Swift利用Decodable解析JSON的一个小问题详解

//: Playground
import Foundation

//Book.swift
struct Book: Codable {
 var id: Int
 var name: String
 var cover: BookCover?

 enum CodingKeys: String, CodingKey {
 case id
 case name
 case cover
 }

 struct BookCover: Codable {

 var url: String
 var thumbURL: String

 enum CodingKeys: String, CodingKey {
 case url
 case thumbURL = "thumb_url"
 }
 }
}

//JSON
let bookJSON1 =
"""
{
 "id": 1,
 "name": "fake name 1",
 "cover": {
 "url": "a.png",
 "thumb_url": "b.png"
 }
}
"""

let bookJSON2 =
"""
{
 "id": 2,
 "name": "fake name 2",
 "cover": {

 }
}
"""

//解析
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let book1 = try? decoder.decode(Book.self, from: bookJSON1.data(using: .utf8)!)

// 解析正常
print(book1)
let book2 = try? decoder.decode(Book.self, from: bookJSON2.data(using: .utf8)!)
// 输出 nil，cover已经是 Optional，为何整个book都解析失败？
print(book2)

extension KeyedDecodingContainer {
 public func decodeIfPresent<T>(_ type: T.Type, forKey key: K) throws -> T? where T : Decodable {
 return try? decode(type, forKey: key)
 }
}