保存 NavigationLink 选择

Question

I'm building a desktop application that has a sidebar. It should behave like this:

1. The user is able to add and remove elements to a list that is displayed in the sidebar.
2. A selected list element needs to remain selected when restarting the application.
3. Upon adding a new element, the new element gets selected.

I'm having issues with achieving the 2. and 3. point.

This is my view:

import SwiftUI
import CoreData

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

    @FetchRequest(entity: Element.entity(),  sortDescriptors: [NSSortDescriptor(keyPath: \Element.title, ascending: false)])
    private var elements: FetchedResults<Element>
    
    @State var selectedElementId: NSManagedObjectID?

    var body: some View {
        VStack (alignment: .leading) {
            List(self.elements) { element in
                NavigationLink(destination: ContentView(), tag: element.objectID, selection: self.$selectedElementId) {
                    Text(element.title!)
                }
            }
            .listStyle(SidebarListStyle())
            
            HStack {
                Button(action: addElement) {
                    Image(systemName: "plus")
                }
                
                Button(action: removeElement) {
                    Image(systemName: "minus")
                }
            }
        }
    }
    
    private func addElement() {
        let element = Element(context: viewContext)
        
        element.title = "My title"
      
        do {
            try viewContext.save()
        } catch {
            print("Unresolved error \(error)")
        }
    }
    
    private func removeElement() {
        if self.$selectedElementId.wrappedValue == nil {
            return
        }
        
        let element = viewContext.object(with: self.$selectedElementId.wrappedValue!)
        
        viewContext.delete(element)
        
        do {
            try viewContext.save()
        } catch {
            print("Unresolved error \(error)")
        }
    }
}

To achieve the 2. and 3. point, I tried this:

I'm using core data as my storage. My idea is to have an attribute Element.selection which would tell me what list item was last selected, but I don't know how to update the $selectedElementId . I tried this in addElement but it doesn't work as expected:

private func addElement() {
    let element = Element(context: viewContext)
            
    element.title = "My title"

    if self.$selectedElementId.wrappedValue != nil {
        let selectedElement = viewContext.object(with: self.$selectedElementId.wrappedValue!)
            
        selectedElement.entity.selected = false
    }
       
    self.$selectedElementId.wrappedValue = element.objectID

    do {
        try viewContext.save()
    } catch {
        print("Unresolved error \(error)")
    }
}

This has two issues:

1. I want to set the new element as the selected element, therefore I need to update self.$selectedElementId to element.objectId . How can I do that? wrappedValue seems to be read-only.
2. I need to set Element.selection to false for the currently selected attribute. I've fetched the element in selectedElement , but I don't know how to update any values as it's of type NSManagedObject , not Element .

I'm new to Swift. Please let me know if there are also other elements of my approach that are wrong or could be done better.

Answer 1

You'll need to fix a few things to get this working.

First, your addElement() method isn't working because you need to set your state var directly to trigger an update, ie:

// Do this
self.selectedElementId = element.objectID

// Not this
self.$selectedElementId.wrappedValue = element.objectID

Second, you're trying to store application state — the user's selection — in your database. In general, databases are for user data and app state should be stored elsewhere. On Mac and iOS that place is usually UserDefaults (aka @AppStorage in SwiftUI), but for what you want SwiftUI has a specific solution: @SceneStorage . Anything you put in SceneStorage is saved per- scene , which is perfect for your case because users can open multiple windows and have different selection in each window.

And better yet, SceneStorage is super simple, just add the property @SceneStorage with a unique key to whatever property should be saved:

@SceneStorage("com.appname.selected_element") var selectedElementId: NSManagedObjectID?

… oh except that won't compile because SceneStorage only works for a few basic types like Int and String. If you add a string property identifier to Element , then you can use it everywhere instead of NSManagedObjectID:

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

    @FetchRequest(entity: Element.entity(),  sortDescriptors: [NSSortDescriptor(keyPath: \Element.title, ascending: false)])
    private var elements: FetchedResults<Element>
    
    @SceneStorage("com.appname.selected_element") var selectedElementId: String?

    var body: some View {
        VStack (alignment: .leading) {
            List(self.elements) { element in
                NavigationLink(destination: ContentView(),
                               tag: element.identifier!,
                               selection: self.$selectedElementId) {
                    Text(element.title!)
                }
            }
            .listStyle(SidebarListStyle())
            
            HStack {
                Button(action: addElement) {
                    Image(systemName: "plus")
                }
                
                Button(action: removeElement) {
                    Image(systemName: "minus")
                }
            }
        }
    }
    
    private func addElement() {
        let element = Element(context: viewContext)
        element.title = "My title"

        // set identifier to a random UUID
        element.identifier = UUID().uuidString
        do {
            try viewContext.save()
        } catch {
            print("Unresolved error \(error)")
        }

        // select our newly-added element
        self.selectedElementId = element.identifier
    }
    
    private func removeElement() {
        // TODO: fetch selected object with identifier == $selectedElementId
    }
}

One more thing that may help is setting the selection in your List , but this doesn't seem to be required:

List(self.elements, id: \.identifier, selection:$selectedElementId) {