简介
在不改变原有类的前提下，可以新增不同的访问器，每种访问器都可自定义访问策略。
如超市购物车里什么物品都有，顾客遍历访问商品主要查看商品是否过期，包装是否完整，收银员遍历商品主要查看价格。顾客和收银员可以理解为两种 Visitor，各自关注的重点就是 Visitor 中的 visit 方法实现逻辑。
原始数据结构就像数据库，而访问器就像基于该数据库的业务逻辑，不同访问器按照不同业务规则使用数据。
角色
类图
如图，Element 中的 accept 方法以 Visitor 做参数，其中调用 visitor.visit 方法，将主动权交给 Visitor 去处理。实现每新增一种 visitor，对原 Element 无感知
类图
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);
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output:
The client code works with all visitors via the base Visitor interface:A + ConcreteVisitor1B + ConcreteVisitor1It allows the same client code to work with different types of visitors:A + ConcreteVisitor2B + ConcreteVisitor2
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