我在Project中有一个带有@escaping方法的iOS项目（AppDelegate），用于为此项目创建NSPersistentContainer。然后在didFinishLaunchingWithOptions方法中调用此方法，如下所示：
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
var window: UIWindow?
var persistentContainer: NSPersistentContainer!
func createProjectContainer(completion: @escaping (NSPersistentContainer) -> ()) {
let container = NSPersistentContainer(name: "Project")
container.loadPersistentStores { (_, error) in
guard error == nil else { fatalError("Failed to load store: \(error!)") }
DispatchQueue.main.async { completion(container) }
}
}
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
createProjectContainer { container in
self.persistentContainer = container
let storyboard = self.window?.rootViewController?.storyboard
guard let vc = storyboard?.instantiateViewController(withIdentifier: "NavigationController") as? NavigationController else {
fatalError("Cannot instantiate root view controller")
}
vc.managedObjectContext = container.viewContext
self.window?.rootViewController = vc
}
return true
}
}
NavigationController类只是UINavigationController的子类，它嵌入了UIViewController（ChildViewController）并且有一个NSManagedObjectContext变量（{{1} }），我想传递给第一个managedObjectContext：
ChildViewController现在class NavigationController: UINavigationController {
var managedObjectContext: NSManagedObjectContext!
override func viewDidLoad() {
super.viewDidLoad()
guard let vc = self.childViewControllers.first as? ChildViewController else { fatalError("") }
vc.managedObjectContext = managedObjectContext
print("NavigationController: viewDidLoad")
print("managedObjectContext ---> \(managedObjectContext)")
}
}
似乎被调用了两次。在第一次通话时，print("NavigationController: viewDidLoad")，在第二次通话中已分配。应用程序加载这两次是否重要？
由于关闭的managedObjectContext = nil属性，storyboard?.instantiateViewController(withIdentifier: "NavigationController")第一次加载NavigationController后，@escaping被称为之后的似乎正在发生。但是，如果我排除该行，只需获取对NavigationController的引用即可显示managedObjectContext。