行为型设计模式-责任链 Chain Of Responsibility

简介

使多个对象都有机会处理请求，从而避免请求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请求，直到有一个对象处理它为止。

角色

• Handler 接口

  定义处理方法签名，设置nextHandler方法

• 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 处理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.