How to conform UIImage to Codable?

问题

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

'required' initializer must be declared directly in class 'UIImage' (not in an extension)
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?

回答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. :)

回答2:

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

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!
        if quality == .png {
            imageData = value.pngData()
        } else {
            imageData = value.jpegData(compressionQuality: quality.rawValue)
        }
        try encode(imageData, forKey: key)
    }

}

extension KeyedDecodingContainer {

    public 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 SDKError.imageConversionError
        }
    }

}

Here is an usage example:

class DocumentScan: Codable {

    private enum CodingKeys: String, CodingKey {
        case scanDate
        case image
    }

    let scanDate: Date
    let image: UIImage

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        scanDate = try container.decode(Date.self, forKey: .scanDate)
        image = try container.decode(UIImage.self, forKey: .image)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(scanDate, forKey: .scanDate)
        try container.encode(image, forKey: .image, quality: .png)
    }
}

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

回答3:

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)

回答4:

Properly most 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:

UIImage(data: instanceOfSomeImage.photo)!

来源：https://stackoverflow.com/questions/46197785/how-to-conform-uiimage-to-codable

标签

uiimage

swift4

codable