NotificationCenter.default not trigger UserDefaults.didChangeNotification at extension widget?

expect:

1. app update UserDefaults use group value.
2. widget use NotificationCenter.default observe same group UserDefaults, then reloadTimeline

actual:

1. successfully set
2. no reload happen, and I try set same key to another value inside the selector function to test whether it's been called. provided the selector not work.

steps:

1. click a app button to set a key of UserDefaults
2. back home , widget no change - expect it update view based one new value
3. rebuild widget , can see the ui use new value - prove set call successfully but notification of UserDefaults.didChangeNotification not send

host app :

UserDefaults(suiteName: "group.a")
store?.setValue("a", forKey: "target")

widget:

class NotiWatch {

   init(){

        NotificationCenter.default.addObserver(self, selector: #selector(onchange), name: UserDefaults.didChangeNotification, object: nil)

    }

    @objc func onchange(notification: NSNotification) { 
        UserDefaults(suiteName: "group.a")?.set("cccc", forKey: "target") 
       WidgetCenter.shared.reloadAllTimelines()

    }

    deinit{

        NotificationCenter.default.removeObserver(self, name: UserDefaults.didChangeNotification, object: nil)

    }

}



struct Provider: TimelineProvider {

    let def:UserDefaults? ;

    let watcher = NotiWatch()

    init(){

        let fmt = DateFormatter()

        fmt.dateFormat = "yyyy-MM-dd HH:mm"

        dd = fmt.date(from: "2070-12-20 23:59")!

        def = UserDefaults(suiteName: "group.a") ; 

    }

    var dd =  Date()

    func placeholder(in context: Context) -> SimpleEntry {

        SimpleEntry(date: Date(), unit: "Y",left: 0,tar: "")

    }



    func getSnapshot(  in context: Context, completion: @escaping (SimpleEntry) -> ()) {

        let date = Date()

        let left = dd.timeIntervalSince(date).minutes

        let entry = SimpleEntry(date: date, unit: "M",left: left,tar: "")

         

        completion(entry)

    }



    func getTimeline( in context: Context, completion: @escaping (Timeline<Entry>) -> ()) {

        var entries: [SimpleEntry] = []



        // Generate a timeline consisting of five entries an hour apart, starting from the current date.

        let tar = def?.string(forKey: "target") ?? "" 

        let currentDate = Date() 

        for offset in 0 ..< 60 {

            let entryDate = Calendar.current.date(byAdding: .minute, value: offset, to: currentDate)!

            

            let left = dd.timeIntervalSince(entryDate).minutes

            let entry = SimpleEntry(date: currentDate, unit: "minutes",left:left,tar: tar)

            entries.append(entry)

        }



        let timeline = Timeline(entries: entries, policy: .atEnd)

        completion(timeline)

    }

}