使用 JSONDecoder 解码 PascalCase JSON

Question

I need to decode a JSON with uppercased first letters (aka PascalCase or UppperCamelCase) like this :

{
    "Title": "example",
    "Items": [
      "hello",
      "world"
    ]
}

So I created a model conforming to Codable :

struct Model: Codable {
    let title: String
    let items: [String]
}

But JSONDecoder raises an error because the case is different.

Swift.DecodingError.keyNotFound(CodingKeys(stringValue: "title", intValue: nil), Swift.DecodingError.Context(codingPath: [], debugDescription: "No value associated with key CodingKeys(stringValue: \"title\", intValue: nil) (\"title\").", underlyingError: nil))

I would like to keep my model's properties in camelCase but I can't change the JSON fomat.

Answer 1

A nice solution I found is to create a KeyDecodingStrategy similar to the .convertFromSnakeCase available in Foundation.

extension JSONDecoder.KeyDecodingStrategy {
    static var convertFromPascalCase: JSONDecoder.KeyDecodingStrategy {
        return .custom { keys -> CodingKey in
            // keys array is never empty
            let key = keys.last!
            // Do not change the key for an array
            guard key.intValue == nil else {
                return key
            }

            let codingKeyType = type(of: key)
            let newStringValue = key.stringValue.firstCharLowercased()

            return codingKeyType.init(stringValue: newStringValue)!
        }
    }
}

private extension String {
    func firstCharLowercased() -> String {
        prefix(1).lowercased() + dropFirst()
    }
}

It can be used easily like so :

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromPascalCase
let model = try! decoder.decode(Model.self, from: json)

Full example on Gist