SwiftUI: How to persist @Published variable using UserDefaults?

问题

I want a @Published variable to be persisted, so that it's the same every time when I relaunch my app.

I want to use both the @UserDefault and @Published property wrappers on one variable. For example I need a '@PublishedUserDefault var isLogedIn'.

I have the following propertyWrapper

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)
        }
    }
}

This is my Settings 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
                    }
               }
         }

}

My View class

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

Is there a way to make a single property wrapper that covers both the persisting and the publishing?

回答1:

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
    ...
}

Sample: https://youtu.be/TXdAg_YvBNE

回答2:

It should be possible to compose a new property wrapper:

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?

Ref: Property WrappersProposal: SE-0258

回答3:

You currently can't wrap @UserDefault around @Published since that is not currently allowed.

The way to implement @PublishedUserDefault is to pass an objectWillChange into the wrapper and call it before setting the variable.

来源：https://stackoverflow.com/questions/57611658/swiftui-how-to-persist-published-variable-using-userdefaults