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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