SwiftUI 中的值没有被传递

Posted 2023-02-24

技术标签:

【中文标题】SwiftUI 中的值没有被传递【英文标题】：Value in SwiftUI not being passed 【发布时间】：2021-01-29 20:01:54 【问题描述】：

我正在尝试传递一个值，让我的项目知道它应该添加到 Core Data 中的哪个组。

这是我的应用程序的一些图片。

请注意，itemsInGroup 会获取应该在组中的所有项目。

在我的应用程序中添加断点后，正在添加 Item 实体的组的值等于 nil。这应该有一个值（在按下添加项目按钮时设置）。提前谢谢你的帮助。 代码主要部分

import SwiftUI
import CoreData

struct ContentView: View 
    @Environment(\.managedObjectContext) private var viewContext
    
    enum ActiveSheet: Identifiable 
        case first, second
        
        var id: Int 
            hashValue
        
    

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

@State var activeSheet: ActiveSheet?
@State private var selectedGroup: Group? = nil

var body: some View 
    NavigationView 
        List 
            ForEach(items)  group in
                //                    Text("Item at \(item.timestamp!, formatter: itemFormatter)")
                Text(group.title ?? "New group")
                ForEach(group.itemsInGroup)  item in
                    Text(item.title ?? "New Item")
                
                Button(action: 
                    selectedGroup = group
                    activeSheet = .second
                , label: 
                    Text("Add Item")
                )
            
            .onDelete(perform: deleteItems)
        
        .toolbar 
            Button(action: 
                activeSheet = .first
            ) 
                Label("Add Item", systemImage: "plus")
            
        
    
    .sheet(item: $activeSheet)  item in
        switch(item) 
        case .first: AddGroupName()
        case .second: AddItemView(group: selectedGroup)
        
    


private func addItem() 
    withAnimation 
        let newItem = Group(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)

我在哪里添加新项目

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

let group: Group?
@State var title = ""
var body: some View 
    VStack 
        Form 
            TextField("Title", text: $title)
            Button(action: 
                let item = Item(context: viewContext)
                item.group = group
                item.title = title
                try? viewContext.save()
            , label: 
                Text("Save")
            )

我在哪里添加新组

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

@State var title = ""

var body: some View 
    VStack 
        Form 
            TextField("Title", text: $title)
            Button(action: 
                let item = Group(context: viewContext)
                item.title = title
                item.timestamp = Date()
                try? viewContext.save()
            , label: 
                Text("Save")
            )

我的核心数据模型

为什么没有正确传递和保存组值？它应该保存在代码主要部分的 selectedGroup 变量中。

当我尝试添加一个项目并将其保存到核心数据数据库时，我收到此错误“非法尝试在不同上下文中的对象之间建立关系‘组’”

请注意，我尝试将 selectedGroup 设置为 nil 以外的值，但是在尝试添加项目时会使用此初始值。

【问题讨论】：

【参考方案1】：

您应该使用AddItemView 中的@Binding 属性包装器声明您的group 属性