关于后端:行为型设计模式责任链-Chain-Of-Responsibility

简介

使多个对象都有机会解决申请，从而防止申请的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该申请，直到有一个对象解决它为止。

角色

• Handler 接口

  定义解决办法签名，设置 nextHandler 办法

• Concrete Handler 具体类

  实现各自 handler 逻辑

• BaseHandler 封装一层 handler，可有可无

类图

如图，在 client 中，将 handler 一个个串起来，每个 handler 解决完后可决定是否向后传递。

代码

interface Handler
{public function setNext(Handler $handler): Handler;

    public function handle(string $request): string;
}

abstract class AbstractHandler implements Handler
{
    private $nextHandler;

    public function setNext(Handler $handler): Handler
    {
        $this->nextHandler = $handler;
        
        return $handler;
    }

    public function handle(string $request): string
    {if ($this->nextHandler) {return $this->nextHandler->handle($request);
        }

        return "";
    }
}

class MonkeyHandler extends AbstractHandler
{public function handle(string $request): string
    {if ($request === "Banana") {return "Monkey: I'll eat the ". $request .".\n";} else {return parent::handle($request);
        }
    }
}

class SquirrelHandler extends AbstractHandler
{public function handle(string $request): string
    {if ($request === "Nut") {return "Squirrel: I'll eat the ". $request .".\n";} else {return parent::handle($request);
        }
    }
}

class DogHandler extends AbstractHandler
{public function handle(string $request): string
    {if ($request === "MeatBall") {return "Dog: I'll eat the ". $request .".\n";} else {return parent::handle($request);
        }
    }
}

function clientCode(Handler $handler)
{foreach (["Nut", "Banana", "Cup of coffee"] as $food) {
        echo "Client: Who wants a" . $food . "?\n";
        $result = $handler->handle($food);
        if ($result) {echo " " . $result;} else {echo "" . $food ." was left untouched.\n";}
    }
}

$monkey = new MonkeyHandler();
$squirrel = new SquirrelHandler();
$dog = new DogHandler();

$monkey->setNext($squirrel)->setNext($dog);

echo "Chain: Monkey > Squirrel > Dog\n\n";
clientCode($monkey);

echo "\nSubchain: Squirrel > Dog\n\n";
clientCode($squirrel);

output:

Chain: Monkey > Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Monkey: I'll eat the Banana.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

Subchain: Squirrel > Dog

Client: Who wants a Nut?
  Squirrel: I'll eat the Nut.
Client: Who wants a Banana?
  Banana was left untouched.
Client: Who wants a Cup of coffee?
  Cup of coffee was left untouched.

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