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Observer Design Pattern

Posted by Srihari Chinna Articles | Design & Architecture September 20, 2010
The Observer pattern defines a one to many relationships between objects so that when one changes its state, all the others are notified accordingly.
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Definition

A way of notifying dependent classes of changes.

The Observer pattern defines a one to many relationships between objects so that when one changes its state, all the others are notified accordingly.

Encapsulate the core components in a Subject abstraction, and the variable components in an Observer hierarchy.

Design

The Observer pattern is composed of two classes. The Subject/Target is a class whose objects change their state at an independent rate. Observers may indicate that they wish to be informed of these changes so the Subject/Target will send them notifications.



Code

//Notifier

    public delegate void ChangedEventHandler(object sender, ChangedEventArgs e);

    //Subject

    public class Target: ArrayList

    {

        public event ChangedEventHandler Changed;

        public override int Add(object value)

        {

            if (Changed != null)

            {

Changed(this, new ChangedEventArgs() { PropertName = "Add", Value = value });

            }

            return base.Add(value);

        }
    }
 
    //Custom Event args

    public class ChangedEventArgs : EventArgs

    {

        public string PropertName { get; set; }

        public object Value { get; set; }
   
    }
public class ObserverOne
    {
        public ObserverOne(Target target)
        {
            target.Changed += new ChangedEventHandler(A);
        }
        public void A(object sender, ChangedEventArgs e)
        {
      
     Console.WriteLine("Tagret added with " +  e.Value.ToString());

               }
    }

    public class ObserverTwo
    {
        public ObserverTwo(Target target)
        {
            target.Changed += new ChangedEventHandler(A);
        }

        public void A(object sender, ChangedEventArgs e)
        {
            Console.WriteLine("Tagret added with  " + e.Value.ToString());

        }
    }
//Client
static void Main(string[] args)
        {
            Target target = new Target();

            ObserverOne observer1 = new ObserverOne(target);
            ObserverTwo observer2 = new ObserverTwo(target);

            target.Add("Chinna");
        }
 

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