[英]Swift 4 JSON decode with configurable keys
I'm new to Swift and I need to parse a JSON with some configurable keys . 我是Swift的新手,我需要使用一些可配置的密钥解析JSON。 Opposite to many examples I've seen here, the keys are known before the decode operation is started, they just depend on some parameters passed to endpoint.
与我在这里看到的许多例子相反,密钥在解码操作开始之前就已知,它们只依赖于传递给端点的一些参数。
Example: 例:
https://some.provider.com/endpoint/?param=XXX
and 和
https://some.provider.com/endpoint/?param=YYY
will answer, respectively: 将分别回答:
[
{
"fixed_key1": "value1",
"fixed_key2": "value2",
"variable_key_1_XXX": "some value",
"variable_key_2_XXX": "some other value"
},
...
]
and 和
[
{
"fixed_key1": "value1",
"fixed_key2": "value2",
"variable_key_1_YYY": "some value",
"variable_key_2_YYY": "some other value"
},
...
]
Given that those keys are known before decoding , I was hoping to get away with some clever declaration of a Decodable structure and/or CodingKeys , without the need to write the 鉴于这些密钥在解码之前是已知的,我希望能够通过一些巧妙的Decodable结构和/或CodingKeys声明 ,而无需编写
init(from decoder: Decoder)
Unfortunately, I was not able to come up with such a declaration. 不幸的是,我无法提出这样的声明。
Of course I don't want to write one Decodable/CodingKeys structure for every possible parameter value :-) 当然我不想为每个可能的参数值编写一个Decodable / CodingKeys结构:-)
Any suggestion ? 有什么建议吗?
Unless all your JSON keys are compile-time constants, the compiler can't synthesize the decoding methods. 除非所有JSON键都是编译时常量,否则编译器无法合成解码方法。 But there are a few things you can do to make manual decoding a lot less cumbersome.
但是,您可以采取一些措施来减轻手动解码的麻烦。
First, some helper structs and extensions: 首先,一些辅助结构和扩展:
/*
Allow us to initialize a `CodingUserInfoKey` with a `String` so that we can write:
decoder.userInfo = ["param": "XXX"]
Instead of:
decoder.userInfo = [CodingUserInfoKey(rawValue:"param")!: "XXX"]
*/
extension CodingUserInfoKey: ExpressibleByStringLiteral {
public typealias StringLiteralType = String
public init(stringLiteral value: StringLiteralType) {
self.rawValue = value
}
}
/*
This struct is a plain-vanilla implementation of the `CodingKey` protocol. Adding
`ExpressibleByStringLiteral` allows us to initialize a new instance of
`GenericCodingKeys` with a `String` literal, for example:
try container.decode(String.self, forKey: "fixed_key1")
Instead of:
try container.decode(String.self, forKey: GenericCodingKeys(stringValue: "fixed_key1")!)
*/
struct GenericCodingKeys: CodingKey, ExpressibleByStringLiteral {
// MARK: CodingKey
var stringValue: String
var intValue: Int?
init?(stringValue: String) { self.stringValue = stringValue }
init?(intValue: Int) { return nil }
// MARK: ExpressibleByStringLiteral
typealias StringLiteralType = String
init(stringLiteral: StringLiteralType) { self.stringValue = stringLiteral }
}
Then the manual decoding: 然后手动解码:
struct MyDataModel: Decodable {
var fixedKey1: String
var fixedKey2: String
var variableKey1: String
var variableKey2: String
enum DecodingError: Error {
case missingParamKey
case unrecognizedParamValue(String)
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: GenericCodingKeys.self)
// Decode the fixed keys
self.fixedKey1 = try container.decode(String.self, forKey: "fixed_key1")
self.fixedKey2 = try container.decode(String.self, forKey: "fixed_key2")
// Now decode the variable keys
guard let paramValue = decoder.userInfo["param"] as? String else {
throw DecodingError.missingParamKey
}
switch paramValue {
case "XXX":
self.variableKey1 = try container.decode(String.self, forKey: "variable_key_1_XXX")
self.variableKey2 = try container.decode(String.self, forKey: "variable_key_2_XXX")
case "YYY":
self.variableKey1 = try container.decode(String.self, forKey: "variable_key_1_YYY")
self.variableKey2 = try container.decode(String.self, forKey: "variable_key_2_YYY")
default:
throw DecodingError.unrecognizedParamValue(paramValue)
}
}
}
And finally here's how you use it: 最后这是你如何使用它:
let jsonData = """
[
{
"fixed_key1": "value1",
"fixed_key2": "value2",
"variable_key_1_XXX": "some value",
"variable_key_2_XXX": "some other value"
}
]
""".data(using: .utf8)!
// Supplying the `userInfo` dictionary is how you "configure" the JSON-decoding
let decoder = JSONDecoder()
decoder.userInfo = ["param": "XXX"]
let model = try decoder.decode([MyDataModel].self, from: jsonData)
print(model)
Taking a similar approach to @Code Different's answer , you can pass the given parameter information through the decoder's userInfo
dictionary, and then pass this onto the key type that you use to decode from the keyed container. 对@Code Different的答案采用类似的方法,您可以通过解码器的
userInfo
字典传递给定的参数信息,然后将其传递给您用来从密钥容器解码的密钥类型。
First, we can define a new static member on CodingUserInfoKey
to use as the key in the userInfo
dictionary: 首先,我们可以在
CodingUserInfoKey
上定义一个新的静态成员,以用作userInfo
字典中的键:
extension CodingUserInfoKey {
static let endPointParameter = CodingUserInfoKey(
rawValue: "com.yourapp.endPointParameter"
)!
}
(the force unwrap never fails; I regard the fact the initialiser is failable as a bug ). (力量展开永远不会失败;我认为初始化器可以作为一个错误使用 )。
Then we can define a type for your endpoint parameter, again using static members to abstract away the underlying strings: 然后我们可以为您的端点参数定义一个类型,再次使用静态成员抽象出底层字符串:
// You'll probably want to rename this to something more appropriate for your use case
// (same for the .endPointParameter CodingUserInfoKey).
struct EndpointParameter {
static let xxx = EndpointParameter("XXX")
static let yyy = EndpointParameter("YYY")
// ...
var stringValue: String
init(_ stringValue: String) { self.stringValue = stringValue }
}
Then we can define your data model type: 然后我们可以定义您的数据模型类型:
struct MyDataModel {
var fixedKey1: String
var fixedKey2: String
var variableKey1: String
var variableKey2: String
}
And then make it Decodable
like so: 然后像它这样使它可
Decodable
:
extension MyDataModel : Decodable {
private struct CodingKeys : CodingKey {
static let fixedKey1 = CodingKeys("fixed_key1")
static let fixedKey2 = CodingKeys("fixed_key2")
static func variableKey1(_ param: EndpointParameter) -> CodingKeys {
return CodingKeys("variable_key_1_\(param.stringValue)")
}
static func variableKey2(_ param: EndpointParameter) -> CodingKeys {
return CodingKeys("variable_key_2_\(param.stringValue)")
}
// We're decoding an object, so only accept String keys.
var stringValue: String
var intValue: Int? { return nil }
init?(intValue: Int) { return nil }
init(stringValue: String) { self.stringValue = stringValue }
init(_ stringValue: String) { self.stringValue = stringValue }
}
init(from decoder: Decoder) throws {
guard let param = decoder.userInfo[.endPointParameter] as? EndpointParameter else {
// Feel free to make this a more detailed error.
struct EndpointParameterNotSetError : Error {}
throw EndpointParameterNotSetError()
}
let container = try decoder.container(keyedBy: CodingKeys.self)
self.fixedKey1 = try container.decode(String.self, forKey: .fixedKey1)
self.fixedKey2 = try container.decode(String.self, forKey: .fixedKey2)
self.variableKey1 = try container.decode(String.self, forKey: .variableKey1(param))
self.variableKey2 = try container.decode(String.self, forKey: .variableKey2(param))
}
}
You can see we're defining the fixed keys using static properties on CodingKeys
, and for the variable keys we're using static methods that take the given parameter as an argument. 您可以看到我们使用
CodingKeys
上的静态属性定义固定键,对于变量键,我们使用将给定参数作为参数的静态方法。
Now you can perform a decode like so: 现在您可以像这样执行解码:
let jsonString = """
[
{
"fixed_key1": "value1",
"fixed_key2": "value2",
"variable_key_1_XXX": "some value",
"variable_key_2_XXX": "some other value"
}
]
"""
let decoder = JSONDecoder()
decoder.userInfo[.endPointParameter] = EndpointParameter.xxx
do {
let model = try decoder.decode([MyDataModel].self, from: Data(jsonString.utf8))
print(model)
} catch {
print(error)
}
// [MyDataModel(fixedKey1: "foo", fixedKey2: "bar",
// variableKey1: "baz", variableKey2: "qux")]
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