设计模式（13）责任链模式

模式介绍

责任链设计模式试图通过给不止一个对象处理特定请求的机会，来避免将请求耦合到特定接收器。
本质上，我们沿着该对象的潜在处理程序的“链”传递对象，直到其中一个处理程序处理请求。

示例

我们还是以餐馆建模。
想一下，当厨房需要采购新设备，厨师长需要像Boss请示。

我们的店里的采购流程是这样的：

1. 1000元以内，厨师长自己就可以做决定
2. 1000-2500元，必须报采购经理审批
3. 2500-10000元，必须报总经理审批
4. 超过10000元，总经理需要开个会议决定了

我们可以看得出来层级结构：厨师长-->采购经理-->总经理
下面我们就使用该模式进行采购系统的实现：

采购单：

/// <summary>
/// The details of the purchase request.  
/// </summary>
class PurchaseOrder
{

    // Constructor
    public PurchaseOrder(int number, double amount, double price, string name)
    {
        RequestNumber = number;
        Amount = amount;
        Price = price;
        Name = name;

        Console.WriteLine("Purchase request for " + name + " (" + amount + " for $" + price.ToString() + ") has been submitted.");
    }

    public int RequestNumber { get; set; }
    public double Amount { get; set; }
    public double Price { get; set; }
    public string Name { get; set; }
}

批准人（抽象类）：

/// <summary>
/// The Handler abstract class.  Every class which inherits from this will be responsible for a kind of request for the restaurant.
/// </summary>
abstract class Approver
{
    protected Approver Supervisor;

    public void SetSupervisor(Approver supervisor)
    {
        this.Supervisor = supervisor;
    }

    public abstract void ProcessRequest(PurchaseOrder purchase);
}

具体的批准人：

/// <summary>
/// A concrete Handler class
/// </summary>
class HeadChef : Approver
{
    public override void ProcessRequest(PurchaseOrder purchase)
    {
        if (purchase.Price < 1000)
        {
            Console.WriteLine("{0} approved purchase request #{1}",
                this.GetType().Name, purchase.RequestNumber);
        }
        else if (Supervisor != null)
        {
            Supervisor.ProcessRequest(purchase);
        }
    }
}

/// <summary>
/// A concrete Handler class
/// </summary>
class PurchasingManager : Approver
{
    public override void ProcessRequest(PurchaseOrder purchase)
    {
        if (purchase.Price < 2500)
        {
            Console.WriteLine("{0} approved purchase request #{1}",
                this.GetType().Name, purchase.RequestNumber);
        }
        else if (Supervisor != null)
        {
            Supervisor.ProcessRequest(purchase);
        }
    }
}

/// <summary>
/// A concrete Handler class
/// </summary>
class GeneralManager : Approver
{
    public override void ProcessRequest(PurchaseOrder purchase)
    {
        if (purchase.Price < 10000)
        {
            Console.WriteLine("{0} approved purchase request #{1}",
                this.GetType().Name, purchase.RequestNumber);
        }
        else
        {
            Console.WriteLine(
                "Purchase request #{0} requires an executive meeting!",
                purchase.RequestNumber);
        }
    }
}

客户端调用：

static void Main(string[] args)
{
    //Create the chain links
    Approver jennifer = new HeadChef();
    Approver mitchell = new PurchasingManager();
    Approver olivia = new GeneralManager();

    //Create the chain
    jennifer.SetSupervisor(mitchell);
    mitchell.SetSupervisor(olivia);

    // Generate and process purchase requests
    PurchaseOrder p = new PurchaseOrder(1, 20, 69, "Spices");
    jennifer.ProcessRequest(p);

    p = new PurchaseOrder(2, 300, 1389, "Fresh Veggies");
    jennifer.ProcessRequest(p);

    p = new PurchaseOrder(3, 500, 4823.99, "Beef");
    jennifer.ProcessRequest(p);

    p = new PurchaseOrder(4, 4, 12099, "Ovens");
    jennifer.ProcessRequest(p);

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

总结

责任链模式允许链中的多个对象在处理请求对象时进行传递。请求流经链，直到链中的一个环节处理它。

源代码

https://github.com/exceptionnotfound/DesignPatterns/tree/master/ChainOfResponsibility

原文

https://www.exceptionnotfound.net/chain-of-responsibility-the-daily-design-pattern/