设计模式 - 21）职责链模式

graph LR client--调用-->Handler ConcreteHandlerA--继承-->Handler ConcreteHandlerB--继承-->Handler Handler--引用-->Handler

当 Handler 处理请求时，可以选择自己处理或调用继任者处理该请求。

abstract class Handler
{
    protected Handler _successor;

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

    public abstract void HandlerRequest(int request);
}

class ConcreteHandlerA : Handler
{
    public override void HandlerRequest(int request)
    {
        if (request < 10)
        {
            Console.WriteLine(string.Format("{0} handle thing which is {1}", this.GetType().Name, request));
        }
        else if (_successor != null)
        {
            Console.WriteLine(string.Format("{0} cannot handle thing which is {1}", this.GetType().Name, request));
            _successor.HandlerRequest(request);
        }
    }
}

class ConcreteHandlerB : Handler
{
    public override void HandlerRequest(int request)
    {
        if (request < 20)
        {
            Console.WriteLine(string.Format("{0} handle thing which is {1}", this.GetType().Name, request));
        }
        else if (_successor != null)
        {
            Console.WriteLine(string.Format("{0} cannot handle thing which is {1}", this.GetType().Name, request));
            _successor.HandlerRequest(request);
        }
    }
}

class ConcreteHandlerC : Handler
{
    public override void HandlerRequest(int request)
    {
        if (request < 30)
        {
            Console.WriteLine(string.Format("{0} handler thing which is {1}", this.GetType().Name, request));
        }
        else
        {
            Console.WriteLine(string.Format("{0} say {1} 太高了，再说吧", this.GetType().Name, request));
        }
    }
}

class Boy : Handler
{
    public override void HandlerRequest(int request)
    {
        _successor.HandlerRequest(request);
    }
}


// 业务代码：
ConcreteHandlerC c = new ConcreteHandlerC();
oncreteHandlerB b = new ConcreteHandlerB();
b.SetSuccessor(c);

ConcreteHandlerA a = new ConcreteHandlerA();
a.SetSuccessor(b);

int[] requests = {5, 15, 25, 35};
foreach (int res in requests)
{
    boy.HandlerRequest(res);
}

output:
ConcreteHandlerA handle thing which is 5

ConcreteHandlerA cannot handle thing which is 15
ConcreteHandlerB handle thing which is 15

ConcreteHandlerA cannot handle thing which is 25
ConcreteHandlerB cannot handle thing which is 25
ConcreteHandlerC handler thing which is 25

ConcreteHandlerA cannot handle thing which is 35
ConcreteHandlerB cannot handle thing which is 35
ConcreteHandlerC say 35 太高了，再说吧