How to conform UIImage to Codable?

You can use very elegant solution using extension for KeyedDecodingContainer and KeyedEncodingContainer classes:

enum ImageEncodingQuality {
  case png
  case jpeg(quality: CGFloat)
}

extension KeyedEncodingContainer {
  mutating func encode(
    _ value: UIImage,
    forKey key: KeyedEncodingContainer.Key,
    quality: ImageEncodingQuality = .png
  ) throws {
    let imageData: Data?
    switch quality {
    case .png:
      imageData = value.pngData()
    case .jpeg(let quality):
      imageData = value.jpegData(compressionQuality: quality)
    }
    guard let data = imageData else {
      throw EncodingError.invalidValue(
        value,
        EncodingError.Context(codingPath: [key], debugDescription: "Failed convert UIImage to data")
      )
    }
    try encode(data, forKey: key)
  }
}

extension KeyedDecodingContainer {
  func decode(
    _ type: UIImage.Type,
    forKey key: KeyedDecodingContainer.Key
  ) throws -> UIImage {
    let imageData = try decode(Data.self, forKey: key)
    if let image = UIImage(data: imageData) {
      return image
    } else {
      throw DecodingError.dataCorrupted(
        DecodingError.Context(codingPath: [key], debugDescription: "Failed load UIImage from decoded data")
      )
    }
  }
}

PS: You can use such way to adopt Codable to any class type

One way to pass an UIImage is to convert it to something that conforms to Codable, like String.

To convert the UIImage to String inside func encode(to encoder: Encoder) throws:

let imageData: Data = UIImagePNGRepresentation(image)!
let strBase64 = imageData.base64EncodedString(options: .lineLength64Characters)
try container.encode(strBase64, forKey: .image)

To convert the String back to UIImage inside required init(from decoder: Decoder) throws:

let strBase64: String = try values.decode(String.self, forKey: .image)
let dataDecoded: Data = Data(base64Encoded: strBase64, options: .ignoreUnknownCharacters)!
image = UIImage(data: dataDecoded)

Properly the easiest way is to just make the property Data instead of UIImage like this:

public struct SomeImage: Codable {

    public let photo: Data
    
    public init(photo: UIImage) {
        self.photo = photo.pngData()!
    }
}

Deserialize the image:

UIImage(data: instanceOfSomeImage.photo)!

A solution: roll your own wrapper class conforming to Codable.

One solution, since extensions to UIImage are out, is to wrap the image in a new class you own. Otherwise, your attempt is basically straight on. I saw this done beautifully in a caching framework by Hyper Interactive called, well, Cache.

Though you'll need to visit the library to drill down into the dependencies, you can get the idea from looking at their ImageWrapper class, which is built to be used like so:

let wrapper = ImageWrapper(image: starIconImage)
try? theCache.setObject(wrapper, forKey: "star")

let iconWrapper = try? theCache.object(ofType: ImageWrapper.self, forKey: "star")
let icon = iconWrapper.image

Here is their wrapper class:

// Swift 4.0
public struct ImageWrapper: Codable {
  public let image: Image

  public enum CodingKeys: String, CodingKey {
    case image
  }

  // Image is a standard UI/NSImage conditional typealias
  public init(image: Image) {
    self.image = image
  }

  public init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    let data = try container.decode(Data.self, forKey: CodingKeys.image)
    guard let image = Image(data: data) else {
      throw StorageError.decodingFailed
    }

    self.image = image
  }

  // cache_toData() wraps UIImagePNG/JPEGRepresentation around some conditional logic with some whipped cream and sprinkles.
  public func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    guard let data = image.cache_toData() else {
        throw StorageError.encodingFailed
    }

    try container.encode(data, forKey: CodingKeys.image)
  }
}

I'd love to hear what you end up using.

UPDATE: It turns out the OP wrote the code that I referenced (the Swift 4.0 update to Cache) to solve the problem. The code deserves to be up here, of course, but I'll also leave my words unedited for the dramatic irony of it all. :)