swift-codable-KeyedEncodingContainer.swift

enum ImageEncodingQuality: CGFloat {
    case png = 0
    case jpegLow = 0.2
    case jpegMid = 0.5
    case jpegHigh = 0.75
}

extension KeyedEncodingContainer {
    
    mutating func encode(_ value: UIImage,
                         forKey key: KeyedEncodingContainer.Key,
                         quality: ImageEncodingQuality = .png) throws {
        
        var imageData: Data?
        let prefix: String
        if quality == .png {
            imageData = value.pngData()
            prefix = "data:image/png;base64,"
        } else {
            imageData = value.jpegData(compressionQuality: quality.rawValue)
            prefix = "data:image/jpeg;base64,"
        }
        
        guard let base64String = imageData?.base64EncodedString(options: .lineLength64Characters) else {
            throw EncodingError.invalidValue(value, EncodingError.Context(codingPath: codingPath, debugDescription: "Can't encode image to base64 string."))
        }
        
        try encode(prefix + base64String, forKey: key)
    }
    
}

extension KeyedDecodingContainer {
    
    public func decode(_ type: UIImage.Type, forKey key: KeyedDecodingContainer.Key) throws -> UIImage {
        let base64String = try decode(String.self, forKey: key)
        
        // data:image/svg+xml;base64,PD.....
        let components = base64String.split(separator: ",")
    
        if components.count != 2 {
            throw DecodingError.typeMismatch(type, DecodingError.Context(codingPath: codingPath, debugDescription: "Unsupported format"))
        }
        
        let dataString = String(components[1])
        if let dataDecoded = Data(base64Encoded: dataString, options: .ignoreUnknownCharacters), let image = UIImage(data: dataDecoded) {
            return image
        } else {
            throw DecodingError.typeMismatch(type, DecodingError.Context(codingPath: codingPath, debugDescription: "Unsupported format"))
        }
    }
    
}