问题描述
import Foundation
protocol ObjectSavable {
func setToObject<Object>(_ object: Object,forKey: String) throws where Object: encodable
func getToObject<Object>(forKey: String,castTo type: Object.Type) throws -> Object where Object: Decodable
}
extension UserDefaults: ObjectSavable {
func setToObject<Object>(_ object: Object,forKey: String) throws where Object: encodable {
let encoder = JSONEncoder()
do {
let data = try encoder.encode(object)
set(data,forKey: forKey)
} catch {
throw ObjectSavableError.unabletoEncode
}
}
func getToObject<Object>(forKey: String,castTo type: Object.Type) throws -> Object where Object: Decodable {
guard let data = data(forKey: forKey) else { throw ObjectSavableError.novalue }
let decoder = JSONDecoder()
do {
let object = try decoder.decode(type,from: data)
return object
} catch {
throw ObjectSavableError.unabletoDecode
}
}
}
enum ObjectSavableError: String,LocalizedError {
case unabletoEncode = "Unable to encode object into data"
case novalue = "No data object found for the given key"
case unabletoDecode = "Unable to decode object into given type"
}
我有这个Person
结构:
struct Person: encodable,Decodable {
var firstName: String
var lastName: String
var birthday: Date
init() {
self.firstName = "Tim"
self.lastName = "Cook"
self.birthday = Date()
}
}
我也有这段代码用于保存/加载Person
结构(正在使用上面的代码)
保存:
print("Saving object...")
let person: Person = Person()
do {
try UserDefaults.standard.setToObject(person,forKey: "person")
print("Object saved successfully")
} catch let err {
print("Error while saving object:\n\(err.localizedDescription)")
}
正在加载:
print("Loading object...")
do {
self.person = try UserDefaults.standard.getToObject(forKey: "person",castTo: Person.self)
print("Successfully load object:\n\(self.person!)")
} catch let err {
print("Error while loading object:\n\(err.localizedDescription)")
}
现在,所有这些都有效。
但是,假设我以这种方式发布了我的应用,然后我想向Person
添加一个新变量,例如,我将添加一个favorite
:
struct Person: encodable,Decodable {
var firstName: String
var lastName: String
var birthday: Date
var favorite: Bool = false
init() {
self.firstName = "Tim"
self.lastName = "Cook"
self.birthday = Date()
}
}
在更新之前,应用程序(在favorite
中没有Person
变量)将被保存,而没有favorite
变量。更新之后,应用程序将尝试使用Person
变量加载先前保存的favorite
。这就是失败的原因,因为较旧版本的数据中没有favorite
变量。因此会引发错误。
我的问题是,有没有办法在从用户默认值中解码Person
时,如果找不到任何匹配的变量(例如:favorite
),而不是抛出错误,它将尝试自动创建吗? (来自var favorite
= false
)?
我的项目:https://github.com/orihpt/Encodable
谢谢。
解决方法
一种方法是将自定义解码代码添加到Person
中:
enum CodingKeys : CodingKey {
case firstName
case lastName
case birthday
case favorite
}
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)
birthday = try container.decode(Date.self,forKey: .birthday)
favorite = try container.decodeIfPresent(Bool.self,forKey: .favorite) ?? false
}
请注意,对于favorite
,我使用了decodeIfPresent
,默认值为false
。
另一种方法是将favorite
声明为可选:
var favorite: Bool?
如果数据中不存在favorite
,而不是您想要的nil
中没有false
,则会将其设置为false
。如果您确实想要Bool?
,则可以使用一个隐式解包的可选nil
,并且每次解码时都需要将false
更改为self.person = try UserDefaults.standard.getToObject(forKey: "person",castTo: Person.self)
if self.person.favorite == nil {
self.person.favorite = false
}
:
getToObject
如果您担心自己可能会忘记这样做,可以使protocol HasDefaults {
func changeNilsToDefaults()
}
extension UserDefaults {
func getToObject<Object: HasDefaults>(forKey: String,castTo type: Object.Type) throws -> Object where Object: Decodable {
guard let data = data(forKey: forKey) else { throw ObjectSavableError.noValue }
let decoder = JSONDecoder()
do {
let object = try decoder.decode(type,from: data)
object.changeNilsToDefaults() // notice this line!
return object
} catch {
throw ObjectSavableError.unableToDecode
}
}
}
仅接受符合此协议的对象:
getToObject(forKey: "person",castTo: Person.self)
除非您使Person
符合HasDefaults
,否则您将无法执行extension Person : HasDefaults {
func changeNilsToDefaults() {
if self.person.favorite == nil {
self.person.favorite = false
}
}
}
:
{{1}},
@Sweeper的建议是可行的解决方案。您还可以将其“可选”方法与DTO解决方案合并。
将对象另存为DTO对象,并在首次发行后将新属性添加为“可选”:
struct PersonDTO: Codable {
let firstName: String
let lastName: String
let birthday: Date
let favorite: Bool?
}
从UserDefaults获取DTO对象后,使用它初始化您的Person对象。
struct Person {
var firstName: String
var lastName: String
var birthday: Date
var favorite: Bool
init(_ dto: PersonDTO) {
self.firstName = dto.firstName
self.lastName = dto.lastName
self.birthday = dto.birthday
self.favorite = dto.favorite ?? false
}
}
,
创建一个 protected
变量:
struct Person: Encodable,Decodable {
var firstName: String
var lastName: String
var birthday: Date
var favorite: Bool {
get {
return favoriteProtected ?? false
}
set {
favoriteProtected = newValue
}
}
private var favoriteProtected: Bool? = nil
init() {
self.firstName = "Tim"
self.lastName = "Cook"
self.birthday = Date()
}
}
这样您就不必实施 init(from decoder: Decoder)
,如果您的 struct
很大,这可能需要很长时间。