ios - 如何观察 UserDefaults 的变化？

Question

我有一个 @ObservedObject在我看来:

struct HomeView: View {

    @ObservedObject var station = Station()

    var body: some View {
        Text(self.station.status)
    }
 

它基于 String 更新文本来自 Station.status :

class Station: ObservableObject {
    @Published var status: String = UserDefaults.standard.string(forKey: "status") ?? "OFFLINE" {
        didSet {
            UserDefaults.standard.set(status, forKey: "status")
        }
    }      

但是，我需要更改 status 的值在我的 AppDelegate ，因为那是我收到我的 Firebase Cloud Messages 的地方:

func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                 fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
  // If you are receiving a notification message while your app is in the background,
  // this callback will not be fired till the user taps on the notification launching the application.

  // Print full message.
  let rawType = userInfo["type"]

  // CHANGE VALUE OF status HERE
}

但是如果我改变 status UserDefaults值在 AppDelegate - 在我看来它不会更新。
我的怎么可以 @ObservedObject在我看来 status 时会收到通知变化？
编辑:忘记提及 SwiftUI 的 2.0 测试版用于上述示例。

Answer 1

这是可能的解决方案

import Combine

// define key for observing
extension UserDefaults {
    @objc dynamic var status: String {
        get { string(forKey: "status") ?? "OFFLINE" }
        set { setValue(newValue, forKey: "status") }
    }
}

class Station: ObservableObject {
    @Published var status: String = UserDefaults.standard.status {
        didSet {
            UserDefaults.standard.status = status
        }
    }

    private var cancelable: AnyCancellable?
    init() {
        cancelable = UserDefaults.standard.publisher(for: \.status)
            .sink(receiveValue: { [weak self] newValue in
                guard let self = self else { return }
                if newValue != self.status { // avoid cycling !!
                    self.status = newValue
                }
            })
    }
}

注意:SwiftUI 2.0 允许您使用/观察 UserDefaults直接通过 AppStorage查看，因此如果您只需要查看该状态，则可以使用

struct SomeView: View {
    @AppStorage("status") var status: String = "OFFLINE"
    ...