Core Data CRUD operation

Core Data CRUD operation

To Do CRUD operation in Core Data, we need to do the following things:

1. Get reference of NSPersistentContainer.
eg. lazy var persistentContainer: NSPersistentContainer (inside Appdelegate)

2. Get context from persistentContainer
eg. let context = persistentContainer.viewContext (inside Appdelegate)
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext (inside ViewController)

3. Call fetch, save and delete over this context
eg. 
Save :
func saveItem(){
    do {
        try context.save()
        fetchItems()
    } catch {
        print("Error while saving :\(error.localizedDescription)")
    }
}

Fetch : 
func fetchItems(){
    let request: NSFetchRequest<Item> = Item.fetchRequest()
    request.predicate = NSPredicate(format: "name CONTAINS[cd] %@", "some text")    //  here name is attribute of the Entity
    request.sortDescriptors = [NSSortDescriptor(key: "title", ascending: true)]
    do {
        itemsArray = try context.fetch(request)
    } catch {
        print("Error while retriving items :\(error)")
    }
}

Delete :
func deleteItem(item: Item){
    context.delete(item)
    saveItem()
}

Edit :
func update(){
    let item = itemsArray[index of managed object that is to be updated]
    item.name = field.text
    self.saveItem()
}

Relationship

Lets take an example of Items and Category

Each category will have list of items. In case of a relationship each Category 

ViewController.swift

//
//  ViewController.swift
//  CoreDataDemo
//
//  Created by Ajeet N on 23/08/20.
//  Copyright © 2020 Ajeet N. All rights reserved.
//

import UIKit
import CoreData

class ViewController: UIViewController {
    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    var itemsArray = [Item](){
        didSet{
            relatedItems.items = itemsArray
        }
    }
    
    let relatedItems = ItemsTableView()

    override func viewDidLoad() {
        super.viewDidLoad()
        loadItem()
        
        relatedItems.relatedItemsDelegate = self
        relatedItems.headerTitles = ["Related Items"]
        relatedItems.items = itemsArray
        view.addAutolayoutSubview(relatedItems)
        
        NSLayoutConstraint.activate([
            relatedItems.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 0),
            relatedItems.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 0),
            relatedItems.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: 0),
            relatedItems.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: 0),
        ])
    }

    @IBAction func addAction(_ sender: UIBarButtonItem) {
        showAlertWithTextField(itm: nil)
    }
    
}

extension ViewController {
    func saveItem(){
        do {
            try context.save()
            loadItem()
        } catch {
            print("Error while saving :\(error.localizedDescription)")
        }
    }
    
    func loadItem(){
        let request: NSFetchRequest<Item> = Item.fetchRequest()
        do {
            itemsArray = try context.fetch(request)
        } catch {
            print("Error while retriving items :\(error)")
        }
    }
    
    func deleteItem(item: Item){
        context.delete(item)
        saveItem()
    }
}

extension ViewController{
    func showAlertWithTextField(itm: Item?){
        var field = UITextField()
        let alertController = UIAlertController(title: "Add Category", message: "", preferredStyle: UIAlertController.Style.alert)
        alertController.addTextField { (textField : UITextField!) -> Void in
            textField.placeholder = "Enter Category Name"
            textField.text = itm?.name
            field = textField
        }
        let saveAction = UIAlertAction(title: "Save", style: UIAlertAction.Style.default, handler: { alert -> Void in
            if let ittm = itm{
                //  While Editing we need to pass the reference of NSManagedObject from the array of fetched NSManagedObject
                ittm.name = field.text
                self.saveItem()
            } else {
                //  While Saving we need to create a new instance of NSManagedObject, set its properties and call context.save()
                let item = Item(context: self.context)
                item.name = field.text
                self.saveItem()
            }
            
        })
        let cancelAction = UIAlertAction(title: "Cancel", style: UIAlertAction.Style.default, handler: nil)
        
        alertController.addAction(saveAction)
        alertController.addAction(cancelAction)
        
        self.present(alertController, animated: true, completion: nil)
    }
    
    func removeItem(item: Item){
        deleteItem(item: item)
    }
}