如何在Swift中实现一个通用结构来管理UserDefaults的键值对? [英] How can I implement a generic struct that manages key-value pairs for UserDefaults in Swift?
问题描述
如何在Swift中实现一种管理UserDefaults映射的结构?现在,我具有一些不同类型的计算属性a,b,c,d和对应的键,如下所示:
How would one implement a struct that manages UserDefaults mappings in Swift? Right now I have some computed properties a, b, c, d of different types and corresponding keys that look like this:
enum UserDefaultsKeys {
a_key
b_key
...
}
var a: String {
get { UserDefaults.standard.string(forKey: UserDefaultsKeys.a_key.rawValue) }
set { UserDefaults.standard.set(newValue, forKey: UserDefaultsKeys.a_key.rawValue) }
}
var b: Int {
get { UserDefaults.standard.integer(forKey: UserDefaultsKeys.b_key.rawValue) }
set { UserDefaults.standard.set(newValue, forKey: UserDefaultsKeys.b_key.rawValue) }
}
...
我想实现的是实现一个具有String类型的键和泛型类型值的结构.值get函数应该-根据其类型-选择是否使用 UserDefaults.standard.string
或 UserDefaults.standard.integer
,这样我就可以创建带有某些内容的DefaultsVar密钥,其他所有内容都会自动为我管理.到目前为止,我的情况是这样:
What I would like to achieve instead is to implement a struct that has a key of type String and a generic type value.
The value get function should - depending on its type - chose wether to use UserDefaults.standard.string
or UserDefaults.standard.integer
so that I can just create a DefaultsVar with some key and everything else is managed automatically for me.
What I have so far is this:
struct DefaultsVar<T> {
let key: String
var value: T {
get {
switch self {
case is String: return UserDefaults.standard.string(forKey: key) as! T
case is Int: return UserDefaults.standard.integer(forKey: key) as! T
default: return UserDefaults.standard.float(forKey: key) as! T
}
}
set { UserDefaults.standard.set(newValue, forKey: key) }
}
}
我收到以下错误:从DefaultsVar转换为不相关的类型'String'总是失败.我是新手(对编程来说是相对较新的),而且并不真正了解如何以正确的方式实现这种方法,或者即使这是一种被认为是一种好的做法,我也不是很了解.有人可以照一下吗?
I get the following error: "Cast from DefaultsVar to unrelated Type 'String' always fails. I am completely new to swift (and relatively new to programming) and don't really understand how to implement this the right way - or if this is even an approach that would be considered a good practice. Could someone please shine some light on this?
提前谢谢!
推荐答案
您可以使用自定义
@propertyWrapper
struct UserDefaultStorage<T: Codable> {
private let key: String
private let defaultValue: T
private let userDefaults: UserDefaults
init(key: String, default: T, store: UserDefaults = .standard) {
self.key = key
self.defaultValue = `default`
self.userDefaults = store
}
var wrappedValue: T {
get {
guard let data = userDefaults.data(forKey: key) else {
return defaultValue
}
let value = try? JSONDecoder().decode(T.self, from: data)
return value ?? defaultValue
}
set {
let data = try? JSONEncoder().encode(newValue)
userDefaults.set(data, forKey: key)
}
}
}
此包装器可以将任何一种可编码的代码存储/恢复到用户默认设置中.
This wrapper can store/restore any kind of codable into/from the user defaults.
@UserDefaultStorage(key: "myCustomKey", default: 0)
var myValue: Int
iOS 14
SwiftUI有一个类似的包装(仅适用于iOS 14),名为 @AppStorage
,它可以用作状态.使用它的好处是它可以直接用作 State
.但是它需要 SwiftUI
,并且只能在iOS 14上运行.
iOS 14
SwiftUI has a similar wrapper (only for iOS 14) called @AppStorage
and it can be used as a state. The advantage of using this is that it can be used directly as a State
. But it requires SwiftUI
and it only works from the iOS 14.
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