如何使 ObservableObject 符合 Codable 协议？

Question

在 SwiftUI beta 5 中，Apple 引入了 @Published 注解。此注释当前阻止此类符合 Codable 协议。

我怎样才能遵守这些协议，以便我可以将这个类编码和解码为 JSON？您现在可以忽略 image 属性。

class Meal: ObservableObject, Identifiable, Codable {

    enum CodingKeys: String, CodingKey {
        case id
        case name
        case ingredients
        case numberOfPeople
    }

    var id = Globals.generateRandomId()
    @Published var name: String = "" { didSet { isInputValid() } }
    @Published var image = Image("addImage")
    @Published var ingredients: [Ingredient] = [] { didSet { isInputValid() } }
    @Published var numberOfPeople: Int = 2
    @Published var validInput = false

    func isInputValid() {
        if name != "" && ingredients.count > 0 {
            validInput = true
        }
    }
}

Answer 1

将init() 和encode() 方法添加到您的类中：

required init(from decoder: Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)

    id = try values.decode(Int.self, forKey: .id)
    name = try values.decode(String.self, forKey: .name)
    ingredients = try values.decode([Ingredient].self, forKey: .ingredients)
    numberOfPeople = try values.decode(Int.self, forKey: .numberOfPeople)
}

func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(id, forKey: .id)
    try container.encode(name, forKey: .name)
    try container.encode(ingredients, forKey: .ingredients)
    try container.encode(numberOfPeople, forKey: .numberOfPeople)
}

Answer 2

经过多次修改后，我设法将 Codable 直接添加到 @Published

注意我必须为 iOS14 更新这个。这说明了挖掘未记录类型的危险......

只需将以下代码添加到文件中，您的 @Published 变量将自动可编码（前提是它们基于可编码类型）

更多信息在这里 https://blog.hobbyistsoftware.com/2020/01/adding-codeable-to-published/

代码在这里：

import Foundation
import SwiftUI

extension Published:Decodable where Value:Decodable {
    public init(from decoder: Decoder) throws {
        let decoded = try Value(from:decoder)
        self = Published(initialValue:decoded)
    }
}

 extension Published:Encodable where Value:Decodable {

    private var valueChild:Any? {
        let mirror = Mirror(reflecting: self)
        if let valueChild = mirror.descendant("value") {
            return valueChild
        }
        
        //iOS 14 does things differently...
        if let valueChild = mirror.descendant("storage","value") {
            return valueChild
        }
        
        //iOS 14 does this too...
        if let valueChild = mirror.descendant("storage","publisher","subject","currentValue") {
            return valueChild
        }

        return nil
    }
   
    public func encode(to encoder: Encoder) throws {
        
        guard let valueChild = valueChild else {
            fatalError("Mirror Mirror on the wall - why no value y'all : \(self)")
        }
        
        if let value = valueChild.value as? Encodable {
            do {
                try value.encode(to: encoder)
                return
            } catch let error {
                assertionFailure("Failed encoding: \(self) - \(error)")
            }
        }
        else {
            assertionFailure("Decodable Value not decodable. Odd \(self)")
        }
    }
}

Answer 3

困惑的沃隆有正确的想法！我正在尝试与他们合作以使某些东西变得强大。我相信这应该是你所需要的，从 Xcode 12 构建。

Published 的 storage 和 Published.Publisher's subject 是私有 API，因此镜像是在需要的地方挖掘的最佳选择：

import struct Combine.Published

extension Published: Encodable where Value: Encodable {
  public func encode(to encoder: Encoder) throws {
    guard
      let storageValue =
        Mirror(reflecting: self).descendant("storage")
        .map(Mirror.init)?.children.first?.value,
      let value =
        storageValue as? Value
        ??
        (storageValue as? Publisher).map(Mirror.init)?
        .descendant("subject", "currentValue")
        as? Value
    else { throw EncodingError.invalidValue(self, codingPath: encoder.codingPath) }
    
    try value.encode(to: encoder)
  }
}

extension Published: Decodable where Value: Decodable {
  public init(from decoder: Decoder) throws {
    self.init(
      initialValue: try .init(from: decoder)
    )
  }
}

extension EncodingError {
  /// `invalidValue` without having to pass a `Context` as an argument.
  static func invalidValue(
    _ value: Any,
    codingPath: [CodingKey],
    debugDescription: String = .init()
  ) -> Self {
    .invalidValue(
      value,
      .init(
        codingPath: codingPath,
        debugDescription: debugDescription
      )
    )
  }
}

Answer 4

没有Mirror的更高效的变体

已发布+Value.swift

private class PublishedWrapper<T> {
    @Published private(set) var value: T

    init(_ value: Published<T>) {
        _value = value
    }
}

extension Published {
    var unofficialValue: Value {
        PublishedWrapper(self).value
    }
}

已发布+Codable.swift

extension Published: Decodable where Value: Decodable {
    public init(from decoder: Decoder) throws {
        self.init(wrappedValue: try .init(from: decoder))
    }
}

extension Published: Encodable where Value: Encodable {
    public func encode(to encoder: Encoder) throws {
        try unofficialValue.encode(to: encoder)
    }
}