在 SwiftUI 中更改对象的子项时刷新视图

Question

我正在开发一个包含两个实体 MyList 和 MyListItem 的 CoreData 应用程序。 MyList 可以有多个 MyListItem（一对多）。当应用程序启动时，我可以看到所有列表。我可以点击列表以转到列表项。在该屏幕上，我点击一个按钮将一个项目添加到所选列表中。之后，当我返回所有列表屏幕时添加项目，我看不到计数中反映的项目数。原因是 MyListsView 没有再次渲染，因为列表的数量没有改变。

完整代码如下：

import SwiftUI
import CoreData

extension MyList {
    
    static var all: NSFetchRequest<MyList> {
        let request = MyList.fetchRequest()
        request.sortDescriptors = []
        return request
    }
}

struct DetailView: View {
    
    @Environment(\.managedObjectContext) var viewContext
    let myList: MyList
    
    var body: some View {
        VStack {
            Text("Detail View")
            Button("Add List Item") {
                
                let myListP = viewContext.object(with: myList.objectID) as! MyList
                
                let myListItem = MyListItem(context: viewContext)
                myListItem.name = randomString()
                myListItem.myList = myListP
                
                try? viewContext.save()
                
            }
        }
    }
    
    func randomString(length: Int = 8) -> String {
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        return String((0..<length).map{ _ in letters.randomElement()! })
    }
}

class ViewModel: NSObject, ObservableObject {
    
    @Published var myLists: [MyList] = []
    
    private var fetchedResultsController: NSFetchedResultsController<MyList>
    private(set) var context: NSManagedObjectContext
    
    override init() {
        self.context = CoreDataManager.shared.context
        
        fetchedResultsController = NSFetchedResultsController(fetchRequest: MyList.all, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
        super.init()
        fetchedResultsController.delegate = self
        
        do {
            try fetchedResultsController.performFetch()
            guard let myLists = fetchedResultsController.fetchedObjects else { return }
            self.myLists = myLists
            
        }  catch {
            print(error)
        }
    }
}

extension ViewModel: NSFetchedResultsControllerDelegate {
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        guard let myLists = controller.fetchedObjects as? [MyList] else { return }
        self.myLists = myLists
    }
}

struct MyListsView: View {
    
    let myLists: [MyList]
    
    var body: some View {
        List(myLists) { myList in
            NavigationLink {
                DetailView(myList: myList)
            } label: {
                HStack {
                    Text(myList.name ?? "")
                    Spacer()
                    
                    Text("\((myList.items ?? []).count)")
                }
            }
        }
    }
}

struct ContentView: View {
    
    @StateObject private var vm = ViewModel()
    @Environment(\.managedObjectContext) var viewContext
    
    var body: some View {
        NavigationView {
            VStack {
               
                // when adding an item to the list the MyListView view is
                // not re-rendered
                MyListsView(myLists: vm.myLists)
                    
                Button("Change List") {
                   
                }
            }
        }
    }
    
    func randomString(length: Int = 8) -> String {
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        return String((0..<length).map{ _ in letters.randomElement()! })
    }
}

在 ContentView 中有一个名为“MyListsView”的视图。添加项目时不会呈现该视图。因为，根据那个观点，没有任何改变，因为列表的数量仍然相同。

你是怎么解决这个问题的？

更新：

如果我为 ListCellView 添加更多级别的视图，会发生什么情况，如下所示：

struct MyListCellView: View {
    
    @StateObject var vm: ListCellViewModel
    
    init(vm: ListCellViewModel) {
        _vm = StateObject(wrappedValue: vm)
    }
    
    var body: some View {
        HStack {
            Text(vm.name)
            Spacer()
            
            Text("\((vm.items).count)")
        }
    }
}

@MainActor
class ListCellViewModel: ObservableObject {
    
    let myList: MyList
    
    init(myList: MyList) {
        self.myList = myList
        self.name = myList.name ?? ""
        self.items = myList.items!.allObjects as! [MyListItem]
        print(self.items.count)
    }
    
    @Published var name: String = ""
    @Published var items: [MyListItem] = []
}

struct MyListsView: View {
    
    @StateObject var vm: ViewModel
    
    init(vm: ViewModel) {
        _vm = StateObject(wrappedValue: vm)
    }
    
    var body: some View {
        let _ = Self._printChanges()
        List(vm.myLists) { myList in
            NavigationLink {
                DetailView(myList: myList)
            } label: {
                MyListCellView(vm: ListCellViewModel(myList: myList))
            }
        }
    }
}

现在计数不再更新。

Answer 1

您的 ViewModel 是一个 ObserveableObject，但您没有在 MyListsView 中观察到它。当你初始化 MyListsView 时，你设置了一个 let 常量。当然不会更新。改为这样做：

struct MyListsView: View {
    
    @ObservedObject var vm: ViewModel

    init(viewModel: ViewModel) {
        self.vm = viewModel
    }
    
    var body: some View {
        List(vm.myLists) { myList in
            NavigationLink {
                DetailView(myList: myList)
            } label: {
                HStack {
                    Text(myList.name ?? "")
                    Spacer()
                    
                    Text("\((myList.items ?? []).count)")
                }
            }
        }
    }
}

现在 ViewModel 中的 @Published 会导致 MyListView 发生变化，其中包括添加相关实体。

Answer 2

首先摆脱你的视图模型，我们不在 SwiftUI 中使用 MVVM，我们使用视图数据结构来处理我们的视图数据和属性包装器使它们表现得像对象。在您的情况下，对列表使用 @FetchRequest 属性包装器，对细节使用 @ObservedObject 包装器，并且将在对模型数据进行任何更改时调用正文。检查 Xcode 中的应用程序模板中的代码，并检查核心数据。它看起来像这样：

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 {
                        DetailView(item: item)
                    } label: {
                        Text(item.timestamp!, formatter: itemFormatter)
                    }
                }
                .onDelete(perform: deleteItems)
            }
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    EditButton()
                }
                ToolbarItem {
                    Button(action: addItem) {
                        Label("Add Item", systemImage: "plus")
                    }
                }
            }
            Text("Select an item")
        }
    }
...
struct DetailView: View {
    @ObservedObject var item: Item
    
    var body: some View {
        Text("Item at \(item.timestamp!, formatter: itemFormatter)")

在 SwiftUI 中更改对象的子项时刷新视图

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swiftui