0

我正在学习 Big Nerd Ranch 的 iOS 编程书中的 UITableView 教程。在本教程中，我们为 UIViewController 分配了一个新属性，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，为什么应用程序委托要为其他视图控制器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，还是应该通过这种方式实例化，itemStore 对象在应用程序的持续时间内保持活动状态？

4

4 回答 4

1

这样做是为了使ItemsViewController更可重用。ItemsViewController不需要知道对象的详细信息，包括要实例化ItemStore哪个子类。ItemStore它只需要知道该对象所遵循的接口。通过在ItemStore外部设置，ItemsViewController可以很容易地与其他子类一起使用，ItemStore而无需更改。

这在第 10 章中，在iOS Programming: The Big Nerd Ranch Guide, 6th Edition书的让控制器访问商店的部分中进行了介绍。

于 2018-03-31T21:16:50.520 回答
0

在ItemsViewController类中，itemStore定义为

var itemStore: ItemStore!

这itemStore是一个未包装的变量。所以它不接受nil价值。否则会导致运行时崩溃。出乎意料nil。

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

let itemStore = ItemStore()
let itemsController = window?.rootViewController as! ItemsViewController
itemsController.itemStore = itemStore
于 2018-03-31T20:56:53.877 回答
0

像这样声明itemStore并不意味着它会保留应用程序生命，它是内部的局部变量didFinishLaunchingWithOptions，如果可以在内部初始化，这种类外初始化样式是无用的ItemsViewController，这可能是因为数据逻辑在 Appdelegate 内部总是发生在 coredata 中，或者它将从本地/远程通知单击初始化的项目launchingOptions

于 2018-03-31T21:01:44.407 回答
0

不管这是否是 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。这种模式称为依赖注入。

这个想法是您可以ItemStore在单元测试期间为您的视图控制器提供 mocked 。（这假设您进行单元测试，您应该这样做！:)

使用模拟，您可以模拟视图控制器在ItemStorenil、空或包含某些项目时的行为。

于 2018-03-31T22:57:38.767 回答