decoding a nested array in swift 4 [duplicate]

Question

take the following JSON:

let rawJson =
"""
[
      {
          "id": 1,
          "name":"John Doe"
      },
      {
          "id": 2,
          "name":"Luke Smith"
      },
 ]
"""

and User model:

struct User: Decodable {
    var id: Int
    var name: String
}

It's pretty simple to decode, like so:

let data = rawJson.data(using: .utf8)
let decoder = JSONDecoder()
let users = try! decoder.decode([User].self, from: data!)

But what if the JSON looks like this, where the top level is a dictionary and need to fetch the array of users:

let json =
"""
{
    "users":
    [
        {
          "id": 1,
          "name":"John Doe"
        },
        {
          "id": 2,
          "name":"Luke Smith"
        },
    ]
}
"""

What's the most effective solution to reading the JSON? I could definitely create another struct like this:

struct SomeStruct: Decodable {
    var posts: [Post]
}

and decode like so:

let users = try! decoder.decode(SomeStruct.self, from: data!)

but it doesn't feel right doing it that way, creating a new model object just because the array is nested inside a dictionary.

Check this article under the "wrapper keys" heading. It appears this is the correct way to parse that structure out. benscheirman.com/2017/06/… — Alex McPherson
– Alex McPherson, Commented Oct 10, 2017 at 15:27

vadian · Accepted Answer · 2017-10-10 15:27:52Z

6

If you want to take advantage of JSONDecoder you have to create a nested struct(ure).

I recommend to use the name Root for the root object and put the child struct into Root

struct Root : Decodable {

    struct User : Decodable {
        let id: Int
        let name: String
    }

    let users : [User]
}

let json = """
{
    "users": [{"id": 1, "name":"John Doe"},
              {"id": 2, "name":"Luke Smith"}]
}
"""

let data = Data(json.utf8)
do {
    let root = try JSONDecoder().decode(Root.self, from: data)
    print(root.users)
} catch {
    print(error)
}

answered Oct 10, 2017 at 15:27

vadian

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2 Comments

Leo Dabus Over a year ago

There is no need to have the User struct nested into it the Root struct struct User : Decodable { let id: Int let name: String } struct Root : Decodable { let users : [User] } The result would be the same

vadian Over a year ago

Yes there is no need, but if the struct is not used somewhere else it's cleaner (imho) and it illustrates the equivalent of the JSON structure.

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