How to conform UIImage to Codable?

Question

Swift 4 has Codable and it's awesome. But UIImage does not conform to it by default. How can we do that?

I tried with singleValueContainer and unkeyedContainer

extension UIImage: Codable {
  // 'required' initializer must be declared directly in class 'UIImage' (not in an extension)
  public required init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    let data = try container.decode(Data.self)
    guard let image = UIImage(data: data) else {
      throw MyError.decodingFailed
    }

    // A non-failable initializer cannot delegate to failable initializer 'init(data:)' written with 'init?'
    self.init(data: data)
  }

  public func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    guard let data = UIImagePNGRepresentation(self) else {
      return
    }

    try container.encode(data)
  }
}

I get 2 errors

1. 'required' initializer must be declared directly in class 'UIImage' (not in an extension)
2. A non-failable initializer cannot delegate to failable initializer 'init(data:)' written with 'init?'

A workaround is to use wrapper. But are there any other ways?

Solution 1

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. :)

Solution 2

Properly the easiest way is to just make it Data instead of UIImage:

public struct SomeImage: Codable {

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

Deserialize the image:

UIImage(data: instanceOfSomeImage.photo)!

Solution 3

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

Solution 4

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)

Solution 5

The existing answers all appear to be incorrect. If you compare the deserialized image with the original, you will find they may well not be equal in any sense. This is because the answers are all throwing away the scale information.

You have to encode the image scale as well as its pngData(). Then when you decode the UIImage, combine the data with the scale by calling init(data:scale:).

View more solutions

Comments

[onmyway133]

thanks. Did you know I implemented that 😉 Please see the commits

[AmitaiB]

Ah, well I certainly know now! 😂 It hadn't occurred to me that the world could be that small. Lesson learned. Hmmm, doesn't that also mean that 'my' answer is the Accepted one, hmmm?

[onmyway133]

Hi, I would like to see if there's any clever solution than mine

[Axel Guilmin]

Data implements Codable, so you don't need to convert to a String ;) developer.apple.com/documentation/foundation/data/…

[freytag]

This is a great answer! +1 I would however change the ImageEncodingQuality enum to enum ImageType { case png; case jpeg(CGFloat) }

[Vitalii Gozhenko]

@freytag you are reading my mind, I already switched to custom enum for ImageType in my projects :-)

[Rob]

FWIW, I might suggest throwing DecodingError.dataCorruptedError(forKey:in:debugDescription:‌​), etc., instead.

[Jessy]

Pretty good! But you should use DecodingError and EncodingError.

[ChuckZHB]

Thanks for introducing this wrapper, work like a charm. I'd like to see how encoder/decoder's container works in this context.

[matt]

I believe that this is wrong. A UIImage is more than just its data. In particular, by throwing away the scale information, you can end up deserializing a very different image from what you serialized.

[matt]

See stackoverflow.com/a/68137443/341994 for an actual example of a codable image wrapper that passes the equality test.