使用 JSONDecoder 和 Swift 将 nil 转换为空字符串

Question

I am using Swift 4 and JSONDecoder . I have the following structure:

struct Customer: Codable {
    var id: Int!
    var cnum: String!
    var cname: String!
}

Note: the fields cannot be made optional.

Now I have a JSON string:

[
    {
        "id": 1,
        "cnum": "200",
        "cname": "Bob Smith"
    },
    {
        "id": 2,
        "cnum": "201",
        "cname": null
    }
]

And to decode it, I use the following:

let decoder = JSONDecoder()
let customers = try decoder.decode([Customer].self, from: json)

Everything works fine except the null data gets converted to nil. My question is, what would be the easiest way to convert incoming nil to an empty string ("")?

I would like to do this with the minimum amount of code but I'm not sure about the correct approach and at what point can the nil be converted to an empty string. Thank you beforehand.

Answer 1

You can use backing ivars:

struct Customer: Codable {
    var id: Int
    var cnum: String {
        get { _cnum ?? "" }
        set { _cnum = newValue }
    }
    var cname: String {
        get { _cname ?? "" }
        set { _cname = newValue }
    }
    
    private var _cnum: String?
    private var _cname: String?
    private enum CodingKeys: String, CodingKey {
        case id, _cnum = "cnum", _cname = "cname"
    }
}

Due to the custom CodingKeys , the JSON decoder will actually decode to _cnum and _cname , which are optional strings. We convert nil to empty string in the property getters.

Answer 2

You can use decodeIfPresent method.

struct Source : Codable {

    let id : String?


    enum CodingKeys: String, CodingKey {
        case id = "id"

    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        id = try values.decodeIfPresent(String.self, forKey: .id) ?? "Default value pass"

    }
}

Answer 3

You should make a computed variable that will have the original value if it's not nil and an empty string if it is.

var cnameNotNil: String {
    return cname ?? ""
}

Answer 4

The usual way is to write an initializer which handles the custom behavior. The null value is caught in an extra do - catch block.

struct Customer: Codable {
    var id: Int
    var cnum: String
    var cname: String

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        cnum = try container.decode(String.self, forKey: .cnum)
        do { cname = try container.decode(String.self, forKey: .cname) }
        catch { cname = "" }
    }
}

Answer 5

Use decodeIfPresent if the value from response might be null

cnum = try container.decode(String.self, forKey: .cnum)
cname = try container.decodeIfPresent(String.self, forKey: .cname)