多态持久性似乎被设计破坏了。
错误报告SR-5331引用了他们在雷达上得到的响应。
与现有的 NSCoding API (NSKeyedArchiver) 不同，为了灵活性和安全性，新的 Swift 4 Codable 实现不会将有关编码类型的类型信息写入生成的档案中。因此，在解码时，API 只能使用您提供的具体类型来解码值（在您的情况下是超类类型）。
这是设计使然——如果您需要执行此操作所需的动力，我们建议您采用 NSSecureCoding 并使用 NSKeyedArchiver/NSKeyedUnarchiver
我不为所动，从所有热情洋溢的文章中想到 Codable 是我的一些祈祷的答案。作为对象工厂的一组并行 Codable 结构是我正在考虑的一种解决方法，以保留类型信息。
更新我已经使用管理重新创建多态类的单个结构编写了一个示例。在GitHub上可用。
我无法让它轻松地与子类化一起工作。但是，符合基本协议的类可以申请Codable默认编码。回购包含键控和非键控方法。比较简单的不加键，复制在下面
// Demo of a polymorphic hierarchy of different classes implementing a protocol
// and still being Codable
// This variant uses unkeyed containers so less data is pushed into the encoded form.
import Foundation
protocol BaseBeast {
func move() -> String
func type() -> Int
var name: String { get }
}
class DumbBeast : BaseBeast, Codable {
static let polyType = 0
func type() -> Int { return DumbBeast.polyType }
var name:String
init(name:String) { self.name = name }
func move() -> String { return "\(name) Sits there looking stupid" }
}
class Flyer : BaseBeast, Codable {
static let polyType = 1
func type() -> Int { return Flyer.polyType }
var name:String
let maxAltitude:Int
init(name:String, maxAltitude:Int) {
self.maxAltitude = maxAltitude
self.name = name
}
func move() -> String { return "\(name) Flies up to \(maxAltitude)"}
}
class Walker : BaseBeast, Codable {
static let polyType = 2
func type() -> Int { return Walker.polyType }
var name:String
let numLegs: Int
let hasTail: Bool
init(name:String, legs:Int=4, hasTail:Bool=true) {
self.numLegs = legs
self.hasTail = hasTail
self.name = name
}
func move() -> String {
if numLegs == 0 {
return "\(name) Wriggles on its belly"
}
let maybeWaggle = hasTail ? "wagging its tail" : ""
return "\(name) Runs on \(numLegs) legs \(maybeWaggle)"
}
}
// Uses an explicit index we decode first, to select factory function used to decode polymorphic type
// This is in contrast to the current "traditional" method where decoding is attempted and fails for each type
// This pattern of "leading type code" can be used in more general encoding situations, not just with Codable
//: **WARNING** there is one vulnerable practice here - we rely on the BaseBeast types having a typeCode which
//: is a valid index into the arrays `encoders` and `factories`
struct CodableRef : Codable {
let refTo:BaseBeast //In C++ would use an operator to transparently cast CodableRef to BaseBeast
typealias EncContainer = UnkeyedEncodingContainer
typealias DecContainer = UnkeyedDecodingContainer
typealias BeastEnc = (inout EncContainer, BaseBeast) throws -> ()
typealias BeastDec = (inout DecContainer) throws -> BaseBeast
static var encoders:[BeastEnc] = [
{(e, b) in try e.encode(b as! DumbBeast)},
{(e, b) in try e.encode(b as! Flyer)},
{(e, b) in try e.encode(b as! Walker)}
]
static var factories:[BeastDec] = [
{(d) in try d.decode(DumbBeast.self)},
{(d) in try d.decode(Flyer.self)},
{(d) in try d.decode(Walker.self)}
]
init(refTo:BaseBeast) {
self.refTo = refTo
}
init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
let typeCode = try container.decode(Int.self)
self.refTo = try CodableRef.factories[typeCode](&container)
}
func encode(to encoder: Encoder) throws {
var container = encoder.unkeyedContainer()
let typeCode = self.refTo.type()
try container.encode(typeCode)
try CodableRef.encoders[typeCode](&container, refTo)
}
}
struct Zoo : Codable {
var creatures = [CodableRef]()
init(creatures:[BaseBeast]) {
self.creatures = creatures.map {CodableRef(refTo:$0)}
}
func dump() {
creatures.forEach { print($0.refTo.move()) }
}
}
//: ---- Demo of encoding and decoding working ----
let startZoo = Zoo(creatures: [
DumbBeast(name:"Rock"),
Flyer(name:"Kookaburra", maxAltitude:5000),
Walker(name:"Snake", legs:0),
Walker(name:"Doggie", legs:4),
Walker(name:"Geek", legs:2, hasTail:false)
])
startZoo.dump()
print("---------\ntesting JSON\n")
let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let encData = try encoder.encode(startZoo)
print(String(data:encData, encoding:.utf8)!)
let decodedZoo = try JSONDecoder().decode(Zoo.self, from: encData)
print ("\n------------\nAfter decoding")
decodedZoo.dump()
更新 2020-04 经验
与使用 Codable 相比，这种方法继续更加灵活和优越，但会花费更多的程序员时间。它在 Touchgram 应用程序中被大量使用，该应用程序在 iMessage 中提供了丰富的交互式文档。
在那里，我需要编码多个多态层次结构，包括不同的传感器和动作。通过存储解码器的签名，它不仅提供了子类化，还允许我将旧解码器保留在代码库中，以便旧消息仍然兼容。