JSONDecoder 解析不连贯

Question

I went across a problem when decoding a JSON file.

The file structure is as follows:

{"grocery":
    [
        { "vegetables":
            {"quantity":"some",
            "compostable":true,
            "stockageSite":"string",
            "package":"bag"
            },

        "meats":
            {"quantity":"string",
            "compostable":true,
            "stockageSite":"string",
            "package":"box"
            },

        "site":"string",
        "active":true,
        "name":"string"
        }
    ]
}

for which I have the following structs

struct Goods: Codable {
    
    var quantity: String
    var compostable: Bool
    var stockageSite: String
    var package: String
}

struct Details: Codable {
    
    var vegetables: Goods
    var meats: Goods
    var site: String
    var active: Bool
    var name: String
}

struct Grocery: Codable {

    var grocery: [Details]
}

the decoding (do-catch with

 URLSession.shared.dataTask(with: request){data, response, error in
            if let error = error {
                print("Error took place: \(error)")
                return
            }
            if let response = response as? HTTPURLResponse {
                if response.statusCode != 200{
                    print("Response HTTP Status code: \(response.statusCode)")
                }
            }
            if let data = data{
              
            let decoder = JSONDecoder()
 do {
            let jsonParse: Grocery =  try decoder.decode(Grocery.self, from: data)
            print("parsed \(jsonParse)")
}
catch let jsonError as NSError{
                    
                    print("Failed \(jsonError)")
                }

) parses very well with no errors throwed as long as I remove the "quantity" or "stockage site" or "package" from Goods (-> ie, the ones with string values, exactly as if more than 2 string items doesn't work there). Otherwise, there's no parsing and the code execution stops there. No error throwed (except in Playgrounds:

error: Execution was interrupted, reason: EXC_BAD_ACCESS (code=EXC_I386_GPFLT)

The string type is the good one, there are no typos. The "compostable" (bool) is well recognized.

(The same thing happens if I add a string element to "Details" and change my json accordingly.)

here are the results printed after correct parsing (whichever of the 3 string elements is removed, so "package" can be parsed correctly):

parsed Grocery(grocery: [__lldb_expr_5.Details(vegetables: __lldb_expr_5.Goods(quantity: "some", compostable: true, stockageSite: "string"), meats: __lldb_expr_5.Goods(quantity: "string", compostable: true, stockageSite: "string"), site: "string", active: true, name: "string")])

and (sometimes! but why?!) the error message printed :

Failed Swift.DecodingError.valueNotFound(__lldb_expr_7.Details, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "grocery", intValue: nil), _JSONKey(stringValue: "Index 0", intValue: 0), _JSONKey(stringValue: "Index 0", intValue: 0)], debugDescription: "Expected Details but found null instead.", underlyingError: nil))

just as if the third string element was never found. Would you have a hint of what is going on?

Answer 1

The Question needs more information but I will guess that the keys in those objects sometimes are missing so here is how you can handle that, by simply using init(from decoder: Decoder) and giving default values for missing keys using decodeIfPresent change your Goods struct to this

    struct Goods: Codable {
    var quantity: String
    var compostable: String
    var stockageSite: String
    
   private enum CodingKeys: String, CodingKey {
        case quantity
        case compostable
        case stockageSite
   }
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        
        self.quantity = try container.decodeIfPresent(String.self, forKey: .quantity) ?? "Empty"
        self.compostable = try container.decodeIfPresent(String.self, forKey: .compostable) ?? "Empty"
        self.stockageSite = try container.decodeIfPresent(String.self, forKey: .stockageSite) ?? "Empty"

        }
}