Swift 4 decoding the rest of an array when one of the items in the array throws a DecodingError

Question

Is it possible to return the successfully decoded objects of an array if one of the objects throws a DecodingError? I'm communicating with quite a ropey API which intermittently sends me some malformed JSON which causes a DecodingError to be thrown and the entire array of objects (including valid entries) is not parsed.

I don't know if there's some way to do it manually, but given an array of JSON with some duff data in it, I'd like to somehow still get at the objects which can be successfully decoded.

EDIT: As requested, code sample:

let json = """
    [
        {"prop1": 1, "prop2": 2, "prop3": 3},
        {"prop1": 1, "prop2": 2, "prop3": 3},                                   
        {"prop1": 1, "prop2": 2}
    ]
    """.data(using: .utf8, allowLossyConversion: false)!

struct MyStruct: Codable {

    let prop1: Int
    let prop2: Int
    let prop3: Int
}

// Throws keyNotFound (uncaught obviously) and returns nil because prop3 in 3rd object is missing
let decoded = try? JSONDecoder().decode(MyStruct.self, from: json)

Obviously a contrived example, but hopefully illustrates my point. If a mandatory property is missing from an object in the response array then I'd like that object to be removed from the list of decoded objects rather than bombing out and returning nothing.

This also applies to all the other DecodingErrors (dataCorrupted, typeMismatch, valueNotFound).

can you put your code here? – Luiz Fernando Salvaterra Oct 25 '17 at 09:23 — Luiz Fernando Salvaterra, Oct 25 '17 at 09:23
please past your code here and screenshot please – swift2geek Oct 25 '17 at 10:16 — swift2geek, Oct 25 '17 at 10:16

score 0 · Answer 1 · answered Oct 25 '17 at 09:31

How about decoding item per item with error Handling embbeded ? This way your program would continue, plus you could display a decoding error for a given item.

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/ErrorHandling.html

score 0 · Answer 2 · answered Oct 25 '17 at 09:34

You can custom decoding.

JSONDecoder().dataDecodingStrategy = .custom({ (decoder) -> Data in
    // custom decode
    return Data()
})

Or handle with unexpected float.

JSONDecoder().nonConformingFloatDecodingStrategy = .convertFromString(positiveInfinity: "s1", negativeInfinity: "s2", nan: "s3")

Swift 4 decoding the rest of an array when one of the items in the array throws a DecodingError

2 Answers2