Definition

A way of notifying a change to a number of dependent classes.

The Observer pattern defines a one to many relationship 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 and the Subject/Target will send them notifications.

UML Diagram

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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");
}