How to handle JSON format inconsistencies with Swift 4 Codable? - ios

Question

I need to parse JSON of which one of the field value is either an array:
"list" :
[
{
"value" : 1
}
]
or an empty string, in case there's no data:
"list" : ""
Not nice, but I can't change the format.
I'm looking at converting my manual parsing, for which this was easy, to JSONDecoder and Codable struct's.
How can I handle this nasty inconsistency?

Answer 1

You need to try decoding it one way, and if that fails, decode it the other way. This means you can't use the compiler-generated decoding support. You have to do it by hand. If you want full error checking, do it like this:
import Foundation
struct ListItem: Decodable {
var value: Int
}
struct MyResponse: Decodable {
var list: [ListItem] = []
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
do {
list = try container.decode([ListItem].self, forKey: .list)
} catch {
switch error {
// In Swift 4, the expected type is [Any].self, but I think it will be [ListItem].self in a newer Swift with conditional conformance support.
case DecodingError.typeMismatch(let expectedType, _) where expectedType == [Any].self || expectedType == [ListItem].self:
let dummyString = try container.decode(String.self, forKey: .list)
if dummyString != "" {
throw DecodingError.dataCorruptedError(forKey: .list, in: container, debugDescription: "Expected empty string but got \"\(dummyString)\"")
}
list = []
default: throw error
}
}
}
enum CodingKeys: String, CodingKey {
case list
}
}
If you want no error checking, you can shorten init(from:) to this:
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
list = (try? container.decode([ListItem].self, forKey: .list)) ?? []
}
Test 1:
let jsonString1 = """
{
"list" : [ { "value" : 1 } ]
}
"""
print(try! JSONDecoder().decode(MyResponse.self, from: jsonString1.data(using: .utf8)!))
Output 1:
MyResponse(list: [__lldb_expr_82.ListItem(value: 1)])
Test 2:
let jsonString2 = """
{
"list" : ""
}
"""
print(try! JSONDecoder().decode(MyResponse.self, from: jsonString2.data(using: .utf8)!))
Output 2:
MyResponse(list: [])