Advance json parsing pt1


My notes on adding Codable support for classes such as UIColor and UIFont which doesn’t support Codable by default This example only touches on Decodable, but the same thing can be done to make the class Encodable.

Theme.json

{
   "color":"white",
    "font":{"name":".SFUIText","size":"16"}
}

Theme.swift

/**
 * A simple struct that holds UIColor and UIFont
 */
struct Theme:Decodable{
    enum CodingKeys: String, CodingKey {/*CodingKeys are required when you want to customize your json parsing*/
        case color
        case font
    }
    let color:UIColor
    let font:UIFont
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        color = try container.decode(key: .color, transformer: ColorTransformer())
        font = try container.decode(key: .font, transformer: UIFontTransformer())
    }
}

DecodingTypes.swift

/**
 * The transformer that handles parsing the string value to UIColor
 * TODO: ⚠️️ Add support for 0x00FF00FF (Aka hex with alpha value)
 * EXAMPLE: "color":"white"
 */
class ColorTransformer: DecodingContainerTransformer {
    enum Error: Swift.Error { case cannotCreateColor(hex: String) }
    func decode(input: String) throws -> UIColor {
        guard let color = try? ColorUtils.color(input) else {
            throw Error.cannotCreateColor(hex: input)
        }
        return color
    }
}
/**
 * The transformer that handles parsing the dictionary (Aka fontName and fontSize) value to UIColor
 * EXAMPLE: "font":{"name":".SFUIText","size":"16"}
 */
class UIFontTransformer: DecodingContainerTransformer {
    enum Error: Swift.Error {  case cannotCreateFont(name: [String:String]) }
    func decode(input:[String:String]) throws -> UIFont {
        guard let name = input["name"], let fontSize = input["size"], let fontSizeAsDouble = Double(fontSize), let font = UIFont.init(name: name, size: CGFloat(fontSizeAsDouble)) else {
            throw Error.cannotCreateFont(name: input)
        }
        return font
    }
}

DecodingHelpers.swift

/**
 * Makes the code more reusable/modular
 */
public protocol DecodingContainerTransformer {
    associatedtype DecodingInput
    associatedtype DecodingOutput
    func decode(input: DecodingInput) throws -> DecodingOutput
}
/**
 * Makes the code more reusable/modular
 */
extension KeyedDecodingContainer {
    public func decode<Transformer: DecodingContainerTransformer>(key: Key, transformer: Transformer) throws -> Transformer.DecodingOutput where Transformer.DecodingInput: Decodable {
        return try transformer.decode(input: try decode(Transformer.DecodingInput.self, forKey: key))
    }
    public func decodeIfPresent<Transformer: DecodingContainerTransformer>(key: Key, transformer: Transformer) throws -> Transformer.DecodingOutput? where Transformer.DecodingInput: Decodable {
        return try decodeIfPresent(Transformer.DecodingInput.self, forKey: key).map(transformer.decode)
    }
}

AppDelegate.swift

let urlStr:String = Bundle.main.resourcePath!+"/theme.json"
guard let data:Data = FileParser.data(urlStr) else {fatalError("wrong file path")}
guard let theme:Theme = try? decode(data: data) else {fatalError("can't be converted json to Theme")}
Swift.print("theme.color:  \(theme.color)")//UIExtendedSRGBColorSpace 1 1 1 1 
Swift.print("theme.font:  \(theme.font)")//<UICTFont: 0x7faef7c02850> font-family: ".SFUIText"; font-weight: normal; font-style: normal; font-size: 16.00pt

Conclusion

By using This technique you can make any class Decodable with out resorting to wrappers or shadow DataContainers etc. Thanks to the swift-lang slack community for the help coming up with this solution. The supporting classes in this article can be found here https://github.com/eonist/swift-utils