Decode a PascalCase JSON with JSONDecoder - 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