我已经下载了一个可以帮助我保存和加载自定义变量的 swift 文件:
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()
}
}
而且我还有用于保存/加载 Person
结构的代码(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,所有这些都确实有效。
但是,假设我以这种方式发布我的应用程序,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 Person
中没有 favorite
变量)将在没有 favorite
变量的情况下保存。更新后,应用程序将尝试使用 favorite
变量加载之前保存的 Person
。这就是它失败的地方,因为旧版本数据中没有 favorite
变量。所以它会抛出一个错误。
我的问题是,当它从用户默认值解码 Person
时,如果没有找到任何匹配的变量(例如:favorite
), 而不是抛出错误,它会尝试自动创建它? (来自 var favorite
= false
)?
我的项目:https://github.com/orihpt/Encodable
提前致谢。
最佳答案
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
并默认为 false
。
另一种方法是将favorite
声明为可选:
var favorite: Bool?
这将导致 favorite
被设置为 nil
如果它不存在于数据中,而不是您想要的 false
。如果你真的想要 false
,你可以使用隐式解包的可选 Bool?
,你需要将 nil
更改为 false
每次解码时:
self.person = try UserDefaults.standard.getToObject(forKey: "person", castTo: Person.self)
if self.person.favorite == nil {
self.person.favorite = false
}
如果你担心你可能会忘记这样做,你可以让 getToObject
只接受符合这个协议(protocol)的对象:
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
}
}
}
并且你将无法执行 getToObject(forKey: "person", castTo: Person.self)
除非你使 Person
符合 HasDefaults
:
extension Person : HasDefaults {
func changeNilsToDefaults() {
if self.person.favorite == nil {
self.person.favorite = false
}
}
}
关于swift - 将变量添加到已保存的用户默认值变量,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/64747293/