swift - 将变量添加到已保存的用户默认值变量

Question

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。

Answer 1

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
        }
    }
}