Question

应该如何修改我的代码，以确保在添加新项目时在核心数据层发生异常时，SwiftUI不会继续显示新项目？

背景：当我运行下面的代码时，添加一个新项目执行“ context.save（）”时出现异常，但是新项目请求确实失败了（未保存到Core Data），UI显示一个新项目。就像@FetchRequest行中的“ lists”变量没有动态地表现一样。

问题-如何修复代码以使应用程序正常运行？

代码：

import SwiftUI

struct ContentView: View {
    @Environment(\.managedObjectContext) var context
    @FetchRequest(fetchRequest: List.allListFetchRequest()) var lists: FetchedResults<List>

    private func addListItem() {
        let newList = List(context: context)
        newList.id = 1
        newList.title = "Testing 123"
        do {
            try context.save()
        } catch let e as NSError {
            print("Could not save new List. \(e.debugDescription)")
            return
        }

    }

    var body: some View {
        NavigationView {
            VStack {
                ForEach(lists) { list in
                    Text("List = \(list.title)")
                }
            }
            .navigationBarTitle( Text("My Todo Lists") )
            .navigationBarItems(
                trailing: Button(action: {self.addListItem()} ) {
                    Image(systemName: "plus.square")
                }
            )
        }
    }
}

示例输出：

Could not save new List. Error Domain=NSCocoaErrorDomain Code=133021 "(null)" UserInfo={NSExceptionOmitCallstacks=true, conflictList=(
    "NSConstraintConflict (0x600001fcccc0) for constraint (\n    id\n): database: (null), conflictedObjects: (\n    \"0x600000a7e360 <x-coredata:///List/t2F01130C-0D2A-4E88-A77D-A7BA0E921C213>\",\n    \"0xfb1bb528bb57810c <x-coredata://41B391F1-A95C-4971-9584-A2D3DFFF5380/List/p3>\"\n)"
)}

Answer 1

吉姆·多维（Jim Dovey）通过这个建议给了我前进的方向：

private func addListItem() {  
    context.performBlock { context in  
        let newList = List(context: context)  
        newList.id = 1  
        newList.title = "Testing 123"  
        do {  
            try context.save()  
        }  
        catch {  
            print("Failed to save new item. Error = \(error)")  
            context.delete(newList)  
            // don't need to save here, because we're in `performBlock` and have reverted the only unsaved change.  
        }  
    }  
}

为了确保核心数据@FetchRequest变量动态运行，需要进行哪些代码更改（附加代码）？

1 个答案: