotifyObservers(); 
    }

    //the actual subject, a newspaper which implements
    //the methods declared in the interface and it own method
    //the goal is that whenever the news property changes 'by 
    //calling ChangeNews(string newNews); all the registered observers will
    //get that new news
    public class Newspaper : Subject
    {
        private List<Observer> observers = new List<Observer>(); //list with observers
        private string news = "initial news, nothing new!"; //the news

        public void RegisterObserver(Observer o)
        {
            this.observers.Add(o);
        }

        public void RemoveObserver(Observer o)
        {
            this.observers.Remove(o);
        }

        public void NotifyObservers()
        {
            foreach (Observer o in this.observers)
                o.Update(news); //update all the observers with the news
        }

        public void ChangeNews(string newNews) //method to manually change the news
        {
            this.news = newNews;
            this.NotifyObservers(); //automatically calls the NotifyObservers() method
        }
    }

    //the actual observer, has a method Update that will be
    //called by the Subject when something changes
    public interface Observer
    {
        void Update(string news);
    }

    //grandma is a observer, whenever the news changes she will be 
    //notified. she also has a reference to the subject instance, so
    //that she can cancel her subscription whenever needed
    public class Grandma : Observer
    {
        private Subject subject;

        public Grandma(Subject subject)
        {
            this.subject = subject; //set reference to the subject
            this.subject.RegisterObserver(this); //register to the subject
        }

        public void Update(string news)
        {
            Console.WriteLine("Granny reads the news, very slowly...");
            Console.WriteLine("The news today is... " + news);
        }
    }

    //possibly other observers go here...
}