SwiftUI：@StateObject deinit不调用？

Question

我有以下代码：

struct ContentView: View {
    @Environment(\.managedObjectContext) private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \Item.timestamp, ascending: true)],
        animation: .default)
    private var items: FetchedResults<Item>

    var body: some View {
        NavigationView {
            List {
                ForEach(items) { item in
                    NavigationLink {
                        Text("Item at \(item.timestamp!, formatter: itemFormatter)")
                    } label: {
//                        Text(item.timestamp!, formatter: itemFormatter)
                        ItemCellView(model: ItemCellViewModel(item: item))
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }

    private func addItem() {
        withAnimation {
            let newItem = Item(context: viewContext)
            newItem.timestamp = Date()

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }

    private func deleteItems(offsets: IndexSet) {
        withAnimation {
            offsets.map { items[$0] }.forEach(viewContext.delete)

            do {
                try viewContext.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nsError = error as NSError
                fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
            }
        }
    }
}

private let itemFormatter: DateFormatter = {
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    formatter.timeStyle = .medium
    return formatter
}()

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView().environment(\.managedObjectContext, PersistenceController.preview.container.viewContext)
    }
}


struct ItemCellView: View {
    
    @StateObject var model:ItemCellViewModel
    
    var body: some View {
        
        Text(model.item.timestamp!, formatter: itemFormatter)
            .foregroundColor(.blue)
    }
    
}

class ItemCellViewModel: ObservableObject {
    
    @Published var item:Item
    
    init(item:Item) {
        self.item = item
    }
    
    deinit {
        print("ItemCellViewModel EDINIT \(self)")
    }
    
}

它得出的结论是：

​

​

问题：

在我滑动以删除项目后，不会调用ItemCellViewModel deinit。有人能告诉我为什么即使在ItemCellViewModel离开之后，ItemCellView仍然存在吗？

这是我正在研究的一个代码基的简化版本。当视图被用户“删除”时，我需要这个模型消失。为什么SwiftUI让ItemCellViewModel在身边？？

Answer 1

视图不会在一个事实中被删除(仅从可见区域中删除)，因为List缓存了一些视图(可见区域+ ~2)，而StateObject是保持其状态的持久视图存储。所以观察到的行为是精心设计的。

Answer 2

去掉视图模型对象，在SwiftUI中我们使用值类型，视图结构是SwiftUI用来代表我们创建和更新UIKit/AppKit视图的视图模型。在SwiftUI要点WWDC 2019中学习这个。而且，您也不能在ObservableObject中嵌套ObservableObject。若要修复此问题，请将ItemCellView更改为：

struct ItemCellView: View {
    
    @ObservedObject var item: Item
    
    var body: some View {
        
        Text(item.timestamp!, formatter: itemFormatter)
            .foregroundColor(.blue)
    }
    
}