在AppDelegate中，主UIWindow是如何实例化的？

问题描述

Master-Detail Xcode项目中默认代码的片段

A snippet of the default code in a Master-Detail Xcode project

AppDelegate.m

AppDelegate.m

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
 UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;  // *** here ***
    MasterViewController *controller = (MasterViewController *)navigationController.topViewController;
    controller.managedObjectContext = self.managedObjectContext;
    return YES;
}

AppDelegate.h

AppDelegate.h

@property (strong, nonatomic) UIWindow *window;

我知道@synthesize只设置了访问器方法，并且没有自动执行初始化。但是， window 如果从未显式初始化，那么 rootViewController 如何？这只是Xcode在幕后启动吗？

I am aware that @synthesize just sets the accessor methods, and no initialization happens automagically. But how does window have a non-nil rootViewController if it is never explicitly initialized? Is this just Xcode init'ing behind the scenes?

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