Wrapper for a modal view controller that should be assigned to rootViewController of a new UIWindow, so that the modal view controller can be presented out of the application flow (based for example on an external event).

TransparentViewController.m

@protocol TransparentViewControllerDelegate;

@interface TransparentViewController : UIViewController

- (instancetype)initWithRootViewController:(UIViewController *)rootViewController;

@property (nonatomic, weak) id<TransparentViewControllerDelegate> delegate;

@end

@protocol TransparentViewControllerDelegate <NSObject>

- (void)transparentViewControllerDidDismissRootViewController:(TransparentViewController *)transparentViewController;

@end

@interface TransparentViewController ()

@property (nonatomic, strong) UIViewController *rootViewController;
@property (nonatomic) BOOL didAlreadyAppear;

@end

@implementation TransparentViewController

- (instancetype)initWithRootViewController:(UIViewController *)rootViewController
{
    self = [super init];
    if (self) {
        _rootViewController = rootViewController;
    }
    return self;
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    if (self.didAlreadyAppear) {
        [self.view.window resignKeyWindow];
        self.view.window.hidden = YES;

        if ([self.delegate respondsToSelector:@selector(transparentViewControllerDidDismissRootViewController:)])
            [self.delegate transparentViewControllerDidDismissRootViewController:self];
    } else {
        if (self.rootViewController)
            [self presentViewController:self.rootViewController animated:YES completion:nil];

        self.rootViewController = nil;
    }

    self.didAlreadyAppear = YES;
}

@end

Usage is as follows:

- (void)presentMyModalViewController
{
    UIViewController *myViewController = ...

    TransparentViewController *transparentViewController =
            [[TransparentViewController alloc] initWithRootViewController:myViewController];
    transparentViewController.delegate = self;

    UIWindow *specialWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    specialWindow.rootViewController = transparentViewController;
    [specialWindow makeKeyAndVisible];

    self.specialWindow = specialWindow;
}

- (void)transparentViewControllerDidDismissRootViewController:
                                 (TransparentViewController *)transparentViewController
{
    self.specialWindow = nil;
}