即使参数是可选的，使用可编码数据解码时出错

Question

我有以下struct:

struct Recipe: Codable {
@DocumentID var id: String?
var minutes: Int?

init(id: String, minutes: Int) {

self.id = id
self.minutes = minutes
}

init(from decoder: Decoder) throws {

enum DecodingKeys: CodingKey {
case minutes
}

let container = try decoder.container(keyedBy: DecodingKeys.self)
minutes = try container.decode(Int.self, forKey: .minutes)
}
}

我从minutes为空的源进行解码，因此我得到以下错误消息：

Error parsing response data: keyNotFound(DecodingKeys(stringValue: "minutes", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key DecodingKeys(stringValue: "minutes", intValue: nil) ("minutes").", underlyingError: nil))

然而，我认为在struct Recipe中将minutes标记为可选是否足以处理这种情况？我还有什么需要实施的吗？

Answer 1

手动实现init(from:)时，需要对可选属性使用decodeIfPresent(_:forKey:)变体。如果JSON数据中没有可为null的字段，而decodeIfPresent(_:forKey:)方法只返回nil，则decode(_:forKey:)方法会抛出错误。

试试这个：

init(from decoder: Decoder) throws {
enum DecodingKeys: CodingKey {
case minutes
}
let container = try decoder.container(keyedBy: DecodingKeys.self)
minutes = try container.decodeIfPresent(Int.self, forKey: .minutes)
}