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