在带有继承的Swift 4中使用Decodable [英] Using Decodable in Swift 4 with Inheritance
问题描述
使用类继承是否应该破坏类的可解码性。例如,以下代码
class服务器:可编码{
var id:整数?
}
类开发:服务器{
var name:字符串?
var userId:整数?
}
var json = {\ id\:1,\ name\:\大型建筑开发\}
let jsonDecoder = JSONDecoder()
let item =尝试jsonDecoder.decode(Development.self,from:json.data(using:.utf8)!)as Development
print(item。 id ?? id is nil)
print(item.name ?? name is nil)在这里
输出为:
1
名称为nil
现在,如果我将其取反,名称会解码,但id不会。
类服务器{
var id:Int?
}
类开发:服务器,可编码{
var name:字符串?
var userId:整数?
}
var json = {\ id\:1,\ name\:\大型建筑开发\}
let jsonDecoder = JSONDecoder()
let item =尝试jsonDecoder.decode(Development.self,from:json.data(using:.utf8)!)as Development
print(item。 id ?? id is nil)
print(item.name ?? name is nil)
输出为:
id为nil
大型建筑开发
而且两种语言都无法表达Codable。
我相信在继承的情况下,您必须自己实现 Coding
。也就是说,您必须指定 CodingKeys
并实现 init(from:)
和 encode(to :)
在超类和子类中。根据
所需的init(来自解码器:解码器)throw {
//获取此子类的编码键$ b $的容器b让容器=尝试解码器。容器(keyedBy:CodingKeys.self)
myVar =尝试容器。解码(MyType.self,forKey:.myVar)
//其他变量= ...
//获取超类的superDecoder并调用super.init(from :)
让superDecoder =试试container.superDecoder()
试试super.init(from:superDecoder)
}
视频似乎停止了编码方面的展示(但它是 container.superE ncoder()
用于 encode(to:)
端),但是在 encode( to:)
实现。在这种简单情况下,我可以确认这项工作有效(请参见下面的操场代码)。
我自己仍在为某些奇怪的行为而苦苦挣扎,而我要从 NSCoding
转换一个更复杂的模型,其中有很多新嵌套的类型(包括 struct
和 enum
),它们表现出这种意外的 nil
行为,并且不应。请注意,可能存在涉及嵌套类型的边缘情况。
编辑:嵌套类型在我的测试环境中似乎可以正常工作;我现在怀疑自引用类(认为是树节点的子代)有问题,而自引用类本身也包含该类的各种子类的实例。一个简单的自引用类的测试可以很好地解码(即没有子类),因此我现在将精力集中在子类案例失败的原因上。
17年6月25日更新:我最终向Apple提交了一个有关此问题的错误。 rdar:// 32911973-不幸的是,包含 Subclass:Superclass
元素的 Superclass
数组的编码/解码周期将导致数组中的所有元素都被解码为 Superclass
(子类的 init(from:)
从未被调用,
//:完全实现继承
类FullSuper:可编码{
var id:UUID?
init(){}
私有枚举CodingKeys:字符串,CodingKey {case id}
init(来自解码器:Decoder)抛出{
让容器=尝试解码器。容器(keyedBy:CodingKeys.self)
id =尝试容器.decode(UUID.self,forKey:.id)
}
函数func encode(编码器:Encoder)抛出{
var container = encoder.container(keyedBy:CodingKeys.self)
try container.encode(id ,forKey:.id)
}
}
class FullSub:FullSuper {
var string:String?
私有枚举CodingKeys:字符串,CodingKey {大小写字符串}
覆盖init(){super.init()}
需要init(来自解码器:解码器)抛出{
让容器=尝试解码器。容器(keyedBy:CodingKeys.self)
让superdecoder =尝试容器.superDecoder()
尝试super.init(来自:superdecoder)
字符串=尝试container.decode(String.self,forKey:.string)
}
覆盖func编码(编码器:编码器) throws {
var container = encoder.container(keyedBy:CodingKeys.self)
try container.encode(string,forKey:.string)
let superencoder = container.superEncoder()
试试super.encode(to:superencoder)
}
}
让fullSub = FullSub()
fullSub.id = UUID()
fullSub.string = FullSub
let fullEncoder = PropertyListEncoder()
let fullData =尝试fullEncoder.encode(fullSub)
let fullDecoder = PropertyListDecoder()
let fullSubDecoded:FullSub =试试fullDecoder.decode(FullSub.self,from:fullData)
超类和子类属性均在 fullSubDecoded
中恢复。
Should the use of class inheritance break the Decodability of class. For example, the following code
class Server : Codable {
var id : Int?
}
class Development : Server {
var name : String?
var userId : Int?
}
var json = "{\"id\" : 1,\"name\" : \"Large Building Development\"}"
let jsonDecoder = JSONDecoder()
let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development
print(item.id ?? "id is nil")
print(item.name ?? "name is nil") here
output is:
1
name is nil
Now if I reverse this, name decodes but id does not.
class Server {
var id : Int?
}
class Development : Server, Codable {
var name : String?
var userId : Int?
}
var json = "{\"id\" : 1,\"name\" : \"Large Building Development\"}"
let jsonDecoder = JSONDecoder()
let item = try jsonDecoder.decode(Development.self, from:json.data(using: .utf8)!) as Development
print(item.id ?? "id is nil")
print(item.name ?? "name is nil")
output is:
id is nil
Large Building Development
And you can't express Codable in both classes.
I believe in the case of inheritance you must implement Coding
yourself. That is, you must specify CodingKeys
and implement init(from:)
and encode(to:)
in both superclass and subclass. Per the WWDC video (around 49:28, pictured below), you must call super with the super encoder/decoder.
required init(from decoder: Decoder) throws {
// Get our container for this subclass' coding keys
let container = try decoder.container(keyedBy: CodingKeys.self)
myVar = try container.decode(MyType.self, forKey: .myVar)
// otherVar = ...
// Get superDecoder for superclass and call super.init(from:) with it
let superDecoder = try container.superDecoder()
try super.init(from: superDecoder)
}
The video seems to stop short of showing the encoding side (but it's container.superEncoder()
for the encode(to:)
side) but it works in much the same way in your encode(to:)
implementation. I can confirm this works in this simple case (see playground code below).
I'm still struggling with some odd behavior myself with a much more complex model I'm converting from NSCoding
, which has lots of newly-nested types (including struct
and enum
) that's exhibiting this unexpected nil
behavior and "shouldn't be". Just be aware there may be edge cases that involve nested types.
Edit: Nested types seem to work fine in my test playground; I now suspect something wrong with self-referencing classes (think children of tree nodes) with a collection of itself that also contains instances of that class' various subclasses. A test of a simple self-referencing class decodes fine (that is, no subclasses) so I'm now focusing my efforts on why the subclasses case fails.
Update June 25 '17: I ended up filing a bug with Apple about this. rdar://32911973 - Unfortunately an encode/decode cycle of an array of Superclass
that contains Subclass: Superclass
elements will result in all elements in the array being decoded as Superclass
(the subclass' init(from:)
is never called, resulting in data loss or worse).
//: Fully-Implemented Inheritance
class FullSuper: Codable {
var id: UUID?
init() {}
private enum CodingKeys: String, CodingKey { case id }
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
id = try container.decode(UUID.self, forKey: .id)
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(id, forKey: .id)
}
}
class FullSub: FullSuper {
var string: String?
private enum CodingKeys: String, CodingKey { case string }
override init() { super.init() }
required init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let superdecoder = try container.superDecoder()
try super.init(from: superdecoder)
string = try container.decode(String.self, forKey: .string)
}
override func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(string, forKey: .string)
let superencoder = container.superEncoder()
try super.encode(to: superencoder)
}
}
let fullSub = FullSub()
fullSub.id = UUID()
fullSub.string = "FullSub"
let fullEncoder = PropertyListEncoder()
let fullData = try fullEncoder.encode(fullSub)
let fullDecoder = PropertyListDecoder()
let fullSubDecoded: FullSub = try fullDecoder.decode(FullSub.self, from: fullData)
Both the super- and subclass properties are restored in fullSubDecoded
.
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