Strategy for decoding UpperCamelCase JSONs to preferred lowerCamelCase version.

convertFromUpperCamelCase.swift

public extension JSONDecoder.KeyDecodingStrategy {
    /// Convert from "UpperCamelCaseKeys" to "lowerCamelCaseKeys" before attempting to match a key with the one specified by each type.
    ///
    /// The conversion to upper case uses `Locale.system`, also known as the ICU "root" locale. This means the result is consistent regardless of the current user's locale and language preferences.
    ///
    /// - Copyright: This function is based on this [source](https://gist.github.com/MojtabaHs/1f9a0f19bafaecd0c94f585648244303)
    static var convertFromUpperCamelCase: Self {
        .custom { keys in
            /// - Precondition: `Keys` array must never be empty.
            guard let key = keys.last else { fatalError("Keys should not be empty") }
            /// - Important: `Integer key`means that the json is a first level array. Do not change the key for an array!
            guard key.intValue == nil else { return key }
            
            let codingKeyType = type(of: key)
            let lowerCamelCased = key.stringValue.lowercased(first: 1)
            
            /// - Precondition: There should be no issues to initialize a new coding key.
            guard let transformedKey = codingKeyType.init(stringValue: lowerCamelCased) else {
                fatalError("Could not initialize a coding key for \"\(lowerCamelCased)\"")
            }
            return transformedKey
        }
    }
}

public extension String {
    func lowercased(first count: Int) -> String {
        prefix(count).lowercased() + dropFirst(count)
    }
}

// USAGE:
/*
let jsonDecoder: JSONDecoder = {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromUpperCamelCase
    return decoder
}()
*/