如何观察 UserDefaults 的变化? [英] How to observe changes in UserDefaults?

问题描述

我的视图中有一个 @ObservedObject:

struct HomeView: View {

    @ObservedObject var station = Station()

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

根据来自 Station.status 的 String 更新文本:

which updates text based on a String from Station.status:

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

但是，我需要在我的 AppDelegate 中更改 status 的值，因为那是我收到我的 Firebase 云消息:

However, I need to change the value of status in my AppDelegate, because that is where I receive my Firebase Cloud Messages:

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
}

但是如果我更改 AppDelegate 中的 status UserDefaults 值 - 在我看来它不会更新.

But if I change the status UserDefaults value in AppDelegate - it won't update in my view.

当status发生变化时，如何通知我视图中的@ObservedObject?

How can my @ObservedObjectin my view be notified when status changes?

忘记提及在上述示例中使用了 SwiftUI 的 2.0 测试版.

Forgot to mention that the 2.0 beta version of SwiftUI is used in the said example.

推荐答案

这里有可能的解决方案

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 允许您直接通过 AppStorage 在视图中使用/观察 UserDefaults，因此如果您只需要在视图中显示该状态，你可以使用

Note: SwiftUI 2.0 allows you to use/observe UserDefaults in view directly via AppStorage, so if you need that status only in view, you can just use

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

这篇关于如何观察 UserDefaults 的变化?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！