In an AppDelegate, how is the main UIWindow instantiated?

Question

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

---

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

@property (strong, nonatomic) UIWindow *window;

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?

Answer 1

From my book:

If you choose the Storyboard option as you specify a template, the process works a little differently. The app is given a main storyboard, pointed to by the Info.plist key “Main storyboard file base name” (UIMainStoryboardFile). After UIApplicationMain instantiates the app delegate class, it asks the app delegate for the value of its window property; if that value is nil, the window is created and assigned to the app delegate’s window property. The storyboard’s initial view controller is then instantiated and assigned to the window’s rootViewController property, with the result that its view is placed in the window as its root view; the window is then sent the makeKeyAndVisible message. All of that is done behind the scenes by UIApplicationMain, with no visible code whatever. That is why, in a storyboard template, the application:didFinishLaunchingWithOptions: implementation is empty.

Answer 2

From the UIWindow documentation:

Note: When you use storyboards and the Xcode app templates to create an app, a window is created for you.

If you don't use storyboards, the window is explicitly created, though all the standard project templates do this out of the box. You'll see a line similar to this in the app delegate:

self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];

Using storyboards, the window is created behind the scenes when the main storyboard is loaded (see the View Controller Programming Guide for more info).