KeyDecodingStrategy_convertFromUpperCamelCase.swift

struct Address : Codable {
    var street: String
    var zipCode: String
    var city: String
    var state: String
}

struct AnyCodingKey : CodingKey {
    
    var stringValue: String
    var intValue: Int?
    
    init(_ base: CodingKey) {
        self.init(stringValue: base.stringValue, intValue: base.intValue)
    }
    
    init(stringValue: String) {
        self.stringValue = stringValue
    }
    
    init(intValue: Int) {
        self.stringValue = "\(intValue)"
        self.intValue = intValue
    }
    
    init(stringValue: String, intValue: Int?) {
        self.stringValue = stringValue
        self.intValue = intValue
    }
}

extension JSONEncoder.KeyEncodingStrategy {
    
    static var convertToUpperCamelCase: JSONEncoder.KeyEncodingStrategy {
        return .custom { codingKeys in
            
            var key = AnyCodingKey(codingKeys.last!)
            
            // uppercase first letter
            if let firstChar = key.stringValue.first {
                let i = key.stringValue.startIndex
                key.stringValue.replaceSubrange(
                    i ... i, with: String(firstChar).uppercased()
                )
            }
            return key
        }
    }
}

extension JSONDecoder.KeyDecodingStrategy {
    
    static var convertFromUpperCamelCase: JSONDecoder.KeyDecodingStrategy {
        return .custom { codingKeys in
            
            var key = AnyCodingKey(codingKeys.last!)
            
            // lowercase first letter
            if let firstChar = key.stringValue.first {
                let i = key.stringValue.startIndex
                key.stringValue.replaceSubrange(
                    i ... i, with: String(firstChar).lowercased()
                )
            }
            return key
        }
    }
}

let address = Address(street: "Apple Bay Street", zipCode: "94608",
                     city: "Emeryville", state: "California")

do {
    let encoder = JSONEncoder()
    encoder.keyEncodingStrategy = .convertToUpperCamelCase
    let encoded = try encoder.encode(address)
    print(String(decoding: encoded, as: UTF8.self))
} catch {
    print(error)
}

let jsonString = """
{"Street":"Apple Bay Street","City":"Emeryville","State":"California","ZipCode":"94608"}
"""

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromUpperCamelCase
    let decoded = try decoder.decode(Address.self, from: Data(jsonString.utf8))
    print(decoded)
} catch {
    print(error)
}

// Address(street: "Apple Bay Street", zipCode: "94608",
// city: "Emeryville", state: "California")

Thanks brother for this awesome solution

Hey hachinobu 👋

We would like to use your code for a commercial product, but we couldn't identify any license information. Can you please tell us the terms of use (e.g. commercial use, distribution, modification etc.) or under which license we can use this snippet?

Thank you very much and kind regards.

Roman