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Mediator Pattern

Mediator Pattern 


Gamma Categorization: Behavioral Design Pattern
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.

Sample Code

Problem: We want to apply several commands on the digital camera.

Code

public class Person
{
    public string Name;
    public ChatRoom Room;
    private List<string> chatLog = new List<string>();

    public Person(string name)
    {
        Name = name;
    }

    public void Receive(string sender, string message)
    {
        string s = $"{sender}: '{message}'";
        System.Console.WriteLine($"[{Name}'s chat session] {s}");
        chatLog.Add(s);
    }

    public void Say(string message)
    {
        Room.Broadcast(Name, message);
    }

    public void PrivateMessage(string who, string message)
    {
        Room.Message(Name, who, message);
    }
}

public class ChatRoom
{
    private List<Person> people = new List<Person>();

    public void Broadcast(string source, string message)
    {
        foreach (var p in people)
            if (p.Name != source)
                p.Receive(source, message);
    }

    public void Join(Person p)
    {
        string joinMsg = $"{p.Name} joins the chat";
        Broadcast("room", joinMsg);

        p.Room = this;
        people.Add(p);
    }

    public void Message(string source, string destination, string message)
    {
        people.FirstOrDefault(p => p.Name == destination)?.Receive(source, message);
    }
}

Usage

var room = new ChatRoom();

var admin = new Person("Admin");
var user1 = new Person("User1");
var user2 = new Person("User2");

room.Join(admin);
room.Join(user1);
room.Join(user2);

user1.Say("hi room");
user2.Say("oh, hey user1");

var guru = new Person("guru");
room.Join(guru);
guru.Say("hi everyone!");

user1.PrivateMessage("guru", "I need some info...");

Output

[Admin's chat session] room: 'User1 joins the chat'
[Admin's chat session] room: 'User2 joins the chat'
[User1's chat session] room: 'User2 joins the chat'
[Admin's chat session] User1: 'hi room'
[User2's chat session] User1: 'hi room'
[Admin's chat session] User2: 'oh, hey user1'
[User1's chat session] User2: 'oh, hey user1'
[Admin's chat session] room: 'guru joins the chat'
[User1's chat session] room: 'guru joins the chat'
[User2's chat session] room: 'guru joins the chat'
[Admin's chat session] guru: 'hi everyone!'
[User1's chat session] guru: 'hi everyone!'
[User2's chat session] guru: 'hi everyone!'
[guru's chat session] User1: 'I need some info...'

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