如何使UIImage符合Codable？

Question

Swift 4有Codable，真棒。但UIImage默认不符合它。我们怎么做？

我试过singleValueContainer和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)
  }
}

我得到2个错误

1. 'required'初始值设定项必须直接在'UIImage'类中声明（不在扩展名中）
2. 一个不可用的初始化程序不能委托给用'init？'写的可用的初始化程序'init（data :)'

解决方法是使用包装器。但还有其他方法吗？

Answer 1

解决方案：滚动自己的符合Codable的包装类。

一个解决方案，因为UIImage的扩展已经出来，就是将图像包装在你拥有的新类中。否则，你的尝试基本上是直接的。我在Hyper Interactive的缓存框架中看到了这个很好的结果，叫做Cache。

虽然您需要访问库以深入了解依赖项，但您可以通过查看它们的ImageWrapper类来获得这个想法，该类是为了这样使用而构建的：

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)
  }
}

我很想听听你最终使用的是什么。

更新：事实证明OP写了我引用的代码（Swift 4.0更新到Cache）来解决问题。当然，这个代码应该在这里，但我也会把我的文字保留为未经编辑，因为它具有戏剧性的讽刺意味。 :)

Answer 2

您可以使用非常优雅的解决方案，使用KeyedDecodingContainer和KeyedEncodingContainer类的扩展：

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
        }
    }

}

这是一个用法示例：

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：您可以使用这种方式将Codable用于任何类型

Answer 3

传递UIImage的一种方法是将其转换为符合Codable的东西，比如String。

要在Qazxswpoi中将UIImage转换为String：

func encode(to encoder: Encoder) throws

要将字符串转换回

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

中的UIImage：

required init(from decoder: Decoder) throws

Answer 4

最恰当最简单的方法就是把它变成

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

而不是Data：

UIImage

反序列化：

public struct SomeImage: Codable {

    public let photo: Data

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