Skip to content

Mediator

Summary: 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.

Frequency of use: Medium low

UML class diagram

alt

Participants

The classes and objects participating in this pattern are:

  • Mediator  (IChatroom)
  • defines an interface for communicating with Colleague objects
  • ConcreteMediator  (Chatroom)
  • implements cooperative behavior by coordinating Colleague objects
  • knows and maintains its colleagues
  • Colleague classes  (Participant)
  • each Colleague class knows its Mediator object
  • each colleague communicates with its mediator whenever it would have otherwise communicated with another colleague

Structural code in C# .{10}

This structural code demonstrates the Mediator pattern facilitating loosely coupled communication between different objects and object types. The mediator is a central hub through which all interaction must take place.

using System;

namespace DoFactory.GangOfFour.Mediator.Structural
{
    /// <summary>
    /// MainApp startup class for Structural
    /// Mediator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            ConcreteMediator m = new ConcreteMediator();

            ConcreteColleague1 c1 = new ConcreteColleague1(m);
            ConcreteColleague2 c2 = new ConcreteColleague2(m);

            m.Colleague1 = c1;
            m.Colleague2 = c2;

            c1.Send("How are you?");
            c2.Send("Fine, thanks");

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Mediator' abstract class
    /// </summary>
    abstract class Mediator
    {
        public abstract void Send(string message,
            Colleague colleague);
    }

    /// <summary>
    /// The 'ConcreteMediator' class
    /// </summary>
    class ConcreteMediator : Mediator
    {
        private ConcreteColleague1 _colleague1;
        private ConcreteColleague2 _colleague2;

        public ConcreteColleague1 Colleague1
        {
            set { _colleague1 = value; }
        }

        public ConcreteColleague2 Colleague2
        {
            set { _colleague2 = value; }
        }

        public override void Send(string message,
            Colleague colleague)
        {
            if (colleague == _colleague1)
            {
                _colleague2.Notify(message);
            }
            else
            {
                _colleague1.Notify(message);
            }
        }
    }

    /// <summary>
    /// The 'Colleague' abstract class
    /// </summary>
    abstract class Colleague
    {
        protected Mediator mediator;

        // Constructor
        public Colleague(Mediator mediator)
        {
            this.mediator = mediator;
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class ConcreteColleague1 : Colleague
    {
        // Constructor
        public ConcreteColleague1(Mediator mediator)
            : base(mediator)
        {
        }

        public void Send(string message)
        {
            mediator.Send(message, this);
        }

        public void Notify(string message)
        {
            Console.WriteLine("Colleague1 gets message: "
                + message);
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class ConcreteColleague2 : Colleague
    {
        // Constructor
        public ConcreteColleague2(Mediator mediator)
            : base(mediator)
        {
        }

        public void Send(string message)
        {
            mediator.Send(message, this);
        }

        public void Notify(string message)
        {
            Console.WriteLine("Colleague2 gets message: "
                + message);
        }
    }
}

Output

Colleague2 gets message: How are you?
Colleague1 gets message: Fine, thanks

Real-world code in C# .{10}

This real-world code demonstrates the Mediator pattern facilitating loosely coupled communication between different Participants registering with a Chatroom. The Chatroom is the central hub through which all communication takes place. At this point only one-to-one communication is implemented in the Chatroom, but would be trivial to change to one-to-many.

using System;
using System.Collections.Generic;

namespace DoFactory.GangOfFour.Mediator.RealWorld
{
    /// <summary>
    /// MainApp startup class for Real-World
    /// Mediator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            // Create chatroom
            Chatroom chatroom = new Chatroom();

            // Create participants and register them
            Participant George = new Beatle("George");
            Participant Paul = new Beatle("Paul");
            Participant Ringo = new Beatle("Ringo");
            Participant John = new Beatle("John");
            Participant Yoko = new NonBeatle("Yoko");

            chatroom.Register(George);
            chatroom.Register(Paul);
            chatroom.Register(Ringo);
            chatroom.Register(John);
            chatroom.Register(Yoko);

            // Chatting participants
            Yoko.Send("John", "Hi John!");
            Paul.Send("Ringo", "All you need is love");
            Ringo.Send("George", "My sweet Lord");
            Paul.Send("John", "Can't buy me love");
            John.Send("Yoko", "My sweet love");

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Mediator' abstract class
    /// </summary>
    abstract class AbstractChatroom
    {
        public abstract void Register(Participant participant);
        public abstract void Send(
            string from, string to, string message);
    }

    /// <summary>
    /// The 'ConcreteMediator' class
    /// </summary>
    class Chatroom : AbstractChatroom
    {
        private Dictionary<string,Participant> _participants =
            new Dictionary<string,Participant>();

        public override void Register(Participant participant)
        {
            if (!_participants.ContainsValue(participant))
            {
                _participants[participant.Name] = participant;
            }

            participant.Chatroom = this;
        }

        public override void Send(
            string from, string to, string message)
        {
            Participant participant = _participants[to];

            if (participant != null)
            {
                participant.Receive(from, message);
            }
        }
    }

    /// <summary>
    /// The 'AbstractColleague' class
    /// </summary>
    class Participant
    {
        private Chatroom _chatroom;
        private string _name;

        // Constructor
        public Participant(string name)
        {
            this._name = name;
        }

        // Gets participant name
        public string Name
        {
            get { return _name; }
        }

        // Gets chatroom
        public Chatroom Chatroom
        {
            set { _chatroom = value; }
            get { return _chatroom; }
        }

        // Sends message to given participant
        public void Send(string to, string message)
        {
            _chatroom.Send(_name, to, message);
        }

        // Receives message from given participant
        public virtual void Receive(
            string from, string message)
        {
            Console.WriteLine("{0} to {1}: '{2}'",
                from, Name, message);
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class Beatle : Participant
    {
        // Constructor
        public Beatle(string name)
            : base(name)
        {
        }

        public override void Receive(string from, string message)
        {
            Console.Write("To a Beatle: ");
            base.Receive(from, message);
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class NonBeatle : Participant
    {
        // Constructor
        public NonBeatle(string name)
            : base(name)
        {
        }

        public override void Receive(string from, string message)
        {
            Console.Write("To a non-Beatle: ");
            base.Receive(from, message);
        }
    }
}

Output

To a Beatle: Yoko to John: 'Hi John!'
To a Beatle: Paul to Ringo: 'All you need is love'
To a Beatle: Ringo to George: 'My sweet Lord'
To a Beatle: Paul to John: 'Can't buy me love'
To a non-Beatle: John to Yoko: 'My sweet love'

.NET Optimized code in C# .{10}

alt

using System;
using System.Collections.Generic;

namespace DoFactory.GangOfFour.Mediator.NETOptimized
{
    /// <summary>
    /// MainApp startup class for .NET optimized
    /// Mediator Design Pattern.
    /// </summary>
    class MainApp
    {
        /// <summary>
        /// Entry point into console application.
        /// </summary>
        static void Main()
        {
            // Create chatroom participants
            Participant George = new Beatle { Name = "George" };
            Participant Paul = new Beatle { Name = "Paul" };
            Participant Ringo = new Beatle { Name = "Ringo" };
            Participant John = new Beatle { Name = "John" };
            Participant Yoko = new NonBeatle { Name = "Yoko" };

            // Create chatroom and register participants
            var chatroom = new Chatroom();
            chatroom.Register(George);
            chatroom.Register(Paul);
            chatroom.Register(Ringo);
            chatroom.Register(John);
            chatroom.Register(Yoko);

            // Chatting participants
            Yoko.Send("John", "Hi John!");
            Paul.Send("Ringo", "All you need is love");
            Ringo.Send("George", "My sweet Lord");
            Paul.Send("John", "Can't buy me love");
            John.Send("Yoko", "My sweet love");

            // Wait for user
            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Mediator' interface
    /// </summary>
    interface IChatroom
    {
        void Register(Participant participant);
        void Send(string from, string to, string message);
    }

    /// <summary>
    /// The 'ConcreteMediator' class
    /// </summary>
    class Chatroom : IChatroom
    {
        private Dictionary<string, Participant> _participants =
            new Dictionary<string, Participant>();

        public void Register(Participant participant)
        {
            if (!_participants.ContainsKey(participant.Name))
            {
                _participants.Add(participant.Name, participant);
            }

            participant.Chatroom = this;
        }

        public void Send(string from, string to, string message)
        {
            var participant = _participants[to];
            if (participant != null)
            {
                participant.Receive(from, message);
            }
        }
    }

    /// <summary>
    /// The 'AbstractColleague' class
    /// </summary>
    class Participant
    {
        // Gets or sets participant name
        public string Name { get; set; }

        // Gets or sets chatroom
        public Chatroom Chatroom { get; set; }

        // Send a message to given participant
        public void Send(string to, string message)
        {
            Chatroom.Send(Name, to, message);
        }

        // Receive message from participant
        public virtual void Receive(
            string from, string message)
        {
            Console.WriteLine("{0} to {1}: '{2}'",
                from, Name, message);
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class Beatle : Participant
    {
        public override void Receive(string from, string message)
        {
            Console.Write("To a Beatle: ");
            base.Receive(from, message);
        }
    }

    /// <summary>
    /// A 'ConcreteColleague' class
    /// </summary>
    class NonBeatle : Participant
    {
        public override void Receive(string from, string message)
        {
            Console.Write("To a non-Beatle: ");
            base.Receive(from, message);
        }
    }
}