How to create a NSManagedObject in a modal view using SwiftUI? - core-data

Question

How does one create a new ManagedObject (MO) in a modal view using SwiftUI?
Running into a strange bug where Xcode consumes GBs of memory and fills up the hard drive on the mac through swap files.
When one creates the Modal view in the .sheet modifier there appears to be some kind of infinite loop created that fills the memory with duplicates of the ManagedObject that is injected into that Modal view.
This sample project illustrates at least part of the issue. If you run it you see the method called in the .sheet modifier fires over and over again. A theory is that the screen underneath that displays a list of ManagedObjects is causing some kind of loop between the two screens.
https://github.com/sphericalwave/ChildContextTest
Was hoping to use a childContext in the modal screen so any unsaved changes would be discarded if the modal view was dismissed without saving the childContext. But need to clear this hurdle first and there is some challenge involved in sharing ManagedObject across contexts.
import CoreData
import SwiftUI
struct CrtFdsUI: View
{
#Environment(\.managedObjectContext) var moc
#FetchRequest(entity: CrtFd.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \CrtFd.scale, ascending: true)])
var crtFds: FetchedResults<CrtFd>
#State var showModal = false
#ObservedObject var absFd: AbsFd
func crtFdModal() -> CrtFdUI {
print("func crtFdModal() -> CrtFdUI")
let cF = CrtFd(scale: 1.0, absFd: absFd, moc: moc)
return CrtFdUI(crtFd: cF)
}
var body: some View {
NavigationView {
VStack {
List {
ForEach(self.crtFds, id: \.objectID) {
CrtFdCell(crtFd: $0)
}
}
.navigationBarTitle("CrtFdsUI")
.navigationBarItems(trailing: PlusBtn(showModal: $showModal))
}
.sheet(isPresented: $showModal) { self.crtFdModal() } //FIXME: Called in endless loop?
}
}
}
Here's the list of managedObjects.
import CoreData
import SwiftUI
struct CrtFdsUI: View
{
#Environment(\.managedObjectContext) var moc
#FetchRequest(entity: CrtFd.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \CrtFd.scale, ascending: true)])
var crtFds: FetchedResults<CrtFd>
#State var showModal = false
#ObservedObject var absFd: AbsFd
func crtFdModal() -> CrtFdUI {
print("func crtFdModal() -> CrtFdUI")
let cF = CrtFd(scale: 1.0, absFd: absFd, moc: moc)
return CrtFdUI(crtFd: cF)
}
var body: some View {
NavigationView {
VStack {
List {
ForEach(self.crtFds, id: \.objectID) {
CrtFdCell(crtFd: $0)
}
}
.navigationBarTitle("CrtFdsUI")
.navigationBarItems(trailing: PlusBtn(showModal: $showModal))
}
.sheet(isPresented: $showModal) { self.crtFdModal() }
}
}
}

Answer 1

Right now the code creates a new instance of CrtFd every time the view hierarchy is recalculated. That's probably not a good idea since the hierarchy might be recalculated unexpectedly for reasons you don't directly control, so that even without an infinite loop of creation you'd still probably end up with more new managed objects than you want.
I downloaded your project and I'm not sure what the two Core Data entities represent, but I did notice that when CrtFdsUI first appears, its absFd already has a value for the crtFd property, and that the crtFd property is to-one rather than to-many. That means that when you create a new instance in this code, you're replacing one CrtFd with another that's an exact duplicate.
I'm going to guess that you don't really want to replace one instance with an identical duplicate, so one way of avoiding the problem, then, is to change your crtFdModal() to use the one that already exists, and only create a new copy if there isn't one already:
func crtFdModal() -> CrtFdUI {
print("func crtFdModal() -> CrtFdUI \(showModal)")
let cF = absFd.crtFd ?? CrtFd(scale: 1.0, absFd: absFd, moc: moc)
return CrtFdUI(crtFd: cF)
}
That avoids the problem you describe. It's hard to tell if it's exactly what you need, but since your code creates apparently-unnecessary duplicates it seems likely that it is.