Nested Container nil value custom decoder Swift

Question

So I have this simple json response:

{
    "gender": "male",
    "name": {
        "title": "mr",
        "first": "brad",
        "last": "gibson"
        }
}

And this is my customer decoder:

struct UserModel: Decodable {
    var gender: String
    var title: String?
    var first: String?
    var last: String?
    
    // Top-level coding keys
    enum CodingKeys: String, CodingKey {
        case name, gender
    }
    
    enum NameKeys: CodingKey {
        case title, first, last
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        gender = try container.decode(String.self, forKey: .gender)
        let name = try container.nestedContainer(keyedBy: NameKeys.self, forKey: .name)
        title = try name.decodeIfPresent(String.self, forKey: .title)
        first = try name.decodeIfPresent(String.self, forKey: .first)
        last = try name.decodeIfPresent(String.self, forKey: .last)
    }
}

I know how to handle the error scenarios if one of the keys inside the nested object name (title, first, last) isn't sent. My question is how would I write something to prevent a crash if the "name" object is not sent back from the response?

Create a nested `Codable` type. Which parameters do you require, just `gender`? — George, Dec 02 '21 at 02:29
yeah so for example, say for some reason "name" does't get sent in the response and it's just gender that's returned, it crashes. — Simon McNeil, Dec 02 '21 at 02:34
Should a name _always_ contain `title`, `first`, and `last`? Currently it just defaults them to empty strings rather than `nil` — George, Dec 02 '21 at 02:35

score 2 · Answer 1 · answered Dec 02 '21 at 02:47

You can have nested Codable structs, as such:

struct User: Codable {
    let gender: String
    let name: UserName?
}

struct UserName: Codable {
    let title: String?
    let first: String?
    let last: String?
}

To make the syntax better than user.name?.title, we can change this to just user.title with the use of @dynamicMemberLookup. Change User to this:

@dynamicMemberLookup
struct User: Codable {
    let gender: String
    let name: UserName?

    subscript<T>(dynamicMember k: KeyPath<UserName, T>) -> T? {
        name?[keyPath: k]
    }

    subscript<T>(dynamicMember k: KeyPath<UserName, T?>) -> T? {
        name?[keyPath: k]
    }
}

Now, you can do the short-hand form to get each property. This now mimics your single container with all the properties.

that's a cool trick. Didn't know about dynamicMemberLookup thanks! — Simon McNeil, Dec 02 '21 at 02:50

score 1 · Answer 2 · answered Dec 02 '21 at 03:03

Thanks to Deepika, this is the solution:

struct UserModel: Decodable {
var gender: String
var title: String?
var first: String?
var last: String?

// Top-level coding keys
enum CodingKeys: String, CodingKey {
    case name, gender
}

enum NameKeys: CodingKey {
    case title, first, last
}

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    gender = try container.decode(String.self, forKey: .gender)
    
    if container.contains(.name) {
        let name = try container.nestedContainer(keyedBy: NameKeys.self, forKey: .name)
        title = try name.decode(String.self, forKey: .title)
        first = try name.decode(String.self, forKey: .first)
        last = try name.decode(String.self, forKey: .last)
    }
}

}

score 0 · Answer 3 · answered Dec 02 '21 at 02:47

How about using below?

struct Name: Decodable {

    var title: String?
    var first: String?
    var last: String?

}

struct UserModel: Decodable {
    var name: Name?
    var gender: String
}

Then everything else gets taken care of itself. For e.g all of below will be parsed without issues.

{
    "gender": "male",
    "name": {
        "title": "mr",
        "first": "brad",
        "last": "gibson"
        }
}

{
    "gender": "male"
}

{
    "gender": "male",
    "name": {
        "title": "mr",
        "first": "brad"
        }
}

I tested using below

let str = """
    {
        "gender": "male",
        "name": {
            "title": "mr",
            "first": "brad"
            }
    }
    """

let usermodel = try! JSONDecoder().decode(UserModel.self, from: str.data(using: .utf8)!)
print(usermodel)

yeah this works and was my initial solution, was just playing around with writing nested container in a custom decoder. — Simon McNeil, Dec 02 '21 at 02:48
Check this link https://stackoverflow.com/questions/46740160/swift-decodable-optional-key — Deepika, Dec 02 '21 at 02:51

Nested Container nil value custom decoder Swift

3 Answers3