Thoughts, stories and ideas.

10th March 2019 · 8 minutes · ios swift

Since the introduction of Codable, transforming APIs into data models has been a blessing. All we need is a simple conformance to the Decodable protocol, and let Swift do its magic.

But sometimes, we need more control. Quickly, we end up declaring our coding keys and overriding the default implementation of init(from: Decoder), to state our own business logic.

Eventually we’ll get exactly what we want. Well, almost exactly.

The issue

Let’s say we want to display a list of items, retrieved from an API.

enum Planet: String, Decodable {
  case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune

class PlanetarySystem: Decodable {
    let planets: [Planet]

    enum CodingKeys: String, CodingKey {
        case planets

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.planets = try container.decode([Planet].self, forKey: .planets)

let json = """
            "planets": ["mercury", "mars", "saturn"]
            "planets": ["mercury", "mars", "saturn", "pluto"]
let jsonData = json.data(using: .utf8)!

do {
    let someSystem = try JSONDecoder().decode([PlanetarySystem].self, from: jsonData)
} catch let error {

A naive data model and a quick JSON mock. A PlanetarySystem with a list of Planet. The JSON data contains a list of PlanetarySystem.

Executing this code will throw a DecodingError.dataCorrupted, with the following, rather explicit, debug description.

And this is where the problem lies. We can’t be sure the data will be valid.

Decodable provides a great way to handle that when the data is a simple object, thanks to optional. But it does not provide one when the data is a list. We either get a full list of items, or an error, if any of the item does not conform to the item’s data model.

What if we want something in between? Like the list of valid items, the items that conform to the data model?

In order to achieve this, we need to build a fault tolerant system, one that will allow lossy decoding of array elements.

The idea

The way Decodaleworks when processing a collection is simple : it will throw as soon as one of the children’s collection throws. In our example, Decodable throws at the very end, when reaching the plutovalue. The value is not a part of the enum ; the decoding of that item and so the decoding of the collection. All the data process before is throw away. A fail strategy.

Our first step is to define new strategies. We start simple: either we remove the faulty element, or we apply the standard behavior (fail).

We then need to use the chosen strategy to apply when encountering an invalid element while decoding.

We’ll use a generic enum to represent it. The current decodeItem method will either:

  • return the decoded element
  • return nil when using remove strategy
  • throw an error when using fail strategy
enum InvalidElementStrategy<T> {
    case remove
    case fail

    func decodeItem(decode: () throws -> T) throws -> T? {
        do {
            return try decode()
        } catch {
            switch self {
            case .remove:
                return nil
            case .fail:
                throw error

The second step is a bit tougher. Our strategy is now defined, we want to use it, when processing a collection.

The key here is to rewrite ourselves the decode method of KeyedDecodingContainer, to use our strategy.

Hence, the method has the same signature as the standard with an additional parameter, our strategy. By default, the strategy is the standard one, throw an error.

The job is then as described earlier. We iterate over the container, try to decode our object using our strategy :

  • if we have an element, we keep it, and we continue looping over the container
  • if we don’t have an element
    • and the strategy is removing : nothing happens, we continue looping over the container
    • and the strategy is failing : we throw, as usual, and the decoding stop
extension KeyedDecodingContainer {

    private struct EmptyDecodable: Decodable {}

    public func decode<T: Decodable>(_ type: [T].Type,
                                     forKey key: KeyedDecodingContainer<K>.Key,
                                     invalidElementStrategy: InvalidElementStrategy<T> = .fail) throws -> [T] {

        var container = try nestedUnkeyedContainer(forKey: key)
        var array: [T] = []

        while !container.isAtEnd {
            let element: T? = try invalidElementStrategy.decodeItem(decode: { try container.decode(T.self) })
            if let element = element {
            } else {
                // hack to advance the index
                _ = try? container.decode(EmptyDecodable.self)
        return array

Updating our example code, and we have a rather simple fault tolerant decoding system.

self.planets = try container.decode([Planet].self, forKey: .planets, invalidElementStrategy: .remove)

I love the fact that if all of our objects are corrupted and we choose the remove strategy, we’ll get an empty array and not an error.

Next step

This solution works great when using a wrapper object, like the PlanetarySystem. Not with a plain object like Planet.

let json = """
["mercury", "mars", "saturn", "pluto"]
let jsonData = json.data(using: .utf8)!

do {
    let planets = try JSONDecoder().decode([Planet].self, from: jsonData)
} catch let error {

Executing this code will throw the exact same error as before. And same as before, we would need to create a new method, in JSONDecoder this time, to have something like this :

func decode<T>(_ type: [T].Type, from data: Data, invalidElementStrategy: InvalidElementStrategy<T> = .fail) throws -> [T] where T : Decodable

Unfortunately, we hit the limit of our possibilities. In order to decode the Data we would need access to the internal _JSONDecoder type, which is… internal. Without it, we cannot do more, unless rewriting our own JSONDecoder, which would be a terrible mistake and a waste, if you ask me.

What you can do

Since September 2017, an issue about this subject is open on the Swift bug tracker, SR-5953. Unfortunately, not much has been made since. So, instead of trying to write your own decoder and if this issue is important to you, feel free to upvote and comment on the issue!

Last updated on 01st October 2019