如何解析嵌套的JSON字符串作为Swift中的对象？

Question

我有到位

struct Symbol : Codable { 
    let id:Int 
    let type:String 
    let properties:[String:String]? 
} 

struct Event : Codable { 
    let event:String 
    let timestamp:Int 
    let symbol:Symbol? 
} 

struct LogRow : Codable { 
    let id:Int 
    let user_id:String 
    let question_id:String 
    let actions:[Event] 
    let timestamp:Double 
} 

和以下JSON阵列

[ 
    { 
    "id": 26535754, 
    "user_id": "qhv1i39wsmbkzhjiffk1rrsg", 
    "question_id": "\"trapdoor|186752c1-948e-4c15-b3df-7d39a99fe9d6\"", 
    "actions": "[{\"event\": \"OPEN\", \"timestamp\": 1499802241640}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243567}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243567, \"dockingPoint\": \"right\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 13, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243699, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802244570}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802244570, \"dockingPoint\": \"argument\"}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802245281}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 13, \"type\": \"Fraction\"}, \"timestamp\": 1499802245845}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 13, \"type\": \"Fraction\"}, \"timestamp\": 1499802246826}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802247468}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802248360}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249161}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249161, \"dockingPoint\": \"denominator\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249289, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249797}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249797, \"dockingPoint\": \"denominator\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249906, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802251451}, {\"event\": \"DROP_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802254229}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802255231}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802255231, \"dockingPoint\": \"denominator\"}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802256593}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802257376}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802258062, \"dockingPoint\": \"denominator\"}, {\"event\": \"CLOSE\", \"timestamp\": 1499802259093}]", 
    "timestamp": 1.499802259277E9 
    }, 
    { 
    "id": 26535718, 
    "user_id": "qhv1i39wsmbkzhjiffk1rrsg", 
    "question_id": "\"trapdoor|186752c1-948e-4c15-b3df-7d39a99fe9d6\"", 
    "actions": "[{\"event\": \"OPEN\", \"timestamp\": 1499802175061}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802178936}, {\"event\": \"DOCK_POTENTIAL_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802183785, \"dockingPoint\": \"subscript\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802184864}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802184865, \"dockingPoint\": \"subscript\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802185857, \"dockingPoint\": \"subscript\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802186710}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802186710, \"dockingPoint\": \"subscript\"}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802188430}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802194665}, {\"event\": \"DROP_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802195427}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"timestamp\": 1499802196167}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"timestamp\": 1499802197167, \"dockingPoint\": \"numerator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802198247}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802198247, \"dockingPoint\": \"right\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802199971, \"dockingPoint\": \"right\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802201551}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802202638}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 12, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802209025}, {\"event\": \"DOCK_POTENTIAL_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 12, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802210771, \"dockingPoint\": \"denominator\"}, {\"event\": \"CLOSE\", \"timestamp\": 1499802212398}]", 
    "timestamp": 1.49980221259E9 
    } 
] 

的JSON数据来自一个数据库，通过DataGrip的JSON出口导出的以下数据结构。数据库字段的类型是JSON Blob（它是Postgres，所以没关系）。查询是这样的：

SELECT 
    ... 
    event_details->>'actions' AS actions, 
    ... 
FROM yadda_yadda...; 

万一它是有用的（是的，我也试过单箭头版本，没有区别）。

我的问题是我该如何让Swift将actions字符串解析为[Event]作为JSON对象？或者，有没有办法使DataGrip（或Postgres）将该字段导出为对象而不是序列化的对象？

编辑我改变LogRow这样

struct LogRow : Codable { 
    let id:Int 
    let user_id:String 
    let question_id:String 
    let actions:[Event] 
    let timestamp:Double 

    enum CodingKeys: String, CodingKey { 
     case id 
     case user_id 
     case question_id 
     case actions 
     case timestamp 
    } 

    init(from decoder: Decoder) throws { 
     let values = try decoder.container(keyedBy: CodingKeys.self) 
     id = try values.decode(Int.self, forKey: .id) 
     user_id = try values.decode(String.self, forKey: .user_id) 
     question_id = try values.decode(String.self, forKey: .question_id) 
     timestamp = try values.decode(Double.self, forKey: .timestamp) 

     let actions_string = try values.decode(String.self, forKey: .actions) 
     let actions_data = actions_string.data(using: .utf8)! 
     actions = try JSONDecoder().decode([Event].self, from: actions_data) 
    } 
} 

，现在我得到一个类似的错误之前，但显然对Event，而不是在LogRow了。

fatal error: Error raised at top level: 
Swift.DecodingError.typeMismatch(Swift.String, 
Swift.DecodingError.Context(codingPath: [Foundation.(_JSONKey in 
_12768CA107A31EF2DCE034FD75B541C9)(stringValue: "Index 1", intValue: 
Optional(1)), tree_builder.Event.(CodingKeys in 
_D5964B2C6A943A986EE24818C2C63D9B).symbol, tree_builder.Symbol. 
(CodingKeys in _D5964B2C6A943A986EE24818C2C63D9B).properties, 
Swift._DictionaryCodingKey(stringValue: "allowSubscript", intValue: 
nil)], debugDescription: "Expected to decode String but found a number 
instead.", underlyingError: nil)): file 
/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang- 
900.0.65/src/swift/stdlib/public/core/ErrorType.swift, line 187 

Answer 1

您可以使用可选的链接，这是由苹果公司提供的解决方案：https://developer.apple.com/swift/blog/?id=37

还有库，它会为你，就像https://github.com/Hearst-DD/ObjectMapper，或https://github.com/tristanhimmelman/AlamofireObjectMapper

这是一个基本的例子如何做到这一点，但你应该重构，并看看我给的链接：

guard let event = jsonObject["event"] as? String, let timestamp = jsonObject["timestamp"] as? Int else {return} 
myEvent.event = event 
myEvent.timestamp = timestamp 
if let symbol = jsonObject["symbol"] as? [String:Any] { 
    guard let symbolId = symbol["id"] as? Int, let type = symbol["type"] else {return} 
    let mySymbol = symbol() 
    mySymbol.id = id 
    mySymbol.type = type 
} 

---

编辑

如果您正在使用SWIFT 4和可编码协议，它应该是很容易的，你就必须让你的嵌套对象为可编码的呢！然后只是：

try decoder.decode(yourType.self, from: yourJsonString) 

如果您正确使用CodingKeys它应该这样做。

Answer 2

问题在于properties的声明，因为JSON对象不会对[String:String]字典进行编码，因为某些值可能不是String s。所以，这是这个问题的“答案”，但这并不能解决嵌套问题，所以我创建了一个新问题：How do I parse a JSON object that has type-dependent sub-objects, in Swift?