? 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