Swift Codable init

Question

I would like to do some initialization logic after the Swift Coding/Encoding feature has finished decoding a JSON.

struct MyStruct: Codable {
    let id: Int 
    var name: String

    init() {
       name = "\(id) \(name)" 
    }
}

But I get the compiler error:

Return from initializer without initializing all stored properties

Which is clear to me because init() wants me to initialise all properties. But adding an init() with all needed properties also doesn't solve it because this initializer is not called(!) when Codable kicks in:

init(id: Int, name: String) {
    // This initializer is not called if Decoded from JSON!
    self.id = id 
    self.name = "\(id) \(name)" 
}

Nevertheless - is there a way to do some initialisation logic after the Decoding has finished but without doing all the decoding manually for each property? So without implementing every time init(from decoder: Decoder). In this short example I have just two simple properties but production code consists of thousands of them.

Thanks.

Thanks, but that's just an example. What I need is a way to run initialisation logic. — Darko, Jan 16 '18 at 12:16
I don't think that's possible. I've either caved in and implemented `init(from:)` or resorted to `finializeInit()` methods which I manually call after the JSON decoding. — Gereon, Jan 16 '18 at 12:41
@Gereon Could you explain your finalizeInit approach? It's not clear to me. Thanks. — Darko, Jan 16 '18 at 12:48
`self.name = "\(id) \(name)"` — what does it do? Adds id prefix to the name? — user28434'mstep, Jan 16 '18 at 12:54
As I already said: that's just an example. What I need is a way to run initialisation logic. (which will be way more afterwards) — Darko, Jan 16 '18 at 12:59

vadian · Accepted Answer · 2021-02-26T08:29:54.540

25

Either you get everything for free but standardized or you have to write a custom initializer like

struct MyStruct: Codable  {

    let id: Int 
    var name: String

    private enum CodingKeys : String, CodingKey { case id, name }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        let decodedName = try container.decode(String.self, forKey: .name)
        name = "\(id) \(decodedName)" 
    }
}

You can implement init() but this works independent of the decoding functionality and you have to assign a default value to all non-optional properties, that's what the error says.

edited Feb 26 '21 at 08:29

answered Jan 16 '18 at 13:03

vadian

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getting an error for `CodingKeys` saying `'CodingKeys' is inaccessible due to 'private' protection level`. do you mean `CodingKey` there? – Keval Langalia Feb 26 '21 at 00:11
1

You have to specify CodingKeys, my bad, see the edit. – vadian Feb 26 '21 at 08:30
oh yes, I have figured that out after little research and already implemented the enum `CodingKeys`. Thank you for the solution though! – Keval Langalia Mar 02 '21 at 08:02

score 12 · Answer 2 · answered Jan 16 '18 at 13:17

Use a factory method that first uses init(from:) and then calls your custom initialization code

struct Foo: Decodable {
    let name: String
    let id: Int

    var x: String!

    private mutating func finalizeInit() {
        self.x = "\(name) \(id)"
    }

    static func createFromJSON(_ data: Data) -> Foo? {
        guard var f = try? JSONDecoder().decode(Foo.self, from: data) else { 
            return nil 
        }
        f.finalizeInit()
        return f
    }
}

let sampleData = """
    { "name": "foo", "id": 42 }
    """.data(using: .utf8)!
let f = Foo.createFromJSON(sampleData)

Swift Codable init

2 Answers2

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