PersistenceController.swift

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()
    
    static var preview: PersistenceController = {
        let result = PersistenceController(inMemory: true)

        let viewContext = result.container.viewContext
        
        for number in 0..<10 {
            ToDo(
                on: viewContext,
                title: "hoge\(number)",
                memo: "メモメモ\(number)",
                dueAt: Calendar.current.date(byAdding: .day, value: number, to: .now) ?? .distantPast
            )
        }
        
        do {
            try viewContext.save()
        } catch {
            // 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.
            let nsError = error as NSError
            fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
        }
        return result
    }()
    
    let container: NSPersistentContainer
    
    init(inMemory: Bool = false) {
        let managedObjectModel: NSManagedObjectModel = .init()
        
        managedObjectModel.entities = [
            ToDo.entityDescription
        ]
        
        container = .init(name: "ToDoApp", managedObjectModel: managedObjectModel)

        if inMemory {
            container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
        }

        container.loadPersistentStores { (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
    }
}