JSONDecoder - decoding string values to correct types

Question

I'm consuming a JSON api that returns every field as a string. However some of those string values actually represent a timestamp and a longitude and latitude (as separate fields).

If I specify these values as a Date and Doubles respectively it swift throws an error Expected to decode Double but found a string

Struct Place {
    var created_at: Date?
    var longitude: Double?
    var latitude: Double?

}

I've seen some examples that seem to deal with Date formatting online, but I'm unsure what to do around the Double values?

I tried overriding init(from decoder:Decoder) but this still doesn't give the correct values:

e.g

init(from decoder:Decoder) throws {
    let values = try decoder.container(keyedBy: CodingKeys.self)
    longitude = try values.decode(Double.self, forKey: .longitude) //returns nil but using String works fine
}

I know I can extract the string and convert to a double like:

  longitude = Double(try values.decode(String.self, forKey: .longitude))

But is there another way?

Furthermore, I actually have a number of other properties that convert fine, but by doing this work around for these 3 properties, I now have to add all the other properties in there which seems a bit redundant. Is there a better way to do this?

vadian · Accepted Answer · 2018-10-15 18:51:43Z

1

If the JSON value is String you have to decode String. An implicit type conversion from String to Double is not possible.

You can avoid a custom initializer by adding a computed variable coordinate, in this case the CodingKeys are required

import CoreLocation

struct Place : Decodable {

    private enum CodingKeys: String, CodingKey { case createdAt = "created_at", longitude, latitude}

    let createdAt: Date
    let longitude: String
    let latitude: String

    var coordinate : CLLocationCoordinate2D {
        return CLLocationCoordinate2D(latitude: Double(latitude) ?? 0.0, longitude: Double(longitude) ?? 0.0)
    }
}

answered Oct 15, 2018 at 18:51

vadian

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2 Comments

TommyBs Over a year ago

It looks like I still need the custom init for the date unfortunately as that is also a string. But I like this computed property for the coordinate

vadian Over a year ago

You can add a date decoding strategy to decode the String to Date

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