SwiftUI：如何使用 UserDefaults 保留 @Published 变量？

Question

我希望保留一个@Published 变量，以便每次重新启动我的应用程序时它都相同。

我想在一个变量上同时使用@UserDefault 和@Published 属性包装器。例如我需要一个'@PublishedUserDefault var isLogedIn'。

我有以下属性Wrapper

import Foundation

@propertyWrapper
struct UserDefault<T> {
    let key: String
    let defaultValue: T

    init(_ key: String, defaultValue: T) {
        self.key = key
        self.defaultValue = defaultValue
    }

    var wrappedValue: T {
        get {
            return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
        }
        set {
            UserDefaults.standard.set(newValue, forKey: key)
        }
    }
}

这是我的设置class

import SwiftUI
import Combine

 class Settings: ObservableObject {

   @Published var isLogedIn : Bool = false

 func doLogin(params:[String:String]) {

        Webservice().login(params: params) { response in

            if let myresponse = response {                    
                    self.login = myresponse.login
                    }
               }
         }

}

我的观点class

struct HomeView : View {
    @EnvironmentObject var settings: Settings
    var body: some View {
        VStack {
            if settings.isLogedIn {
            Text("Loged in")
            } else{
            Text("Not Loged in")
            }
        }
    }
}

有没有办法制作一个涵盖持久化和发布的属性包装器？

Answer 1

您目前无法将 @UserDefault 环绕在 @Published 周围，因为目前不允许这样做。

实现@PublishedUserDefault的方法是将一个objectWillChange传递给包装器，并在设置变量之前调用它。

Answer 2

private var cancellables = [String:AnyCancellable]()

extension Published {
    init(wrappedValue defaultValue: Value, key: String) {
        let value = UserDefaults.standard.object(forKey: key) as? Value ?? defaultValue
        self.init(initialValue: value)
        cancellables[key] = projectedValue.sink { val in
            UserDefaults.standard.set(val, forKey: key)
        }
    }
}

class Settings: ObservableObject {
    @Published(key: "isLogedIn") var isLogedIn = false
    ...
}

样本：https://youtu.be/TXdAg_YvBNE

所有 Codable 类型的版本请查看 here

Answer 3

应该可以编写一个新的属性包装器：

Composition was left out of the first revision of this proposal, because one can manually compose property wrapper types. For example, the composition @A @B could be implemented as an AB wrapper:

@propertyWrapper
struct AB<Value> {
  private var storage: A<B<Value>>

  var wrappedValue: Value {
    get { storage.wrappedValue.wrappedValue }
    set { storage.wrappedValue.wrappedValue = newValue }
  }
}

The main benefit of this approach is its predictability: the author of AB decides how to best achieve the composition of A and B, names it appropriately, and provides the right API and documentation of its semantics. On the other hand, having to manually write out each of the compositions is a lot of boilerplate, particularly for a feature whose main selling point is the elimination of boilerplate. It is also unfortunate to have to invent names for each composition---when I try the compose A and B via @A @B, how do I know to go look for the manually-composed property wrapper type AB? Or maybe that should be BA?

参考：Property WrappersProposal: SE-0258

Answer 4

struct HomeView : View {
    @StateObject var auth = Auth()
    @AppStorage("username") var username: String = "Anonymous"

    var body: some View {
        VStack {
            if username != "Anonymous" {
                Text("Logged in")
            } else{
                Text("Not Logged in")
            }
        }
        .onAppear(){
             auth.login()
        }
    }
}


import SwiftUI
import Combine

class Auth: ObservableObject {

 func login(params:[String:String]) {

        Webservice().login(params: params) { response in

            if let myresponse = response {          
                    UserDefaults.standard.set(myresponse.login, forKey: "username")`
                    }
               }
         }

}

Answer 5

要保留您的数据，您可以使用 @AppStorage 属性包装器。但是，如果不使用 @Published，您的 ObservableObject 将不再发布有关更改数据的消息。要解决此问题，只需从属性的 willSet 观察者调用 objectWillChange.send()。

import SwiftUI

class Settings: ObservableObject {
    @AppStorage("Example") var example: Bool = false {
        willSet {
            // Call objectWillChange manually since @AppStorage is not published
            objectWillChange.send()
        }
    }
}

SwiftUI：如何使用 UserDefaults 保留 @Published 变量？

SwiftUI: How to persist @Published variable using UserDefaults?

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