Swift可解码可选键

Question

(这是此问题的后续跟进:使用具有多个键的可解码协议.)

我有以下Swift代码:

let additionalInfo = try values.nestedContainer(keyedBy: UserInfoKeys.self, forKey: .age)
age = try additionalInfo.decodeIfPresent(Int.self, forKey: .realage)

我知道如果我使用decodeIfPresent并且没有该属性,如果它是一个可选变量,它仍将正确处理它.

例如,以下JSON使用上面的代码解析它.

{
    "firstname": "Test",
    "lastname": "User",
    "age": {"realage": 29}
}

以下JSON也适用.

{
    "firstname": "Test",
    "lastname": "User",
    "age": {"notrealage": 30}
}

但以下不起作用.

{
    "firstname": "Test",
    "lastname": "User"
}

我怎样才能使所有3个例子都有效？是否有类似的东西decodeIfPresent了nestedContainer？

Answer 1

您可以使用以下KeyedDecodingContainer功能:

func contains(_ key: KeyedDecodingContainer.Key) -> Bool

返回一个Bool值,指示解码器是否包含与给定键关联的值.与给定密钥相关联的值可以是适合于数据格式的空值.

例如,在请求相应的嵌套容器之前检查"age"密钥是否存在:

struct Person: Decodable {
    let firstName, lastName: String
    let age: Int?

    enum CodingKeys: String, CodingKey {
        case firstName = "firstname"
        case lastName = "lastname"
        case age
    }

    enum AgeKeys: String, CodingKey {
        case realAge = "realage"
        case fakeAge = "fakeage"
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        self.firstName = try values.decode(String.self, forKey: .firstName)
        self.lastName = try values.decode(String.self, forKey: .lastName)

        if values.contains(.age) {
            let age = try values.nestedContainer(keyedBy: AgeKeys.self, forKey: .age)
            self.age = try age.decodeIfPresent(Int.self, forKey: .realAge)
        } else {
            self.age = nil
        }
    }
}

Answer 2

我遇到了这个问题，我找到了这个解决方案，以防万一对其他人有帮助：

let ageContainer = try? values.nestedContainer(keyedBy: AgeKeys.self, forKey: .age)
self.age = try ageContainer?.decodeIfPresent(Int.self, forKey: .realAge)

如果你有一个可选的容器， usingtry? values.nestedContainer(keyedBy:forKey)你不需要使用contains(.