#import <pthread.h>

+ (UIViewController *)topViewController {
    UIViewController *result = nil;
    
    static pthread_mutex_t pLock;
    pthread_mutex_init(&pLock, NULL);
    pthread_mutex_lock(&pLock);
    
    UIWindow * window = [[self class] mainWindow];
    if (window.windowLevel != UIWindowLevelNormal) {
        NSArray *windows = [[UIApplication sharedApplication] windows];
        for (UIWindow * tmpWin in windows) {
            if (tmpWin.windowLevel == UIWindowLevelNormal) {
                window = tmpWin;
                break;
            }
        }
    }
    
    if (window) {
        NSArray *subviews = [window subviews];
        if ([subviews isKindOfClass:[NSArray class]] && subviews.count > 0) {
            UIView *frontView = [subviews firstObject];
            id nextResponder = [frontView nextResponder];
            if ([nextResponder isKindOfClass:[UIViewController class]]) {
                result = nextResponder;
            }
            else {
                result = window.rootViewController;
            }
        }
    }
    
    UIViewController *viewController = [[self class] topViewControllerWithRootViewController:result];
    
    pthread_mutex_unlock(&pLock);
    
    return viewController;
}

+ (UIViewController*)topViewControllerWithRootViewController:(UIViewController*)rootViewController {
    
    if ([rootViewController isKindOfClass:[UITabBarController class]]) {
        UITabBarController* tabBarController = (UITabBarController*)rootViewController;
        return [[self class] topViewControllerWithRootViewController:tabBarController.selectedViewController];
    }
    else if ([rootViewController isKindOfClass:[UINavigationController class]]) {
        UINavigationController* navigationController = (UINavigationController*)rootViewController;
        return [[self class] topViewControllerWithRootViewController:navigationController.topViewController];
    }
    else if (rootViewController.presentedViewController) {
        UIViewController* presentedViewController = rootViewController.presentedViewController;
        if ([presentedViewController isKindOfClass:[UIAlertController class]]) {
            return rootViewController;
        }
        return [[self class] topViewControllerWithRootViewController:presentedViewController];
    }
    else {
        return rootViewController;
    }
}