我Codable第一次使用Swift 4的协议，我无法理解decodeIfPresentfrom的使用Decodable。
Codable
decodeIfPresent
Decodable
/// Decodes a value of the given type for the given key, if present. /// /// This method returns `nil` if the container does not have a value associated with `key`, or if the value is null. The difference between these states can be distinguished with a `contains(_:)` call. /// /// - parameter type: The type of value to decode. /// - parameter key: The key that the decoded value is associated with. /// - returns: A decoded value of the requested type, or `nil` if the `Decoder` does not have an entry associated with the given key, or if the value is a null value. /// - throws: `DecodingError.typeMismatch` if the encountered encoded value is not convertible to the requested type. public func decodeIfPresent(_ type: String.Type, forKey key: KeyedDecodingContainer.Key) throws -> String?
nil如果没有关联的键值，则建议返回。如果这是唯一的原因，那么它与可选属性有何不同，因为可选变量也设置为nil如果响应中不存在值。
nil
这两行代码之间有一个细微但重要的区别：
// Exhibit 1 foo = try container.decode(Int?.self, forKey: .foo) // Exhibit 2 foo = try container.decodeIfPresent(Int.self, forKey: .foo)
图表1将解析：
{ "foo": null, "bar": "something" }
但 不是 ：
{ "bar": "something" }
而展览2将很高兴地解析两者。因此，在正常的JSON解析器用例中，您需要decodeIfPresent模型中的每个可选项。
JSON
