# 14. Chain of Responsibility

###### tags: `DesignPatterns`

## 關於 Chain of Responsibility 本篇將討論以下幾個問題

> ### 1. 關於 Chain of Responsibility
> ### 2. UML
> ### 3. 將 UML 轉為程式碼
> ### 4. 情境

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

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## 1. 關於 Chain of Responsibility
> Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
>
> by [Gang of Four](https://en.wikipedia.org/wiki/Design_Patterns)

- 藉由給一個以上對像處理請求的機會，避免將請求的發送者與接收者耦合
- 鏈接接收對象，並將請求沿著鏈傳遞，直到對象處理該請求

Chain of Responsibility(責任鏈)屬於行為型(Behavioral Patterns)，當遇到**未知請求 & 未知處理者**時，可使用 Chain of Responsibility 來將請求依序對應全部處理者，找尋可處理請求的對象。當遇到**過濾、攔截**的需求時，可使用 Chain of Responsibility 來依照指定順序執行判斷邏輯。

優點：
- 符合 單一職責原則(Single Responsibility Principle)
- 符合 開閉原則(Open Closed Principle)

缺點：
- 用於未知請求 & 未知處理者時，可能會因為處理者過多而影響效能

---
## 2. UML

Class 間關聯：
- Client 關聯 Handler
- ConcreteHandler 1 / 2 繼承 Handler
- ConcreteHandler2 可包含 Handler

Class：
- Client：呼叫端
- Handler：處理請求的介面
- ConcreteHandler：處理請求實作

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## 3. 將 UML 轉為程式碼
處理請求的抽象類別

/// <summary>
/// 處理請求的抽象類別
/// </summary>
public abstract class Handler
{
    protected Handler successor;

    public void SetSuccessor(Handler successor)
    {
        this.successor = successor;
    }

    public abstract void HandleRequest(int request);
}

處理請求實作 1 / 2 / 3

/// <summary>
/// 處理請求實作
/// </summary>
public class ConcreteHandler1 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request >= 0 && request < 10)
        {
            Console.WriteLine($"{GetType().Name} handled request {request}");
        }
        else if (successor != null)
        {
            successor.HandleRequest(request);
        }
    }
}

/// <summary>
/// 處理請求實作
/// </summary>
public class ConcreteHandler2 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request >= 10 && request < 20)
        {
            Console.WriteLine($"{GetType().Name} handled request {request}");
        }
        else if (successor != null)
        {
            successor.HandleRequest(request);
        }
    }
}

/// <summary>
/// 處理請求實作
/// </summary>
public class ConcreteHandler3 : Handler
{
    public override void HandleRequest(int request)
    {
        if (request >= 20 && request < 30)
        {
            Console.WriteLine($"{GetType().Name} handled request {request}");
        }
        else if (successor != null)
        {
            successor.HandleRequest(request);
        }
    }
}

1. 建立`handler`
2. 經由`SetSuccessor`設置接續執行的`handler`
3. 依序傳入測試資料

static void Main(string[] args)
{
    Default.Handler handler1 = new Default.ConcreteHandler1();
    Default.Handler handler2 = new Default.ConcreteHandler2();
    Default.Handler handler3 = new Default.ConcreteHandler3();
    handler1.SetSuccessor(handler2);
    handler2.SetSuccessor(handler3);
    
    var requests = new[]{ 5, 12, 24 };

    Console.WriteLine("Array elements: 5, 12, 24");

    foreach (var request in requests)
    {
        handler1.HandleRequest(request);
    }

    Console.ReadLine();
}

執行結果

Array elements: 5, 12, 24
ConcreteHandler1 handled request 5
ConcreteHandler2 handled request 12
ConcreteHandler3 handled request 24

---
## 4. 情境
我們接到了一個員工請假簽核的需求
- 三天內(包含三天)**店長**同意即可
- 三到七天(包含七天)**店長 & 區主管**同意
- 超過七天則要**店長 & 區主管 & 總經理**同意
- 假單需依序簽核

請假簽核抽象類別

/// <summary>
/// 請假簽核抽象類別
/// </summary>
public abstract class Leave
{
    protected Leave successor;

    public void SetSuccessor(Leave successor)
    {
        this.successor = successor;
    }

    public abstract void TakeALeave(int days);
}

店長 & 區主管 & 總經理簽核實作

/// <summary>
/// 店長簽核實作
/// </summary>
public class StoreManagerApprove : Leave
{
    public override void TakeALeave(int days)
    {
        if (days <= 3)
        {
            Console.WriteLine($"Store Manager : OK~");
        }
        else if (successor != null)
        {
            Console.WriteLine($"Store Manager : OK~");
            successor.TakeALeave(days);
        }
    }
}

/// <summary>
/// 區主管簽核實作
/// </summary>
public class DistrictManagerApprove : Leave
{
    public override void TakeALeave(int days)
    {
        if (days <= 7)
        {
            Console.WriteLine($"District Manager : OK~");
        }
        else if (successor != null)
        {
            Console.WriteLine($"District Manager : OK~");
            successor.TakeALeave(days);
        }
    }
}

/// <summary>
/// 總經理簽核實作
/// </summary>
public class GeneralManagerApprove : Leave
{
    public override void TakeALeave(int days)
    {
        if (days > 7)
        {
            Console.WriteLine($"General Manager : OK~");
        }
    }
}

1. 建立請假流程`handler`
2. 經由`SetSuccessor`設置接續執行的主管
3. 依序傳入請假天數(1、5、10 天)

static void Main(string[] args)
{
    Situation.StoreManagerApprove storeManager = new Situation.StoreManagerApprove();
    Situation.DistrictManagerApprove districtManager = new Situation.DistrictManagerApprove();
    Situation.GeneralManagerApprove generalManager = new Situation.GeneralManagerApprove();
    storeManager.SetSuccessor(districtManager);
    districtManager.SetSuccessor(generalManager);
    
    Console.WriteLine($"請假 1 天");
    storeManager.TakeALeave(1);
    Console.WriteLine($"\n");
    
    Console.WriteLine($"請假 5 天");
    storeManager.TakeALeave(5);
    Console.WriteLine($"\n");
    
    Console.WriteLine($"請假 10 天");
    storeManager.TakeALeave(10);
    Console.WriteLine($"\n");

    Console.ReadLine();
}

執行結果

請假 1 天
Store Manager : OK~

請假 5 天
Store Manager : OK~
District Manager : OK~

請假 10 天
Store Manager : OK~
District Manager : OK~
General Manager : OK~

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## 完整程式碼
GitHub：[Behavioral_01_ChainOfResponsibility](https://github.com/darionnnnnn/blog/tree/master/Blog/Behavioral_01_ChainOfResponsibility)

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## 總結
### 本篇沒有將 abstract class 改為 interface，同樣是因為覺得 Chain of Responsibility 使用繼承的方式能使子類別更加簡潔，將接續處理的動作放在父類別，子類別只專注在處理邏輯上與決定是否需要呼叫接續者繼續執行。

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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