Swift Json decoding with unsure incoming data

Question

I'm struggeling to decode some Json data in swift.

My back-end api will return me either an array of X or an object with (at least) one property named "items" of type array of X.

I've searched but found no solutions. Do you have any?

struct A: Decodable {
   var items: [X]
   // some other optional properties

   public init(from decoder: Decoder) throws {
       // Sometimes I receive the correct A object
       // Sometimes I only receive the array of X without the surrounding object of type A.
   }
}

And to make things worst, I'm bound to decode like I was always receiving an object of typa A... :

myObjectOfTypeA = try decoder.decode(A.self, from: data)

Most of the time, I'll receive a proper A object like this:

{
    "items": 
    [
        {
            "id": 7,
            "startsOn": "2021-03-01",
            "endsOn": "2021-12-31"
        },
        {
            "id": 6,
            "startsOn": "2021-04-19",
            "endsOn": "2022-04-04"
        }
    ],
    "next": null,
    "prev": null,
    "count": 2
}

But sometimes, I'll receive only the items array like this:

[
    {
        "id": 7,
        "startsOn": "2021-03-01",
        "endsOn": "2021-12-31"
    },
    {
        "id": 6,
        "startsOn": "2021-04-19",
        "endsOn": "2022-04-04"
    }
]

Any ideas would be greatly appreciated because I'm clearly out of ideas myself...

Answer 1

You just need to try to decode your enclosing type items and catch the error. If it is a type mismatch error try to decode it again. No need to create a custom decoder:

---

struct A: Decodable {
   let items: [X]
}

---

struct X: Decodable {
    let id: Int
    let startsOn: Date
    let endsOn: Date
}

---

Playground testing:

let json = """
{
    "items":
    [
        {
            "id": 7,
            "startsOn": "2021-03-01",
            "endsOn": "2021-12-31"
        },
        {
            "id": 6,
            "startsOn": "2021-04-19",
            "endsOn": "2022-04-04"
        }
    ],
    "next": null,
    "prev": null,
    "count": 2
}
"""

---

let dateFormatter = DateFormatter()
dateFormatter.calendar = .init(identifier: .iso8601)
dateFormatter.locale = .init(identifier: "en_US_POSIX")
dateFormatter.dateFormat = "yyyy-MM-dd"

let jsonDecoder = JSONDecoder()
jsonDecoder.dateDecodingStrategy = .formatted(dateFormatter)
let items: [X]
do {
    items = try jsonDecoder.decode(A.self, from: Data(json.utf8)).items
} catch DecodingError.typeMismatch {
    items = try jsonDecoder.decode([X].self, from: Data(json.utf8))
} catch {
    items = []
    print(error)
}
print(items)

---

It will print the following for both json inputs:

[X(id: 7, startsOn: 2021-03-01 03:00:00 +0000, endsOn: 2021-12-31 03:00:00 +0000), X(id: 6, startsOn: 2021-04-19 03:00:00 +0000, endsOn: 2022-04-04 03:00:00 +0000)]