SwiftUI ObservableObject 子属性修改不能刷新问题

class MainModel : ObservableObject {
    @Published var child : ChildModel
   
    init(element : ChildModel) {
        self.child = element

        
    }
}

class ChildModel : ObservableObject {
    @Published var value : Int
    init(value : Int) {
        self.value = value
    }
}

struct MainView: View {
    @ObservedObject var model : MainModel
   
    
    var body: some View {
        HStack {
            Text("Detail:")
            Text("\(model.child.value)")
            Button("Change Numbers"){
                model.child.value += 1
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {

    static var previews: some View {
        MainView(model: MainModel(element: ChildModel(value: 0)))
    }
}

struct ChildModel {
    var value : Int
}

struct ChildView: View {
    @ObservedObject var element : ChildModel
    var body: some View {
      
        Text("\(element.value)")
    }
}

struct MainView: View {
    @ObservedObject var model : MainModel
    var body: some View {
        HStack {
            Text("Detail:")
            //Text("\(model.child.value)")
            ChildView(element: model.child)
            Button("Change Numbers"){
                model.child.value += 1
            }
        }
    }
}

class MainModel : ObservableObject {
    @Published var child : ChildModel
    var cancellable : AnyCancellable?
    init(element : ChildModel) {
        self.child = element
        self.cancellable = self.child.$value.sink(
                  receiveValue: { [weak self] _ in
                      self?.objectWillChange.send()
                  }
        )
        
    }
}