Creating a ManagedObject in a SwiftUI NaviagationLink - swift

Question

I want to use a quite complex view to update and create CoreData Managed Objects.
To make the code shorter the edit/create view 'Edit' here is shortened.
In the case of creating a new Person, my problem is, when to create the NSManagedObject Instance.
The code below crashes in Variant 1. It seem, the Edit View in the NavigationLink is called, before the button action, which creates the Object, is performed.
Another approach I tried was to create the Object in the NavigationLink parameter (Variant 2).
Here I have a quite strange beaviour, that the Edit View dismisses, without pressing a button, if job is changed to manager.
What approach would you recommend?
struct ContentView: View {
#FetchRequest( entity: Person.entity(), sortDescriptors: [],
predicate: NSPredicate(format: "job ='manager'"))
var persons: FetchedResults<Person>
#State var newPerson : Person?
#State var selection: Int? = nil
#Environment(\.managedObjectContext) var moc
var body: some View {
NavigationView {
VStack{
Text("\(persons.count) persons")
List(persons, id: \.self) { person in
HStack {
NavigationLink(destination: Edit(person: person)) {
HStack {
Text("\(person.name) -- \(person.job )")
}
Image(systemName: "trash").onTapGesture {
self.moc.delete(person)
try! self.moc.save()
}
}
}
}
/* Variante 1 */
NavigationLink(destination: Edit(person: self.newPerson!, new: true), tag: 1, selection: $selection) {
Button(action: {
print("login tapped")
self.selection = 1
self.newPerson = Person(context: self.moc)
self.newPerson?.job = "manager"
try! self.moc.save()
}) {
Text("New Person V1").bold()
}
}
/* Variant 2
NavigationLink(destination: Edit(person: Person(context: self.moc), new: true)) {
Text("New Person V2").bold()
}
*/
}
}
}
}
struct Edit: View {
#ObservedObject var person : Person
var new = false
#Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>
#Environment(\.managedObjectContext) var moc
var body: some View {
VStack{
TextField("Name", text: $person.name)
TextField("Job", text: $person.job)
Spacer()
Button("Save"){
do {
try self.moc.save()
} catch {
print(error)
}
print("====================SAVE PRESSED")
self.presentationMode.wrappedValue.dismiss()
}
Button("Cancel"){
print("====================CANCEL PRESSED")
self.moc.refresh(self.person, mergeChanges: false)
self.presentationMode.wrappedValue.dismiss()
}
}
.navigationBarTitle("\(self.new ? "New" : "Edit")")
.navigationBarBackButtonHidden(true)
}
}
What approach would you recommend?

Answer 1

Here is possible modification of Variant1 (it looks more appropriate to go). The idea is to hide navigation link and make it active only on button click. Also make destination conditional to avoid early creation of Edit view.
See also comments inline.
Button(action: {
print("login tapped")
self.newPerson = Person(context: self.moc)
self.newPerson?.job = "manager"
try! self.moc.save()
self.selection = 1 // activate link at the end !!
}) {
Text("New Person V1").bold()
}
.background(NavigationLink(destination:
Group { // safe variant, can be separated into computed property
if self.newPerson != nil {
Edit(person: self.newPerson!, new: true)
} else { EmptyView() }
},
tag: 1, selection: $selection) { EmptyView() })