关于后端:行为型设计模式访问器-Visitor

简介

在不扭转原有类的前提下，能够新增不同的拜访器，每种拜访器都可自定义拜访策略。

如超市购物车里什么物品都有，顾客遍历拜访商品次要查看商品是否过期，包装是否残缺，收银员遍历商品次要查看价格。顾客和收银员能够了解为两种 Visitor，各自关注的重点就是 Visitor 中的 visit 办法实现逻辑。

原始数据构造就像数据库，而拜访器就像基于该数据库的业务逻辑，不同拜访器依照不同业务规定应用数据。

角色

• 形象 Visitor

  定义拜访每一种 Element 的办法签名 visit

• 具体 Visitor

  实现具体拜访办法 visit

• 形象 Element

  定义 accept 办法，反对将 visitor 做参数，通过调用 accept 办法，实现 accept→visitor→visit 的调用

• 具体 Element

  实现具体方法

类图

如图，Element 中的 accept 办法以 Visitor 做参数，其中调用 visitor.visit 办法，将主动权交给 Visitor 去解决。实现每新增一种 visitor，对原 Element 无感知

代码

interface Component
{public function accept(Visitor $visitor): void;
}

class ConcreteComponentA implements Component
{public function accept(Visitor $visitor): void
    {$visitor->visitConcreteComponentA($this);
    }

    public function exclusiveMethodOfConcreteComponentA(): string
    {return "A";}
}

class ConcreteComponentB implements Component
{public function accept(Visitor $visitor): void
    {$visitor->visitConcreteComponentB($this);
    }

    public function specialMethodOfConcreteComponentB(): string
    {return "B";}
}

interface Visitor
{public function visitConcreteComponentA(ConcreteComponentA $element): void;

    public function visitConcreteComponentB(ConcreteComponentB $element): void;
}

class ConcreteVisitor1 implements Visitor
{public function visitConcreteComponentA(ConcreteComponentA $element): void
    {echo $element->exclusiveMethodOfConcreteComponentA() . "+ ConcreteVisitor1\n";
    }

    public function visitConcreteComponentB(ConcreteComponentB $element): void
    {echo $element->specialMethodOfConcreteComponentB() . "+ ConcreteVisitor1\n";
    }
}

class ConcreteVisitor2 implements Visitor
{public function visitConcreteComponentA(ConcreteComponentA $element): void
    {echo $element->exclusiveMethodOfConcreteComponentA() . "+ ConcreteVisitor2\n";
    }

    public function visitConcreteComponentB(ConcreteComponentB $element): void
    {echo $element->specialMethodOfConcreteComponentB() . "+ ConcreteVisitor2\n";
    }
}

function clientCode(array $components, Visitor $visitor)
{foreach ($components as $component) {$component->accept($visitor);
    }
}

$components = [new ConcreteComponentA(),
    new ConcreteComponentB(),];

echo "The client code works with all visitors via the base Visitor interface:\n";
$visitor1 = new ConcreteVisitor1();
clientCode($components, $visitor1);

echo "It allows the same client code to work with different types of visitors:\n";
$visitor2 = new ConcreteVisitor2();
clientCode($components, $visitor2);

output:

The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1
It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2

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