问题
In a SwiftUI View i have a List based on @FetchRequest showing data of a Primary entity and the via relationship connected Secondary entity. The View and its List is updated correctly, when I add a new Primary entity with a new related secondary entity.
The problem is, when I update the connected Secondary item in a detail view, the database gets updated, but the changes are not reflected in the Primary List. Obviously, the @FetchRequest does not get triggered by the changes in another View.
When I add a new item in the primary view thereafter, the previously changed item gets finally updated.
As a workaround, i additionally update an attribute of the Primary entity in the detail view and the changes propagate correctly to the Primary View.
My question is: How can I force an update on all related @FetchRequests in SwiftUI Core Data? Especially, when I have no direct access to the related entities/@Fetchrequests?
import SwiftUI
extension Primary: Identifiable {}
// Primary View
struct PrimaryListView: View {
@Environment(\.managedObjectContext) var context
@FetchRequest(
entity: Primary.entity(),
sortDescriptors: [NSSortDescriptor(key: "primaryName", ascending: true)]
)
var fetchedResults: FetchedResults<Primary>
var body: some View {
List {
ForEach(fetchedResults) { primary in
NavigationLink(destination: SecondaryView(primary: primary)) {
VStack(alignment: .leading) {
Text("\(primary.primaryName ?? "nil")")
Text("\(primary.secondary?.secondaryName ?? "nil")").font(.footnote).foregroundColor(.secondary)
}
}
}
}
.navigationBarTitle("Primary List")
.navigationBarItems(trailing:
Button(action: {self.addNewPrimary()} ) {
Image(systemName: "plus")
}
)
}
private func addNewPrimary() {
let newPrimary = Primary(context: context)
newPrimary.primaryName = "Primary created at \(Date())"
let newSecondary = Secondary(context: context)
newSecondary.secondaryName = "Secondary built at \(Date())"
newPrimary.secondary = newSecondary
try? context.save()
}
}
struct PrimaryListView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
return NavigationView {
PrimaryListView().environment(\.managedObjectContext, context)
}
}
}
// Detail View
struct SecondaryView: View {
@Environment(\.presentationMode) var presentationMode
var primary: Primary
@State private var newSecondaryName = ""
var body: some View {
VStack {
TextField("Secondary name:", text: $newSecondaryName)
.textFieldStyle(RoundedBorderTextFieldStyle())
.padding()
.onAppear {self.newSecondaryName = self.primary.secondary?.secondaryName ?? "no name"}
Button(action: {self.saveChanges()}) {
Text("Save")
}
.padding()
}
}
private func saveChanges() {
primary.secondary?.secondaryName = newSecondaryName
// TODO: ❌ workaround to trigger update on primary @FetchRequest
primary.managedObjectContext.refresh(primary, mergeChanges: true)
// primary.primaryName = primary.primaryName
try? primary.managedObjectContext?.save()
presentationMode.wrappedValue.dismiss()
}
}
回答1:
You need a Publisher which would generate event about changes in context and some state variable in primary view to force view rebuild on receive event from that publisher.
Important: state variable must be used in view builder code, otherwise rendering engine would not know that something changed.
Here is simple modification of affected part of your code, that gives behaviour that you need.
@State private var refreshing = false
private var didSave = NotificationCenter.default.publisher(for: .NSManagedObjectContextDidSave)
var body: some View {
List {
ForEach(fetchedResults) { primary in
NavigationLink(destination: SecondaryView(primary: primary)) {
VStack(alignment: .leading) {
// below use of .refreshing is just as demo,
// it can be use for anything
Text("\(primary.primaryName ?? "nil")" + (self.refreshing ? "" : ""))
Text("\(primary.secondary?.secondaryName ?? "nil")").font(.footnote).foregroundColor(.secondary)
}
}
// here is the listener for published context event
.onReceive(self.didSave) { _ in
self.refreshing.toggle()
}
}
}
.navigationBarTitle("Primary List")
.navigationBarItems(trailing:
Button(action: {self.addNewPrimary()} ) {
Image(systemName: "plus")
}
)
}
回答2:
I tried to touch the primary object in the detail view like this:
// TODO: ❌ workaround to trigger update on primary @FetchRequest
if let primary = secondary.primary {
secondary.managedObjectContext?.refresh(primary, mergeChanges: true)
}
Then the primary list will update. But the detail view has to know about the parent object. This will work, but this is probably not the SwiftUI or Combine way...
Edit:
Based on the above workaround, I modified my project with a global save(managedObject:) function. This will touch all related Entities, thus updating all relevant @FetchRequest's.
import SwiftUI
import CoreData
extension Primary: Identifiable {}
// MARK: - Primary View
struct PrimaryListView: View {
@Environment(\.managedObjectContext) var context
@FetchRequest(
sortDescriptors: [
NSSortDescriptor(keyPath: \Primary.primaryName, ascending: true)]
)
var fetchedResults: FetchedResults<Primary>
var body: some View {
print("body PrimaryListView"); return
List {
ForEach(fetchedResults) { primary in
NavigationLink(destination: SecondaryView(secondary: primary.secondary!)) {
VStack(alignment: .leading) {
Text("\(primary.primaryName ?? "nil")")
Text("\(primary.secondary?.secondaryName ?? "nil")")
.font(.footnote).foregroundColor(.secondary)
}
}
}
}
.navigationBarTitle("Primary List")
.navigationBarItems(trailing:
Button(action: {self.addNewPrimary()} ) {
Image(systemName: "plus")
}
)
}
private func addNewPrimary() {
let newPrimary = Primary(context: context)
newPrimary.primaryName = "Primary created at \(Date())"
let newSecondary = Secondary(context: context)
newSecondary.secondaryName = "Secondary built at \(Date())"
newPrimary.secondary = newSecondary
try? context.save()
}
}
struct PrimaryListView_Previews: PreviewProvider {
static var previews: some View {
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
return NavigationView {
PrimaryListView().environment(\.managedObjectContext, context)
}
}
}
// MARK: - Detail View
struct SecondaryView: View {
@Environment(\.presentationMode) var presentationMode
var secondary: Secondary
@State private var newSecondaryName = ""
var body: some View {
print("SecondaryView: \(secondary.secondaryName ?? "")"); return
VStack {
TextField("Secondary name:", text: $newSecondaryName)
.textFieldStyle(RoundedBorderTextFieldStyle())
.padding()
.onAppear {self.newSecondaryName = self.secondary.secondaryName ?? "no name"}
Button(action: {self.saveChanges()}) {
Text("Save")
}
.padding()
}
}
private func saveChanges() {
secondary.secondaryName = newSecondaryName
// save Secondary and touch Primary
(UIApplication.shared.delegate as! AppDelegate).save(managedObject: secondary)
presentationMode.wrappedValue.dismiss()
}
}
extension AppDelegate {
/// save and touch related objects
func save(managedObject: NSManagedObject) {
let context = persistentContainer.viewContext
// if this object has an impact on related objects, touch these related objects
if let secondary = managedObject as? Secondary,
let primary = secondary.primary {
context.refresh(primary, mergeChanges: true)
print("Primary touched: \(primary.primaryName ?? "no name")")
}
saveContext()
}
}
来源：https://stackoverflow.com/questions/58643094/how-to-update-fetchrequest-when-a-related-entity-changes-in-swiftui