SOLVED: SwiftUI Preview Crashes with CoreData

@andreasara-dev

Jul '23

Hi guys,

Here is the code:

import SwiftUI

struct ContentView: View {
    @Environment(\.managedObjectContext) var moc

    @FetchRequest(sortDescriptors: [
        SortDescriptor(\.date, order: .reverse),
        SortDescriptor(\.title)
    ]) var notes: FetchedResults<Note>

    let relativeDateTimeFormatter = RelativeDateTimeFormatter()

    @State private var showingAlert = false
    @State private var noteTitle = ""
    @State private var noteDescription = ""
    @State private var navigateToNoteView = false

    var body: some View {
        NavigationStack {
            List {
                ForEach(notes) { note in
                    NavigationLink {
                        NoteView(note: note)
                    } label: {
                        HStack {
                            VStack(alignment: .leading) {

                                Text(note.wrappedTitle)
                                    .font(.title2)

                                Text(note.wrappedDescription)
                                    .font(.caption)
                            }

                            Spacer()
                            Text(relativeDateTimeFormatter.localizedString(for: note.wrappedDate, relativeTo: Date()))
                                .font(.caption)
                                .italic()

                        }
                    }

                }
                .onDelete { offsets in
                    DataController().deleteNote(at: offsets, moc: moc, notes: Array(notes))
                }
            }
            .alert("New note", isPresented: $showingAlert) {
                TextField("Note title", text: $noteTitle)
                TextField("Note description", text: $noteDescription)

                Button("Cancel") { showingAlert = false }
                Button("Ok") {
                    if noteTitle.isEmpty || noteDescription.isEmpty {
                        return
                    } else {
                        DataController().addNote(moc: moc, title: noteTitle, description: noteDescription, text: "")
                        noteTitle = ""
                        noteDescription = ""
                        navigateToNoteView = true
                    }
                }
            }
            .navigationTitle("Notes")
            .toolbar {
                ToolbarItem(placement: .navigationBarTrailing) {
                    Button {
                        showingAlert = true
                    } label: {
                        Image(systemName: "square.and.pencil")
                    }
                }
                ToolbarItem(placement: .navigationBarLeading) {
                    EditButton()
                }
            }
            .navigationDestination(isPresented: $navigateToNoteView) {
                let note = DataController().getLastNote(moc: moc)
                NoteView(note: note ?? DataController.exampleNote)
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var dataController = DataController()
    static var previews: some View {
        Group {
            ContentView()
            ContentView()
                .preferredColorScheme(.dark)
        }
        .environment(\.managedObjectContext, dataController.container.viewContext)
    }
}

I know that the line that trigger the crash of the preview is this: NoteView(note: note ?? DataController.exampleNote) used in NavigationDestination.

Do you know how to solve?

Thanks

2

@deirdresm HWS+

Jul '23

Have you tried setting breakpoints to see where it's crashing?

2

@andreasara-dev

Jul '23

@deirdresm Yes, it crashes in this line: NoteView(note: note ?? DataController.exampleNote).

I tried to comment it and it works fine without crashing. But I don't know why...

2

@delawaremathguy

Jul '23

hi,

the thing that sticks out to me is that you are not making calls to a fixed DataController that was passed in, say, through the environment. rather, all your calls have the form

DataController().doSomething( ... )

which means that every such call creates its own instance of a DataController.

for example, the statement

DataController().addNote(moc: moc, title: noteTitle, description: noteDescription, text: "")

creates a new data controller, adds a new note, and then this instance of a data controller goes away.

similarly, i would think that

let note = DataController().getLastNote(moc: moc)

always returns a nil value, since a new data controller instance has (probably) no notes.

(i can't tell exactly what DataController.exampleNote is for the case that note is nil.)

hope that helps,

DMG

2

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