Swift Json实例详细解析

{
 "userId": 1,
 "name": "Jack",
 "height": 1.7,
}

struct User {
 var userId: Int?
 var name: String?
 var height: CGFloat?
}

struct User {
 ...
 init?(json: [String: Any]) {
  guard let userId = json["userId"] as? Int,
  let name = json["name"] as? String,
  let height = json["height"] as? CGFloat else { return nil }
  self.userId = userId
  self.name = name
  self.height = height
 }
}

struct User {
 ...
 init?(json: [String: Any]) {
  guard let userId = json["userId"] as? Int,
  let name = json["name"] as? String else { return nil }
  self.userId = userId
  self.name = name
  self.height = json["height"] as? CGFloat
 }
}

struct User: Decodable {
 var userId: Int?
 var name: String?
 var height: CGFloat?
}

let decoder = JSONDecoder()
if let data = jsonString.data(using: String.Encoding.utf8) {
 let user = try? decoder.decode(User.self, from: data)
}

/// A type that can decode itself from an external representation.
public protocol Decodable {
 /// Creates a new instance by decoding from the given decoder.
 ///
 /// This initializer throws an error if reading from the decoder fails, or
 /// if the data read is corrupted or otherwise invalid.
 ///
 /// - Parameter decoder: The decoder to read data from.
 public init(from decoder: Decoder) throws
}

struct User: Decodable {
 var userId: Int?
 var name: String?
 var height: CGFloat? 
 enum CodingKeys: String, CodingKey {
  case userId = "user_id"
  case name
  case height
 }
}

open class JSONDecoder {
 /// The strategy to use for decoding `Date` values.
 public enum DateDecodingStrategy {

  /// Defer to `Date` for decoding. This is the default strategy.
  case deferredToDate

  /// Decode the `Date` as a UNIX timestamp from a JSON number.
  case secondsSince1970

  /// Decode the `Date` as UNIX millisecond timestamp from a JSON number.
  case millisecondsSince1970

  /// Decode the `Date` as an ISO-8601-formatted string (in RFC 3339 format).
  case iso8601

  /// Decode the `Date` as a string parsed by the given formatter.
  case formatted(DateFormatter)

  /// Decode the `Date` as a custom value decoded by the given closure.
  case custom((Decoder) throws -> Date)
 }
 ......
 /// The strategy to use in decoding dates. Defaults to `.deferredToDate`.
 open var dateDecodingStrategy: JSONDecoder.DateDecodingStrategy
}

// 入口方法
JSONDecoder decoder(type:Type data:Data) 
 // 内部类，真实用来解析的
 _JSONDecoder unbox(value:Any type:Type) 
 // Model调用init方法
 Decodable init(decoder: Decoder) 
 // 自动生成的init方法调用container
 Decoder container(keyedBy:CodingKeys) 
 // 解析的容器
 KeyedDecodingContainer decoderIfPresent(type:Type) or decode(type:Type)
  // 内部类，循环调用unbox
  _JSONDecoder unbox(value:Any type:Type)
  ...循环，直到基本类型

extension KeyedDecodingContainer {
 .......
 /// Decode (Int, String) -> Int if possiable
 public func decodeIfPresent(_ type: Int.Type, forKey key: K) throws -> Int? {
  if let value = try? decode(type, forKey: key) {
   return value
  }
  if let value = try? decode(String.self, forKey: key) {
   return Int(value)
  }
  return nil
 }
 
 .......
 
 /// Avoid the failure just when decoding type of Dictionary, Array, SubModel failed
 public func decodeIfPresent<T>(_ type: T.Type, forKey key: K) throws -> T? where T : Decodable {
  return try? decode(type, forKey: key)
 }
}