Example of using custom JSONDecoder().keyDecodingStrategy

CustomKeyDecodingStrategy.swift

import Foundation

// What if we wait lowercased key, but suddenly has come uppercased? JSONDecoder().keyDecodingStrategy!

let json = """
{"NAME": "Vova"}
"""

struct Model: Decodable {
    let name: String
    
    enum CodingKeys: String, CodingKey {
        case name // wait lowercased key
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: CodingKeys.name)
    }
}

struct UppercasedKey: CodingKey {
    var intValue: Int?
    var stringValue: String
    
    init?(intValue: Int) {
        fatalError("init?(intValue:) has not been implemented")
    }
    
    init(stringValue: String) {
        self.stringValue = stringValue.lowercased()
    }
}

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .custom({ (keys) -> CodingKey in
    let key = keys.last!.stringValue
    return UppercasedKey(stringValue: key)
})

do {
    let result = try decoder.decode(Model.self, from: Data(json.utf8))
    print(result.name)
} catch {
    print(error.localizedDescription)
}