Add CoreData to your SwiftUI using only one file.

Persistence.swift

//
//  Persistence.swift
//
//
//  Template from https://gist.github.com/kpostekk/1e7c22f68021f0fd07ce7b481c1ffb11
//

import CoreData

struct PersistenceController {
    static let shared = PersistenceController()
    
    static var preview: PersistenceController = {
        let pc = PersistenceController(inMemory: true)
        let viewContext = pc.container.viewContext
        
        // Whatever preview entities goes here
        
        return pc
    }()
    
    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "WhateverCoreDataName")

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

        container.loadPersistentStores(completionHandler: { _, error in
            if let error = error as NSError? {
                // Whatever errors goes here
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
    }
}