我有一个带有一组值的
JSON：
[
{ "tag": "Foo",… },{ "tag": "Bar",{ "tag": "Baz",]
我想将此数组解码为一个结构数组,其中特定类型取决于标记：
protocol SomeCommonType {}
struct Foo: Decodable,SomeCommonType { … }
struct Bar: Decodable,SomeCommonType { … }
struct Baz: Decodable,SomeCommonType { … }
let values = try JSONDecoder().decode([SomeCommonType].self,from: …)
我怎么做？目前我有这个有点丑陋的包装：
struct DecodingWrapper: Decodable {
let value: SomeCommonType
public init(from decoder: Decoder) throws {
let c = try decoder.singleValueContainer()
if let decoded = try? c.decode(Foo.self) {
value = decoded
} else if let decoded = try? c.decode(Bar.self) {
value = decoded
} else if let decoded = try? c.decode(Baz.self) {
value = decoded
} else {
throw …
}
}
}
然后：
let wrapped = try JSONDecoder().decode([DecodingWrapper].self,from: …)
let values = wrapped.map { $0.value }
有没有更好的办法？
解决方法
你的数组包含有限,可枚举变种的异构对象;听起来像Swift枚举的完美用例.它不适用于多态性,因为从概念上讲,这些“事物”不一定是同一种.他们恰好被标记了.
以这种方式看待它：你有一系列都有标签的东西,有些属于这种类型,有些属于完全不同的类型,还有其他……有时你甚至都不认识标签. Swift enum是捕捉这一想法的完美载体.
struct Foo: Decodable {
let tag: String
let fooValue: Int
}
struct Bar: Decodable {
let tag: String
let barValue: Int
}
struct Baz: Decodable {
let tag: String
let bazValue: Int
}
并且您的数组可以包含上述类型的任何实例,或者包含未知类型的实例.所以你有enum TagggedThing(或更好的名字).
enum TagggedThing {
case foo(Foo)
case bar(Bar)
case baz(Baz)
case unknown
}
你的数组,用Swift术语,是[TagggedThing]类型.所以你将TagggedThing类型与Decodable一致,如下所示：
extension TagggedThing: Decodable {
private enum CodingKeys: String,CodingKey {
case tag
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let tag = try container.decode(String.self,forKey: .tag)
let singleValueContainer = try decoder.singleValueContainer()
switch tag {
case "foo":
// if it's not a Foo,throw and blame the server guy
self = .foo(try singleValueContainer.decode(Foo.self))
case "bar":
self = .bar(try singleValueContainer.decode(Bar.self))
case "baz":
self = .baz(try singleValueContainer.decode(Baz.self))
default:
// this tag is unknown,or known but we don't care
self = .unknown
}
}
}
现在您可以解码以下JSON：
let json: Data! = """
[
{"tag": "foo","fooValue": 1},{"tag": "bar","barValue": 2},{"tag": "baz","bazValue": 3}
]
""".data(using: .utf8)
像这样：
let taggedThings = try? JSONDecoder().decode([TagggedThing].self,from: json)
