Make Swift JSONDecoder not fail when type on key not match

Question

I have one simple struct like that:

struct Object: Codable {
    let year: Int?
    …
}

It's fine when decode JSON like { "year": 10, … } or no year in JSON.
But will fail decode when JSON has different type on key: { "year": "maybe string value" }

How could I not fail decode process when type not match on the Optional property?

Thanks.

Answer 1

Implement init(from:) in struct Object. Create enum CodingKeys and add the cases for all the properties you want to parse.

In init(from:) parse the keys manually and check if year from JSON can be decoded as an Int. If yes, assign it to the Object's year property otherwise don't.

struct Object: Codable {
    var year: Int?

    enum CodingKeys: String,CodingKey {
        case year
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        if let yr = try? values.decodeIfPresent(Int.self, forKey: .year) {
            year = yr
        }
    }
}

Parse the JSON response like,

do {
    let object = try JSONDecoder().decode(Object.self, from: data)
    print(object)
} catch {
    print(error)
}

Example:

1. If JSON is { "year": "10"}, object is: Object(year: nil)
2. If JSON is { "year": 10}, object is: Object(year: Optional(10))