为什么 app delegate 应该为其他 viewController 中使用的属性创建实例？

Question

我正在学习来自 Big Nerd Ranch 的 iOS 编程书籍中的 UITableView 教程。在本教程中，我们将新的属性分配给 UIViewController，如下所示：

class ItemsViewController: UITableViewController {

    var itemStore: ItemStore!  

}

但是，在应用程序的AppDelegate中，我们在AppDelegate中实例化了这个itemStore 属性，如下所示：

class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?


    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.


        let itemStore = ItemStore()


        let itemsController = window?.rootViewController as! ItemsViewController

        itemsController.itemStore = itemStore

        return true
    }

    func applicationWillResignActive(_ application: UIApplication) {
        // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
        // Use this method to pause ongoing tasks, disable timers, and invalidate graphics rendering callbacks. Games should use this method to pause the game.
    }

    func applicationDidEnterBackground(_ application: UIApplication) {
        // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
        // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
    }

    func applicationWillEnterForeground(_ application: UIApplication) {
        // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
    }

    func applicationDidBecomeActive(_ application: UIApplication) {
        // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    }

    func applicationWillTerminate(_ application: UIApplication) {
        // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    }


}

我的问题是，应用委托为什么要为其他 viewController 中使用的属性创建实例？这个属性不应该在实际使用它的 viewController 中实例化，或者通过这种方式实例化，itemStore 对象在应用程序运行期间保持活动状态？

Answer 1

在ItemsViewControllerclass中，itemStore定义为

var itemStore: ItemStore!

这里itemStore是一个展开的变量。所以它不接受 nil 值。否则会导致运行时崩溃。意外 nil.

这就是为什么在 AppDelegate 中我们需要确保 itemStore 对象不是 nil

let itemStore = ItemStore()
let itemsController = window?.rootViewController as! ItemsViewController
itemsController.itemStore = itemStore

Answer 2

像这样声明itemStore并不意味着它会保持应用程序生命，它是didFinishLaunchingWithOptions中的一个局部变量，这种class初始化风格如果可以的话是没有用的在 ItemsViewController 内初始化，这可能是因为数据逻辑在 Appdelegate 中，它总是与 coredata 一起发生，或者它的项目将从 launchingOptions launchingOptions [=14= 的 local/remote 通知点击中初始化]

Answer 3

这样做是为了让 ItemsViewController 更易于重用。 ItemsViewController 不需要知道 ItemStore 对象的详细信息，包括要实例化哪个 ItemStore 子类。它只需要知道该对象所依附的接口。通过在外部设置 ItemStore，ItemsViewController 可以很容易地与 ItemStore 的其他子类一起使用，而无需更改。

这在 iOS 编程：大书呆子一书的第 10 章 授予控制器对存储区的访问权限 部分中进行了介绍牧场指南，第 6 版。

Answer 4

无论这是否是 Big Nerd Ranch 的意图，将 ItemStore 实例传递到视图控制器的最佳理由是为了可测试性。这种模式被称为依赖注入。

想法是您可以在单元测试期间向视图控制器提供模拟的 ItemStore。（假设您进行了单元测试，您应该这样做！:)

使用模拟，您可以模拟当 ItemStore 为 nil、空或包含某些项目时视图控制器的行为。

为什么 app delegate 应该为其他 viewController 中使用的属性创建实例？

Why should the app delegate create instances for properties used in other viewControllers?

objectinstantiation

ios

swift