rawValue 用于swift中的enum(枚舉)禾锤，用于取枚舉項(xiàng)的原始值蜻懦，例如：
enum Category: String, CaseIterable, Codable, Hashable {
? ? ? ? case featured = "Featured"
? ? ? ? case lakes = "Lakes"
? ? ? ? case rivers = "Rivers"
? ? ? ? case mountains = "Mountains"
? ? }
Category.lakes.rawValue 就是 "Lakes"
