与核心数据一起使用时预览崩溃

Question

我正在尝试使用 swiftui 和 core-data 创建一个简单的待办事项应用程序，但是在添加视图时遇到了一些问题。我有一个TaskView 视图，它将用于显示有关列表中任务的一些信息（标题、日期等）。但是，当尝试使用演示 Task 对象为视图创建预览时，预览会崩溃（正在运行的应用程序仍然有效）。

对不起，如果这是一个微不足道的问题，对于 swift 和一般编程来说都是新手。

TaskView.swift

import SwiftUI

struct TaskView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    
    var task: Task
    
    var body: some View {
        HStack {
            Image(systemName: task.checked == true ? "circle" : "checkmark.circle.fill")

            VStack {
                Text(task.name ?? "unknown name")
                Text(String(task.streak))
            }
        }
    }
}

struct TaskView_Previews: PreviewProvider {
    static var previews: some View {
        let task = Task()
        task.name = "Washing up"
        task.desc = "Wash the dishes"
        task.checked = false
        task.streak = 2
        
        return TaskView(task: task)
            .previewLayout(.fixed(width: 375, height: 60))
    }
}

ContentView.swift

import SwiftUI

struct ContentView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @FetchRequest(entity: Task.entity(), sortDescriptors: [
        NSSortDescriptor(keyPath: \Task.checked, ascending: false),
        NSSortDescriptor(keyPath: \Task.name, ascending: true)
    ]) var tasks: FetchedResults<Task>
    
    var body: some View {
        NavigationView {
            List {
                ForEach(tasks, id: \.self) { task in
                    TaskView(task: task).environment(\.managedObjectContext, self.managedObjectContext)
                }
                .onDelete(perform: deleteTask)
            }
            .navigationBarTitle("Today")
        }
    }
    
    func deleteTask(at indexSet: IndexSet) {
        indexSet.forEach { TaskUtils.delete(task: tasks[$0], using: self.managedObjectContext)}
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

Answer 1

我通过添加首先设置上下文常量然后使用 init 初始化您在预览中使用的任务常量来修复您的 TaskView

import SwiftUI

struct TaskView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    
    var task: Task
    
    var body: some View {
        HStack {
            Image(systemName: task.checked == true ? "circle" : "checkmark.circle.fill")

            VStack {
                Text(task.name ?? "unknown name")
                Text(String(task.streak))
            }
        }
    }
}

struct TaskView_Previews: PreviewProvider {
    
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext // IMPORTANT
        let task = Task.init(context: context) // IMPORTANT
        task.name = "Washing up"
        task.desc = "Wash the dishes"
        task.checked = false
        task.streak = 2
        
        return TaskView(task: task).environment(\.managedObjectContext, context)
            .previewLayout(.fixed(width: 375, height: 60))
    }
}

至于您的 ContentView，我做了一些小改动以使其正常工作 - 添加了一个按钮以将对象添加到 CoreData 模型。

import SwiftUI

struct ContentView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @FetchRequest(entity: Task.entity(), sortDescriptors: [
        NSSortDescriptor(keyPath: \Task.checked, ascending: false),
        NSSortDescriptor(keyPath: \Task.name, ascending: true)
    ]) var tasks: FetchedResults<Task>
    
    var body: some View {
        NavigationView {
            List {
                ForEach(tasks, id: \.self) { task in
                    TaskView(task: task).environment(\.managedObjectContext, self.managedObjectContext)
                }
                //.onDelete(perform: deleteTask)
            }
            .navigationBarTitle("Today")
            .navigationBarItems(trailing:
                Button("Add") {
                    let task = Task(context: self.managedObjectContext)
                    task.name = "Washing up"
                    task.desc = "Wash the dishes"
                    task.checked = false
                    task.streak = Int32(Int.random(in: 0...10))
                    
                    try? self.managedObjectContext.save()
                }
            )
        }
    }
    
//    func deleteTask(at indexSet: IndexSet) {
//        indexSet.forEach { TaskUtils.delete(task: tasks[$0], using: self.managedObjectContext)}
//    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return ContentView().environment(\.managedObjectContext, context)
    }
}