Definition

Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.

Summary

You should use the pattern in the following cases:

Behavior that is distributed between some objects can be grouped or customized.
Object reuse is difficult because it communicates with other objects.
Objects in the system communicate in well-defined but complex ways.

UML class diagram

Participants

The classes and/or objects participating in this pattern are:

Mediator - defines an interface for communicating with Colleague objects
ConcreteMediator

implements cooperative behavior by coordinating Colleague objects

knows and maintains its colleagues

Colleague classes

each Colleague class knows its Mediator object

each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague

Sample code in C#


region Mediator
 
    public interface Mediator
    {
        #region Methods
 
        void Send(string message, Colleague colleague);
 
        #endregion
    }
 
   #endregion
 
    #region Concrete Mediator
 
    public class ConcreteMediator : Mediator
    {
        #region Properties
 
        public List Colleagues { get; private set; }
 
        #endregion
 
        #region Ctor
 
        public ConcreteMediator()
        {
            Colleagues = new List();
        }
 
        #endregion
 
       #region Mediator Members
 
        public void Send(string message, Colleague colleague)
        {
            foreach (Colleague currentColleague in Colleagues)
            {
                if (!currentColleague.Equals(colleague))
                {
                    currentColleague.Recieve(message);
                }
            }
        }
 
        #endregion
    }
 
    #endregion
 
    #region Colleague
 
    public abstract class Colleague
    {
        #region Members
 
        protected Mediator _mediator;
 
        #endregion
 
        #region Ctor
 
        public Colleague(Mediator mediator)
        {
            _mediator = mediator;
        }
 
        #endregion
 
        #region Methods
 
        /// 
        /// Sends the given message
        /// 
        /// The given message
        public abstract void Send(string message);
 
        /// 
        /// Recieves the given message
        /// 
        /// The given message
        public abstract void Recieve(string message);
 
        #endregion
    }
 
    #endregion
 
    #region Concrete Colleague
 
    public class ConcreteColleague : Colleague
    {
        #region Properties
 
        public int ID { get; set; }
 
        #endregion
 
        #region Ctor
 
        public ConcreteColleague(Mediator mediator, int id)
            : base(mediator)
        {
            ID = id;
        }
 
        #endregion
 
        #region Methods
 
        public override void Send(string message)
        {
            _mediator.Send(message, this);
        }
 
       public override void Recieve(string message)
        {
            Console.WriteLine("{0} recieved the message: {1}",
                ID, message);
        }
 
        #endregion
    }
 
    #endregion

As can be seen in the example, I have a mediator object that sends messages to the concretes of the colleague class. The example is simple but it shows the concepts that are used in the mediator pattern. With few modifications you
can use the example to build a small chat room application.

The following code was used to test the previous example:


  ConcreteMediator mediator = new ConcreteMediator();

   ConcreteColleague colleague1 = new ConcreteColleague(mediator, 1);
   ConcreteColleague colleague2 = new ConcreteColleague(mediator, 2);
 
   mediator.Colleagues.Add(colleague1);
   mediator.Colleagues.Add(colleague2);
 
   colleague1.Send("Hello from colleague 1");
   colleague2.Send("Hello from colleague 2");

   Console.Read();

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Mediator

Definition

Summary

UML class diagram

Participants

Sample code in C#

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