Open Radar

 
Summary:
So if a UIWindow subclass redefines a delegate property and use it then, that leads to unexpected behavior and weird side effects (like interface rotations get disabled) which are difficult to debug.

Private delegates in UIKit must be declared with custom accessors, e.g.:

@property (getter=_delegate, setter=_setDelegate:, nonatomic <UIWindowInternalDelegate> *delegate;

Steps to Reproduce:
Compile and run the following code, which demonstrates harmful side effects:

// UIWindow with redefined delegate.
@interface Window : UIWindow
@property (nonatomic, weak) id<NSObject> delegate;
@end

@implementation Window
@end

// View controller which can change UIWindow delegate.
@interface ViewController : UIViewController
@end

@implementation ViewController

- (instancetype)initWithWindow:(Window *)window {
  self = [super init];
  if (self) {
    window.delegate = self;
  }
  return self;
}

- (void)viewWillTransitionToSize:(CGSize)size
       withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator {
  [super viewWillTransitionToSize:size withTransitionCoordinator:coordinator];

  // Never called because we've overridden UIWindow delegate and set it before setRootViewController.
  assert(NO);
}

@end

// Standard application delegate.
@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (nonatomic) Window *window;

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
  self.window = [[Window alloc] initWithFrame:[UIScreen mainScreen].bounds];
  self.window.rootViewController = [[ViewController alloc] initWithWindow:self.window];
  [self.window makeKeyAndVisible];

  return YES;
}

@end


Expected Results:
Private UIKit properties do not cause collisions with public ones declared in subclasses.
The code above should invoke viewWillTransitionToSize:withTransitionCoordinator: on device rotation.

Actual Results:
Private UIWindow delegate property causes collisions with public one declared in subclasses.
The code above does not invoke viewWillTransitionToSize:withTransitionCoordinator: on device rotation.