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iOS/AppDelegate (XCode 11) .md

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+# AppDelegate
+
+First loaded file. It prepares the app visualiation by calling the *scene* UI.
+
+```swift
+import UIKit
+
+@UIApplicationMain
+class AppDelegate: UIResponder, UIApplicationDelegate {
+
+    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
+        // Override point for customization after application launch.
+        return true
+    }
+
+    // MARK: UISceneSession Lifecycle
+
+    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
+        // Called when a new scene session is being created.
+        // Use this method to select a configuration to create the new scene with.
+        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
+    }
+
+    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
+        // Called when the user discards a scene session.
+        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
+        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
+    }
+
+    // MARK: - Core Data stack
+
+    lazy var persistentContainer: NSPersistentContainer = {
+        /*
+         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 container = NSPersistentContainer(name: "CoreDataExample")
+        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)")
+            }
+        })
+        return container
+    }()
+
+    // MARK: - Core Data Saving support
+
+    func saveContext () {
+        let context = persistentContainer.viewContext
+        if context.hasChanges {
+            do {
+                try context.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)")
+            }
+        }
+    }
+}
+```