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? 访问者(行为型模式)

封装某些作用于某种数据结构中各元素的操作，它可以在不改变数据结构的前提下定义作用于这些元素的新的操作。

示例：

protocol PlanetVisitor {
   func visit(planet: PlanetAlderaan)
   func visit(planet: PlanetCoruscant)
   func visit(planet: PlanetTatooine)
    func visit(planet: MoonJedha)
}

protocol Planet {
   func accept(visitor: PlanetVisitor)
}

final class MoonJedha: Planet {
    func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

final class PlanetAlderaan: Planet {
    func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

final class PlanetCoruscant: Planet {
   func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

final class PlanetTatooine: Planet {
   func accept(visitor: PlanetVisitor) { visitor.visit(planet: self) }
}

final class NameVisitor: PlanetVisitor {
   var name = ""

   func visit(planet: PlanetAlderaan)  { name = "Alderaan" }
   func visit(planet: PlanetCoruscant) { name = "Coruscant" }
   func visit(planet: PlanetTatooine)  { name = "Tatooine" }
    func visit(planet: MoonJedha)      { name = "Jedha" }
}

用法

let planets: [Planet] = [PlanetAlderaan(), PlanetCoruscant(), PlanetTatooine(), MoonJedha()]

let names = planets.map { (planet: Planet) -> String in
   let visitor = NameVisitor()
    planet.accept(visitor: visitor)

    return visitor.name
}

names

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