Difficulty: Beginner | Easy | Normal | Challenging
This article has been developed using Xcode 14.2, and Swift 5.7.2
This article is "inspired" by the current implementation of Decodable. If we have missing fields in an Array which we wish to decode, what are we to do?
Read on to find out what can possibly be done about this!
You will be expected to be aware of how to make a Single View Application in Swift. It would be great to have some background regarding decoding JSON
Decodable: A type that can decode itself from an external representation JSON: JavaScript Object Notation, a lightweight format for storing and transporting data
try this
When we take a response (json, usually) from a backend we can use Decodable to do just that.
func decode<T: Decodable>(data: Data) -> T? {
do {
return try JSONDecoder().decode(T.self, from: data)
} catch {
// will silently fail and return nil
return nil
}
}I can use Swift to make a basic call to the backend
func basicCall() {
let url = URL(string: "https://jsonplaceholder.typicode.com/todos/8")!
httpManager?.get(url: url, completionBlock: {res in
print (res)
switch res {
case .success(let data) :
let _: ToDoModel? = self.decode(data: data)
break
case .failure:
break
}
})
}Which is then processed with this model
struct ToDoModel: Codable {
let completed: Bool
let id: Int
let title: String
let userId: Int
}With those basics out of the way, for the rest of this article I shall use json files within the App bundle. If you are interested, you can checkout the repo and the Bundle-decode file.
We can process more complex responses. Snake cased keys and dates are one example of more difficult data that needs to be processed.
JSONdecoder has keyDecodingStrategy and dateDecodingStrategy to help us out!
Now an "ordinary" ISO8601Date (such as "2020-02-06T00:33:01Z") can be handled with decoder.dateDecodingStrategy = .iso8601!
Glory!
static var snakeCaseISO8601Date: JSONDecoder = {
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
return decoder
}()But what if your endpoint features "2019-01-18T10:15:32.250Z" - like in the following JSON string:
[
{
"timestamp": "2019-01-18T10:15:29.979Z",
"unique_key": "policy:dev_pol_000000BansDm7Jjbj3k4R1IUJwrEe"
},
{
"timestamp": "2019-01-18T10:15:32.250Z",
"unique_key": "transaction:dev_tx_000000BansDm7JjbiFjqm6TTTFPdo"
}
]
still in Zulu time but we are including milliseconds. The standard .iso8601 decoding strategy will not recognise this as ISO8601-formatted.
Oh dear!
So we can use a custom decoder -
extension JSONDecoder {
let decoder = JSONDecoder()
let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
decoder.keyDecodingStrategy = .convertFromSnakeCase
decoder.dateDecodingStrategy = .formatted(dateFormatter)
return decoder
}()
}and since we are trying to get rid of the annoying underscore_ in the variable name when we are converting this into Swift, we can use the CodingKey protocol to make this happen!
struct TimeModel: Codable {
let timeStamp: Date
let uniqueKey: String
enum CodingKeys: String, CodingKey {
case timeStamp = "timestamp"
case uniqueKey = "uniqueKey"
}
}In the repo I decode json Strings from a bundle. In order to use the modified JSONDecoder that I use would be updated too.
The update of that? It's right here:
extension Bundle {
func decode<T: Decodable>(_ type: T.Type, from file: String, with decoder: JSONDecoder = JSONDecoder()) throws -> T {
guard let url = self.url(forResource: file, withExtension: nil) else {
throw ErrorModel(errorDescription: "\(file) missing in \(self).")
}
guard let data = try? Data(contentsOf: url) else {
throw ErrorModel(errorDescription: "\(file) missing in \(self).")
}
do {
let decoded = try decoder.decode(T.self, from: data)
return decoded
} catch {
throw ErrorModel(errorDescription: "\(file) could not be decoded from \(self) with error: \(error).")
}
}
}You might have a field in a rather large list that is missing a field or two.
[
{
"timestamp": "2019-01-18T10:15:29.979Z",
"unique_key": "policy:dev_pol_000000BansDm7Jjbj3k4R1IUJwrEe"
},
{
"unique_key": "transaction:dev_pol_000000BsrbGm3JjbiFjqm6TTTFPdo"
},
{
"timestamp": "2019-01-18T10:15:32.250Z",
"unique_key": "transaction:dev_tx_000000BansDm7JjbiFjqm6TTTFPdo"
}
]It's not enough just to try to process the fields as above.
That is, the following code block:
func missingFieldsFile() {
let times: [TimeModel] = try! Bundle.main.decode([TimeModel].self, from: "TimesMissingTimeStamp.json", with: JSONDecoder.snakeCaseMillisecondsISO8601Date)
print(times)
}will generate a DecodingError and a crash for the user.
We need to think about a better strategy in order to create the right experience for the user.
One way of handling this is to create an optional field in the model for that particular type. The problem comes when we require that field for a view, and therefore just wish to skip the object and move on. There must be a good solution for this.
So could we, I don't know, create our own decoder for this? The following implementation uses the TimeModel declared above, but also RUNS FOREVER IF WE ARE MISSING A SINGLE TIMESTAMP SO DON'T DO THIS.
// Don't do this
struct TimeModelInfiniteLoop: Codable {
var times: [TimeModel]
init(from decoder: Decoder) throws {
var times = [TimeModel]()
var container = try decoder.unkeyedContainer()
while !container.isAtEnd {
if let route = try? container.decode(TimeModel.self) {
times.append(route)
}
}
self.times = times
}
}What is happening here is decoder.unkeyedContainer() does not move onto the next value if one is missing. You'd want an else and skip the value if you can't append the route to the times array.
Can we create a solution?
It seems unfortunate, and like a hack. But we create DummyCodable struct.
struct TimeModelCodable: Codable {
var times: [TimeModel]
init(from decoder: Decoder) throws {
var times = [TimeModel]()
var container = try decoder.unkeyedContainer()
while !container.isAtEnd {
if let route = try? container.decode(TimeModel.self) {
times.append(route)
}
else {
_ = try? container.decode(DummyCodable.self)
}
}
self.times = times
}
}
private struct DummyCodable: Codable {}This suggestion is from swiftlang/swift-corelibs-foundation#4414, and although it seems that there is a better solution in the works from Swift wizards so we shall see.
I hope this article has been of help to you.
Happy programming!
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