Persistent Storage for Previewing

Question

sheeley OP

Created Jun ’20

Replies 3

Boosts 0

Views 1.4k

Participants 2

In the previous version of my app, I used this pattern for previews that depended on a NSManagedObjectContext:

Code Block struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
				 let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return ContentView().environment(\.managedObjectContext, managedObjectContext)
    }
}

With the delegate gone, I've swapped to this:

Code Block class PreviewData {
        static var persistentContainer: NSPersistentContainer = {
            let container = NSPersistentCloudKitContainer(name: "Structure")
            container.loadPersistentStores(completionHandler: { _, error in
                if let error = error { 
                    fatalError("Unresolved error \(error)")
                }
            })
            return container
        }()
        static var previewContext: NSManagedObjectContext {
            persistentContainer.viewContext
        }

This works for previews some of the time. For example, I have this FilteredList:

Code Block import CoreData
import SwiftUI
protocol StructureManagedObject {
    static var defaultSortDescriptors: [NSSortDescriptor] { get }
}
struct FilteredList<T: NSManagedObject & Identifiable & StructureManagedObject, Content: View>: View {
    let fetchRequest: FetchRequest<T>
    var results: FetchedResults<T> { fetchRequest.wrappedValue }
    let content: (T) -> Content
    init(sortDescriptors: [NSSortDescriptor] = [], predicate: NSPredicate? = nil, isActive: @escaping ((T) -> Bool) = { _ in false }, @ViewBuilder content: @escaping (T) -> Content) {
        var sortDescriptors = sortDescriptors
        if sortDescriptors.isEmpty {
            sortDescriptors = T.defaultSortDescriptors
        }
        fetchRequest = FetchRequest(entity: T.entity(), sortDescriptors: sortDescriptors, predicate: predicate)
        self.content = content
    }
    var body: some View {
        Group {
            if results.isEmpty {
                EmptyView()
            } else {
                ForEach(results) { r in
                    self.content(r)
                }
            }
        }
    }
}
struct FilteredList_Previews: PreviewProvider {
    static var previews: some View {
				// This is required or preview will fail
        _ = Tag(context: PreviewData.previewContext, text: "#tagging")
        return VStack {
            FilteredList { (tag: Tag) in
                Text(tag.id)
            }
        }.wrappedPreview()
    }
}
class Tag: NSManagedObject {
 // ...
}

This appears to work, but has issues if I don't explicitly create entities before rendering. Is there a better path I should take?

Answered by sheeley in 619067022

I was able to fix this by moving my data store into its own class and using the same thing for both the app and previews:

Code Block import CloudKit
import CoreData
import Foundation
class DataStore {
    static let shared = DataStore()
    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentCloudKitContainer(name: "Structure")
        container.loadPersistentStores(completionHandler: { _, error in
            if let error = error {
                fatalError("Unresolved error \(error)")
            }
            let context = container.viewContext
            context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
            context.automaticallyMergesChangesFromParent = true
						// other setup here
        })
        return container
    }()
    func saveContext() {
        let context = persistentContainer.viewContext
        if context.hasChanges {
            do {
                try context.save()
            } catch {
                fatalError("Unresolved error \(error)")
            }
        }
    }
}

Boost

Answer 1

sheeley OP

Jun ’20

What I'm still encountering in some cases is this error:

Code Block *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'executeFetchRequest:error: A fetch request must have an entity.'
terminating with uncaught exception of type NSException

1

Answer 2

sheeley OP

Jul ’20

Accepted Answer

I was able to fix this by moving my data store into its own class and using the same thing for both the app and previews:

Code Block import CloudKit
import CoreData
import Foundation
class DataStore {
    static let shared = DataStore()
    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentCloudKitContainer(name: "Structure")
        container.loadPersistentStores(completionHandler: { _, error in
            if let error = error {
                fatalError("Unresolved error \(error)")
            }
            let context = container.viewContext
            context.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
            context.automaticallyMergesChangesFromParent = true
						// other setup here
        })
        return container
    }()
    func saveContext() {
        let context = persistentContainer.viewContext
        if context.hasChanges {
            do {
                try context.save()
            } catch {
                fatalError("Unresolved error \(error)")
            }
        }
    }
}

1

Answer 3

HardyS OP

Sep ’20

I'm struggling with a similar problem and the same error. Is there some "good practice" how a persistent store for SwiftUI 2.0 previews should be set up?

[Edit: Oops, this refers to the error post below from sheeley...]

0