# 18. Mediator

###### tags: `DesignPatterns`

## 關於 Mediator 本篇將討論以下幾個問題

> ### 1. 關於 Mediator
> ### 2. UML
> ### 3. 將 UML 轉為程式碼
> ### 4. 情境

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## 測試環境：
>OS：Windows 10
>IDE：Visual Studio 2019

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## 1. 關於 Mediator
> 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.
>
> by [Gang of Four](https://en.wikipedia.org/wiki/Design_Patterns)

- 定義一個一組物件間互動邏輯的封裝(Mediator)
- 透過防止物件之間直接互相引用來減低耦合，並且可以獨立修改其互動邏輯

Mediator(中介者)屬於行為型(Behavioral Patterns)，當遇到**物件間相互存在複雜引用**時，可藉由 Mediator 來將物件間互動邏輯抽離，減低物件之間耦合，之後若需要修改互動邏輯時則不只需要修改 Mediator 而不需要動到物件本身。

優點：
- 符合 單一職責原則(Single Responsibility Principle)
- 符合 開閉原則(Open Closed Principle)
- 快速解除物件間耦合

缺點：
- 過度使用可能會造成 Mediator 演變成 [God Class](https://zh.wikipedia.org/wiki/%E4%B8%8A%E5%B8%9D%E5%AF%B9%E8%B1%A1)

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## 2. UML

Class 間關聯：
- Colleague 關聯 Mediator
- ConcreteMediator 繼承 Mediator
- ConcreteColleague 繼承 Colleague
- ConcreteMediator 關聯 ConcreteColleague

Class：
- Mediator：中介者的抽象方法或介面
- ConcreteMediator：中介者實作
- Colleague：包含 Mediator 的抽象方法或介面
- ConcreteColleague：Colleague 實作，可經由中介者與其他 ConcreteColleague 互動

---
## 3. 將 UML 轉為程式碼
中介者的介面

/// <summary>
/// 中介者的介面
/// </summary>
public interface IMediator
{
    void Send(string message, IColleague colleague);
}

中介者實作

/// <summary>
/// 中介者實作
/// </summary>
public class ConcreteMediator : IMediator
{
    public IColleague Colleague1 { get; set; }
    public IColleague Colleague2 { get; set; }

    public void Send(string message, IColleague colleague)
    {
        if (colleague == Colleague1)
        {
            Colleague2.Notify(message);
        }
        else
        {
            Colleague1.Notify(message);
        }
    }
}

包含 Mediator 的介面

/// <summary>
/// 包含 Mediator 的介面
/// </summary>
public interface IColleague
{
    void Send(string message);
    void Notify(string message);
}

Colleague 實作，可經由中介者與其他 ConcreteColleague 互動

/// <summary>
/// Colleague 實作，可經由中介者與其他 ConcreteColleague 互動
/// </summary>
public class ConcreteColleague1 : IColleague
{
    private IMediator _mediator { get; }
    public ConcreteColleague1(IMediator mediator)
    {
        _mediator = mediator;
    }

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

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

/// <summary>
/// Colleague 實作，可經由中介者與其他 ConcreteColleague 互動
/// </summary>
public class ConcreteColleague2 : IColleague
{
    private IMediator _mediator { get; }
    public ConcreteColleague2(IMediator mediator)
    {
        _mediator = mediator;
    }

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

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

1. 建立`mediator`
2. 將`mediator`傳入`colleague1` & `colleague2`
3. 再將`colleague1` & `colleague2`指定給`mediator`
4. `colleague1` & `colleague2`經由`mediator`互動

static void Main(string[] args)
{
    #region Default

    Default.ConcreteMediator mediator = new Default.ConcreteMediator();

    Default.ConcreteColleague1 colleague1 = new Default.ConcreteColleague1(mediator);
    Default.ConcreteColleague2 colleague2 = new Default.ConcreteColleague2(mediator);

    mediator.Colleague1 = colleague1;
    mediator.Colleague2 = colleague2;

    colleague1.Send("How are you?");
    colleague2.Send("Fine, thanks.");

    Console.ReadLine();

    #endregion
}

執行結果

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

---
## 4. 情境
我們接到了一個門市取得其他分店庫存的需求
- 不同店之間要能夠取得對方庫存資訊以互相支援
- 各分店販售商品不完全相同，故撈取資料參數不同

中介者的介面

/// <summary>
/// 中介者的介面
/// </summary>
public interface IMediator
{
    void GetOtherStoreInStore(IStore store);
}

中介者實作

/// <summary>
/// 中介者實作
/// </summary>
public class Mediator : IMediator
{
    public IStore StoreDaan { get; set; }
    public IStore StoreXinyi { get; set; }

    public void GetOtherStoreInStore(IStore store)
    {
        if (store == StoreDaan)
        {
            StoreXinyi.GetInStore();
        }
        else
        {
            StoreDaan.GetInStore();
        }
    }
}

包含 Mediator 的門市介面

/// <summary>
/// 包含 Mediator 的門市介面
/// </summary>
public interface IStore
{
    void GetInStore();
}

門市間可透過 Mediator 取得其他分店庫存

/// <summary>
/// 大安店實作，透過 Mediator 取得信義店庫存
/// </summary>
public class StoreDaan : IStore
{
    private IMediator _mediator { get; }
    public StoreDaan(IMediator mediator)
    {
        _mediator = mediator;
    }

    public void GetOtherStoreInStore()
    {
        _mediator.GetOtherStoreInStore(this);
    }

    public void GetInStore()
    {
        Console.WriteLine($"大安店 目前庫存 咖啡 100 杯，蛋糕 5 片");
    }
}

/// <summary>
/// 信義店實作，透過 Mediator 取得大安店庫存
/// </summary>
public class StoreXinyi : IStore
{
    private IMediator _mediator { get; }
    public StoreXinyi(IMediator mediator)
    {
        _mediator = mediator;
    }

    public void GetOtherStoreInStore()
    {
        _mediator.GetOtherStoreInStore(this);
    }

    public void GetInStore()
    {
        Console.WriteLine($"信義店 目前庫存 咖啡 10 杯，蛋糕 120 片");
    }
}

1. 建立`mediator`
2. 將`mediator`傳入`storeDaan` & `storeXinyi`
3. 再將`storeDaan` & `storeXinyi`指定給`mediator`
4. 大安店經由`mediator`取得信義店庫存

static void Main(string[] args)
{
    Situation.Mediator mediator = new Situation.Mediator();

    Situation.StoreDaan storeDaan = new Situation.StoreDaan(mediator);
    Situation.StoreXinyi storeXinyi = new Situation.StoreXinyi(mediator);

    mediator.StoreDaan = storeDaan;
    mediator.StoreXinyi = storeXinyi;

    storeDaan.GetInStore();
    storeDaan.GetOtherStoreInStore();

    Console.ReadLine();
}

執行結果

大安店 目前庫存 咖啡 100 杯，蛋糕 5 片
信義店 目前庫存 咖啡 10 杯，蛋糕 120 片

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## 完整程式碼
GitHub：[Behavioral_05_Mediator](https://github.com/darionnnnnn/blog/tree/master/Blog/Behavioral_05_Mediator)

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## 總結
### Mediator 為了處理物件間的互動，所以必須直接耦合所有物件，雖然在新增物件互動邏輯時不需要修改物件本身，但在物件日漸增多、邏輯日漸複雜之後，Mediator 很容易變成龐大且難以維護的怪物，適時的拆分 & 重構才能避免問題惡化。

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## 參考資料
1. [Design Patterns](https://en.wikipedia.org/wiki/Design_Patterns)
2. [大話設計模式](https://www.tenlong.com.tw/products/9789866761799)
2. [dofactory](https://www.dofactory.com/)
3. [Refactoring.Guru](https://refactoring.guru/)

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## 新手上路，若有錯誤還請告知，謝謝