BooksWidget.swift

import WidgetKit
import SwiftUI
import CoreData

struct Provider: TimelineProvider {
	var managedObjectContext : NSManagedObjectContext
	init(context : NSManagedObjectContext) {
		self.managedObjectContext = context
	}
	
	func placeholder(in context: Context) -> SimpleEntry {
		SimpleEntry(date: Date())
	}
	
	func getSnapshot(in context: Context, completion: @escaping (SimpleEntry) -> ()) {
		let entry = SimpleEntry(date: Date())
		completion(entry)
	}
	
	func getTimeline(in context: Context, completion: @escaping (Timeline<Entry>) -> ()) {
		var entries: [SimpleEntry] = []
		
		// Generate a timeline consisting of five entries an hour apart, starting from the current date.
		let currentDate = Date()
		for hourOffset in 0 ..< 5 {
			let entryDate = Calendar.current.date(byAdding: .hour, value: hourOffset, to: currentDate)!
			let entry = SimpleEntry(date: entryDate)
			entries.append(entry)
		}
		
		let timeline = Timeline(entries: entries, policy: .atEnd)
		completion(timeline)
	}
}

struct SimpleEntry: TimelineEntry {
	let date: Date
}

struct BooksWidgetEntryView : View {
	var entry: Provider.Entry
	@Environment(\.managedObjectContext) var moc
	@FetchRequest(entity: Book.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \Book.date, ascending: true)]) var books: FetchedResults<Book>
	
	var body: some View {
		Text("Test")
	}
}

public extension URL {

	/// Returns a URL for the given app group and database pointing to the sqlite database.
	static func storeURL(for appGroup: String, databaseName: String) -> URL {
		guard let fileContainer = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: appGroup) else {
			fatalError("Shared file container could not be created.")
		}

		return fileContainer.appendingPathComponent("\(databaseName).sqlite")
	}
}

public enum AppGroup: String {
  case facts = "group.app-group-books"

  public var containerURL: URL {
	switch self {
	case .facts:
	  return FileManager.default.containerURL(
	  forSecurityApplicationGroupIdentifier: self.rawValue)!
	}
  }
}

// MARK: - Core Data stack

var persistentContainer: NSPersistentCloudKitContainer = {
	/*
	 The persistent container for the application. This implementation
	 creates and returns a container, having loaded the store for the
	 application to it. This property is optional since there are legitimate
	 error conditions that could cause the creation of the store to fail.
	*/
	let storeURL = AppGroup.facts.containerURL.appendingPathComponent("Books")
	let description = NSPersistentStoreDescription(url: storeURL)
	let container = NSPersistentCloudKitContainer(name: "Books")
	container.persistentStoreDescriptions = [description]
	
	description.setOption(true as NSObject, forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
	
	container.loadPersistentStores(completionHandler: { (storeDescription, error) in
		if let error = error as NSError? {
			// Replace this implementation with code to handle the error appropriately.
			// fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
			 
			/*
			 Typical reasons for an error here include:
			 * The parent directory does not exist, cannot be created, or disallows writing.
			 * The persistent store is not accessible, due to permissions or data protection when the device is locked.
			 * The device is out of space.
			 * The store could not be migrated to the current model version.
			 Check the error message to determine what the actual problem was.
			 */
			fatalError("Unresolved error \(error), \(error.userInfo)")
		}
	})
	
	container.viewContext.automaticallyMergesChangesFromParent = true
	container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
	
	return container
}()

@main
struct BooksWidget: Widget {
	let kind: String = "BooksWidget"
	@Environment(\.managedObjectContext) var moc

	var body: some WidgetConfiguration {
		StaticConfiguration(kind: kind, provider: Provider(context: persistentContainer.viewContext)) { entry in
			BooksWidgetEntryView(entry: entry)
				.environment(\.managedObjectContext, self.moc)
		}
		.configurationDisplayName("My Widget")
		.description("This is an example widget.")
	}
}

struct BooksWidget_Previews: PreviewProvider {
	static var previews: some View {
		BooksWidgetEntryView(entry: SimpleEntry(date: Date()))
			.previewContext(WidgetPreviewContext(family: .systemSmall))
	}
}