How to exclude properties from Swift 4's Codable
The list of keys to encode/decode is controlled by a type called CodingKeys
(note the s
at the end). The compiler can synthesize this for you but can always override that.
Let's say you want to exclude the property nickname
from both encoding and decoding:
struct Person: Codable {
var firstName: String
var lastName: String
var nickname: String?
private enum CodingKeys: String, CodingKey {
case firstName, lastName
}
}
If you want it to be asymmetric (i.e. encode but not decode or vice versa), you have to provide your own implementations of encode(with encoder: )
and init(from decoder: )
:
struct Person: Codable {
var firstName: String
var lastName: String
// Since fullName is a computed property, it's excluded by default
var fullName: String {
return firstName + " " + lastName
}
private enum CodingKeys: String, CodingKey {
case firstName, lastName, fullName
}
// We don't want to decode `fullName` from the JSON
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
firstName = try container.decode(String.self, forKey: .firstName)
lastName = try container.decode(String.self, forKey: .lastName)
}
// But we want to store `fullName` in the JSON anyhow
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(firstName, forKey: .firstName)
try container.encode(lastName, forKey: .lastName)
try container.encode(fullName, forKey: .fullName)
}
}
If we need to exclude decoding of a couple of properties from a large set of properties in the structure, declare them as optional properties. Code to unwrapping optionals is less than writing a lot of keys under CodingKey enum.
I would recommend using extensions to add computed instance properties and computed type properties. It separates codable comforming properties from other logic hence provides better readability.
Another way to exclude some properties from encoder, separate coding container can be used
struct Person: Codable {
let firstName: String
let lastName: String
let excludedFromEncoder: String
private enum CodingKeys: String, CodingKey {
case firstName
case lastName
}
private enum AdditionalCodingKeys: String, CodingKey {
case excludedFromEncoder
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let anotherContainer = try decoder.container(keyedBy: AdditionalCodingKeys.self)
firstName = try container.decode(String.self, forKey: .firstName)
lastName = try container.decode(String.self, forKey: .lastName)
excludedFromEncoder = try anotherContainer(String.self, forKey: . excludedFromEncoder)
}
// it is not necessary to implement custom encoding
// func encode(to encoder: Encoder) throws
// let person = Person(firstName: "fname", lastName: "lname", excludedFromEncoder: "only for decoding")
// let jsonData = try JSONEncoder().encode(person)
// let jsonString = String(data: jsonData, encoding: .utf8)
// jsonString --> {"firstName": "fname", "lastName": "lname"}
}
same approach can be used for decoder