为什么在swift4中的Decode方法给对象nil如果json负载中有一个标签类型不匹配

Question

I've json payload as below

{
    "name": "Dog",
    "type": 1
}

and the class which I want to convert from json to custom type is as below

class Animal: Codable {
      var name: String?
      var type: String?
}

Decoding process:

let decoder = JSONDecoder()
        var animal: Animal?

        do {
            animal = try decoder.decode(Animal.self, from: data)
        } catch DecodingError.keyNotFound(let key, let context) {

        } catch DecodingError.valueNotFound(let type, let context) {

        } catch DecodingError.typeMismatch(let type, let context) {
            print("mismatch")
        }
        catch {
            print("some error \(error.localizedDescription)")
        }

        print(animal?.name ?? "Decode did not happen!")

Animal object is nil. However as per apple WWDC talk( https://developer.apple.com/videos/play/wwdc2017/212/ ) it supposed to assign value to nil for type property. Since there is a mismatch in "type" data. (Expected String but Int has been found)

Can you guess what is the reason behind this. If any one of the tag data type is mismatched then whole object is becoming nil doesn't sounds good to me.

Answer 1

Please read the error message carefully, the reason is very clear (no need to guess )

Expected String but Int has been found

means the (found) value is Int but you declared a String property

All strings in JSON are wrapped in double quotes, the type for type is Int

class Animal: Codable {
    var name: String?
    var type: Int?
}

If the JSON contains always both values declare the properties as non-optional by removing the question marks.

The implicit decoder / initializer fails if any error occurs, you can see this just from the code syntax. If you want to have finer control write your own custom initializer.

Answer 2

To get nil only for type mismatched property :

required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    name = (try? container.decodeIfPresent(String.self, forKey: .name)) ?? nil
    type = (try? container.decodeIfPresent(String.self, forKey: .type)) ?? nil
}