设计模式(19)---职责链模式

一、定义

职责链模式：使得多个对象都有机会处理请求，从而避免请求的发送者和接受者之间的耦合关系。将这个对象连成一条链，并沿着这条链传递该请求，直到有一个对象处理它为止。

解释：简单来说应该是对于一个请求，不同的人根据自己职责范围对它进行处理，同时也因为它一般是从底层向上层传递的请求的，所以将处理该请求的不同的对象比喻成一个链，从相对“低”的地方传递到相对“高”的地方，从而完成请求操作。

二、UML类图及基本代码

基本代码：

abstract class Handler
    {
        protected Handler successor;

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

        public abstract void HandleRequest(int request);
    }

    class ConcreteHandler1 : Handler
    {
        public override void HandleRequest(int request)
        {
            if (request >= 0 && request < 10)
            {
                Console.WriteLine("{0}处理请求{1}", this.GetType().Name, request);
            }
            else if (successor != null)
            {
                successor.HandleRequest(request);
            }
        }
    }

    class ConcreteHandler2 : Handler
    {
        public override void HandleRequest(int request)
        {
            if (request >= 10 && request < 20)
            {
                Console.WriteLine("{0}处理请求{1}", this.GetType().Name, request);
            }
            else if (successor != null)
            {
                successor.HandleRequest(request);
            }
        }
    }

    class ConcreteHandler3 : Handler
    {
        public override void HandleRequest(int request)
        {
            if (request >= 20 && request < 30)
            {
                Console.WriteLine("{0}处理请求{1}", this.GetType().Name, request);
            }
            else if (successor != null)
            {
                successor.HandleRequest(request);
            }
        }
    }

客户端调用及其结果：

Handler h1 = new ConcreteHandler1();
            Handler h2 = new ConcreteHandler2();
            Handler h3 = new ConcreteHandler3();
            h1.SetSuccessor(h2);
            h2.SetSuccessor(h3);

            int[] requests = { 6, 5, 14, 23, 18, 9, 17, 6, 26 };
            foreach (int request in requests)
            {
                h1.HandleRequest(request);
            }

三、具体实例

列举一个能更加说明职责链的实例。john在单位，向领导请假，小于等于2天经理有权利批准，小于等于5天总监有权利批准，其他的则要交到总经理那里批准。具体代码如下：

class Request
    {
        public string RequestType{get;set;}
        public string RequestContent{get;set;}
        public int Number{get;set;}
    }

    abstract class Manager
    {
        protected string name;
        protected Manager superior;

        public Manager(string name)
        {
            this.name = name;
        }

        public void SetSuperior(Manager superior)
        {
            this.superior = superior;
        }

        public abstract void RequestApplications(Request request);
    }

    class CommonManager : Manager
    {
        public CommonManager(string name)
            : base(name)
        { }

        public override void RequestApplications(Request request)
        {
            if (request.RequestType == "leave" && request.Number <= 2)
            {
                Console.WriteLine("{0}:{1} number is {2} ok", name, request.RequestContent, request.Number);
            }
            else
            {
                if (superior != null)
                {
                    superior.RequestApplications(request);
                }
            }
        }
    }

    class Majordome : Manager
    {
        public Majordome(string name)
            : base(name)
        { }

        public override void RequestApplications(Request request)
        {
            if (request.RequestType == "leave" && request.Number <= 5)
            {
                Console.WriteLine("{0}:{1} number is {2} ok", name, request.RequestContent, request.Number);
            }
            else
            {
                if (superior != null)
                {
                    superior.RequestApplications(request);
                }
            }
        }
    }

    class GeneralManager : Manager
    {
        public GeneralManager(string name)
            : base(name)
        { }

        public override void RequestApplications(Request request)
        {
            if (request.RequestType == "leave")
            {
                Console.WriteLine("{0}:{1} number is {2} ok", name, request.RequestContent, request.Number);
            }
            else if (request.RequestType == "add money" && request.Number <= 500)
            {
                Console.WriteLine("{0}:{1} number is {2} ok", name, request.RequestContent, request.Number);
            }
            else if (request.RequestType == "add money" && request.Number > 500)
            {
                Console.WriteLine("{0}:{1} number is {2} not ok", name, request.RequestContent, request.Number);
            }
        }
    }

客户端调用及结果：

CommonManager jinli = new CommonManager("经理");
            Majordome zongjian = new Majordome("总监");
            GeneralManager zhongjingli = new GeneralManager("总经理");
            jinli.SetSuperior(zongjian);
            zongjian.SetSuperior(zhongjingli);

            Request request = new Request();
            request.RequestType = "leave";
            request.RequestContent = "john ask for days";
            request.Number = 2;
            jinli.RequestApplications(request);

            Request request2 = new Request();
            request2.RequestType = "leave";
            request2.RequestContent = "john ask for days";
            request2.Number = 5;
            jinli.RequestApplications(request2);

四、优缺点及适用场景

优点：

1）降低了请求发送者和处理者之间的耦合。

2）把多个条件判定分散到各个处理类中，使得代码更加清晰，责任更加明确。

缺点：

1）在找到正确的处理对象之前，所有的条件判定都要执行一遍，当职责链过长时，可能会引起性能问题。

2）可能导致某个请求不被处理。

适用场景：

1）一个系统的请求需要多个对象进行审批才能完成的情况。

2）代码中有多个if-else时，也可以考虑使用职责链模式对代码进行重构。