Mediatorパターンとは | GoFデザインパターン

Mediatorパターンは、複数のオブジェクト間の複雑なコミュニケーションを管理し、直接的な依存関係を避けることで、オブジェクト同士の結合度を低減させるデザインパターンです。

このパターンを使うことで、システム全体のメンテナンス性や拡張性が向上します。

この記事では、Mediatorパターンの基本概念、実際の使い方、そしてJava、C++、C#、VB.NETでの実装サンプルを提供します。

Mediatorパターンとは

Mediatorパターンは、オブジェクト間の通信を一つのMediator（仲介者）オブジェクトに集中させることで、個々のオブジェクト同士の依存関係を排除し、シンプルな構造を保つデザインパターンです。

これにより、システムの変更が容易になり、柔軟性が向上します。

Mediatorパターンの使い方

Mediatorパターンは、主にオブジェクト間の依存関係を減らしたいときや、複数のオブジェクトが同時にやり取りをする状況で役立ちます。

たとえば、チャットルームのようなシステムでは、各参加者（オブジェクト）がMediatorを介してメッセージを送受信するため、参加者同士が直接やり取りをする必要がなくなります。

Mediatorパターン実装サンプル

以下に、Mediatorパターンの実装サンプルをそれぞれJava、C++、C#、VB.NETで示します。

JavaによるMediatorパターンの実装

public interface Mediator {
    void sendMessage(String message, Colleague colleague);
}

public abstract class Colleague {
    protected Mediator mediator;

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

    public abstract void receiveMessage(String message);
}

public class ConcreteColleague1 extends Colleague {
    public ConcreteColleague1(Mediator mediator) {
        super(mediator);
    }

    public void sendMessage(String message) {
        System.out.println("Colleague1 sending message: " + message);
        mediator.sendMessage(message, this);
    }

    public void receiveMessage(String message) {
        System.out.println("Colleague1 received message: " + message);
    }
}

public class ConcreteColleague2 extends Colleague {
    public ConcreteColleague2(Mediator mediator) {
        super(mediator);
    }

    public void sendMessage(String message) {
        System.out.println("Colleague2 sending message: " + message);
        mediator.sendMessage(message, this);
    }

    public void receiveMessage(String message) {
        System.out.println("Colleague2 received message: " + message);
    }
}

public class ConcreteMediator implements Mediator {
    private ConcreteColleague1 colleague1;
    private ConcreteColleague2 colleague2;

    public void setColleague1(ConcreteColleague1 colleague1) {
        this.colleague1 = colleague1;
    }

    public void setColleague2(ConcreteColleague2 colleague2) {
        this.colleague2 = colleague2;
    }

    public void sendMessage(String message, Colleague colleague) {
        if (colleague == colleague1) {
            colleague2.receiveMessage(message);
        } else {
            colleague1.receiveMessage(message);
        }
    }
}

public class MediatorPatternDemo {
    public static void main(String[] args) {
        ConcreteMediator mediator = new ConcreteMediator();
        ConcreteColleague1 colleague1 = new ConcreteColleague1(mediator);
        ConcreteColleague2 colleague2 = new ConcreteColleague2(mediator);

        mediator.setColleague1(colleague1);
        mediator.setColleague2(colleague2);

        colleague1.sendMessage("Hello from Colleague1");
        colleague2.sendMessage("Hello from Colleague2");
    }
}

C++によるMediatorパターンの実装

#include 
#include 

class Colleague;

class Mediator {
public:
    virtual void sendMessage(const std::string& message, Colleague* colleague) = 0;
};

class Colleague {
protected:
    Mediator* mediator;

public:
    Colleague(Mediator* mediator) : mediator(mediator) {}

    virtual void receiveMessage(const std::string& message) = 0;
};

class ConcreteColleague1 : public Colleague {
public:
    ConcreteColleague1(Mediator* mediator) : Colleague(mediator) {}

    void sendMessage(const std::string& message) {
        std::cout << "Colleague1 sending message: " << message << std::endl;
        mediator->sendMessage(message, this);
    }

    void receiveMessage(const std::string& message) {
        std::cout << "Colleague1 received message: " << message << std::endl;
    }
};

class ConcreteColleague2 : public Colleague {
public:
    ConcreteColleague2(Mediator* mediator) : Colleague(mediator) {}

    void sendMessage(const std::string& message) {
        std::cout << "Colleague2 sending message: " << message << std::endl;
        mediator->sendMessage(message, this);
    }

    void receiveMessage(const std::string& message) {
        std::cout << "Colleague2 received message: " << message << std::endl;
    }
};

class ConcreteMediator : public Mediator {
private:
    ConcreteColleague1* colleague1;
    ConcreteColleague2* colleague2;

public:
    void setColleague1(ConcreteColleague1* colleague) {
        colleague1 = colleague;
    }

    void setColleague2(ConcreteColleague2* colleague) {
        colleague2 = colleague;
    }

    void sendMessage(const std::string& message, Colleague* colleague) {
        if (colleague == colleague1) {
            colleague2->receiveMessage(message);
        } else {
            colleague1->receiveMessage(message);
        }
    }
};

int main() {
    ConcreteMediator mediator;
    ConcreteColleague1 colleague1(&mediator);
    ConcreteColleague2 colleague2(&mediator);

    mediator.setColleague1(&colleague1);
    mediator.setColleague2(&colleague2);

    colleague1.sendMessage("Hello from Colleague1");
    colleague2.sendMessage("Hello from Colleague2");

    return 0;
}

C#によるMediatorパターンの実装

using System;

public interface Mediator {
    void SendMessage(string message, Colleague colleague);
}

public abstract class Colleague {
    protected Mediator mediator;

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

    public abstract void ReceiveMessage(string message);
}

public class ConcreteColleague1 : Colleague {
    public ConcreteColleague1(Mediator mediator) : base(mediator) {}

    public void SendMessage(string message) {
        Console.WriteLine("Colleague1 sending message: " + message);
        mediator.SendMessage(message, this);
    }

    public override void ReceiveMessage(string message) {
        Console.WriteLine("Colleague1 received message: " + message);
    }
}

public class ConcreteColleague2 : Colleague {
    public ConcreteColleague2(Mediator mediator) : base(mediator) {}

    public void SendMessage(string message) {
        Console.WriteLine("Colleague2 sending message: " + message);
        mediator.SendMessage(message, this);
    }

    public override void ReceiveMessage(string message) {
        Console.WriteLine("Colleague2 received message: " + message);
    }
}

public class ConcreteMediator : Mediator {
    private ConcreteColleague1 colleague1;
    private ConcreteColleague2 colleague2;

    public void SetColleague1(ConcreteColleague1 colleague) {
        colleague1 = colleague;
    }

    public void SetColleague2(ConcreteColleague2 colleague) {
        colleague2 = colleague;
    }

    public void SendMessage(string message, Colleague colleague) {
        if (colleague == colleague1) {
            colleague2.ReceiveMessage(message);
        } else {
            colleague1.ReceiveMessage(message);
        }
    }
}

public class MediatorPatternDemo {
    public static void Main(string[] args) {
        ConcreteMediator mediator = new ConcreteMediator();
        ConcreteColleague1 colleague1 = new ConcreteColleague1(mediator);
        ConcreteColleague2 colleague2 = new ConcreteColleague2(mediator);

        mediator.SetColleague1(colleague1);
        mediator.SetColleague2(colleague2);

        colleague1.SendMessage("Hello from Colleague1");
        colleague2.SendMessage("Hello from Colleague2");
    }
}

VB.NETによるMediatorパターンの実装

Public Interface Mediator
    Sub SendMessage(ByVal message As String, ByVal colleague As Colleague)
End Interface

Public MustInherit Class Colleague
    Protected mediator As Mediator

    Public Sub New(ByVal mediator As Mediator)
        Me.mediator = mediator
    End Sub

    Public MustOverride Sub ReceiveMessage(ByVal message As String)
End Class

Public Class ConcreteColleague1
    Inherits Colleague

    Public Sub New(ByVal mediator As Mediator)
        MyBase.New(mediator)
    End Sub

    Public Sub SendMessage(ByVal message As String)
        Console.WriteLine("Colleague1 sending message: " & message)
        mediator.SendMessage(message, Me)
    End Sub

    Public Overrides Sub ReceiveMessage(ByVal message As String)
        Console.WriteLine("Colleague1 received message: " & message)
    End Sub
End Class

Public Class ConcreteColleague2
    Inherits Colleague

    Public Sub New(ByVal mediator As Mediator)
        MyBase.New(mediator)
    End Sub

    Public Sub SendMessage(ByVal message As String)
        Console.WriteLine("Colleague2 sending message: " & message)
        mediator.SendMessage(message, Me)
    End Sub

    Public Overrides Sub ReceiveMessage(ByVal message As String)
        Console.WriteLine("Colleague2 received message: " & message)
    End Sub
End Class

Public Class ConcreteMediator
    Implements Mediator

    Private colleague1 As ConcreteColleague1
    Private colleague2 As ConcreteColleague2

    Public Sub SetColleague1(ByVal colleague As ConcreteColleague1)
        colleague1 = colleague
    End Sub

    Public Sub SetColleague2(ByVal colleague As ConcreteColleague2)
        colleague2 = colleague
    End Sub

    Public Sub SendMessage(ByVal message As String, ByVal colleague As Colleague) Implements Mediator.SendMessage
        If colleague Is colleague1 Then
            colleague2.ReceiveMessage(message)
        Else
            colleague1.ReceiveMessage(message)
        End If
    End Sub
End Class

Module MediatorPatternDemo
    Sub Main()
        Dim mediator As New ConcreteMediator()
        Dim colleague1 As New ConcreteColleague1(mediator)
        Dim colleague2 As New ConcreteColleague2(mediator)

        mediator.SetColleague1(colleague1)
        mediator.SetColleague2(colleague2)

        colleague1.SendMessage("Hello from Colleague1")
        colleague2.SendMessage("Hello from Colleague2")
    End Sub
End Module

まとめ

Mediatorパターンは、複数のオブジェクト間の複雑な相互依存を管理し、メンテナンスを容易にするための重要なデザインパターンです。

システムの柔軟性を高め、拡張性のある設計が可能になります。

上記のように、Java、C++、C#、VB.NETそれぞれでの実装方法を通して、このパターンの有用性を確認できました。

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