转载自：http://www.cnblogs.com/junhuawang/p/6639878.html
当前界面所在的视图控制器 逐步完善的过程
一、

//获取当前屏幕显示的viewcontroller   (这里面获取的相当于rootViewController)
- (UIViewController *)getCurrentVC  
{  
    UIViewController *result = nil;  
      
    UIWindow * window = [[UIApplication sharedApplication] keyWindow];  
    if (window.windowLevel != UIWindowLevelNormal)  
    {  
        NSArray *windows = [[UIApplication sharedApplication] windows];  
        for(UIWindow * tmpWin in windows)  
        {  
            if (tmpWin.windowLevel == UIWindowLevelNormal)  
            {  
                window = tmpWin;  
                break;  
            }  
        }  
    }  
      
    UIView *frontView = [[window subviews] objectAtIndex:0];  
    id nextResponder = [frontView nextResponder];  
      
    if ([nextResponder isKindOfClass:[UIViewController class]])  
        result = nextResponder;  
    else  
        result = window.rootViewController;  
      
    return result;  
} 

二、不够完善

//获取到当前所在的视图
+ (UIViewController *)presentingVC{
    UIWindow * window = [[UIApplication sharedApplication] keyWindow];
    if (window.windowLevel != UIWindowLevelNormal){
        NSArray *windows = [[UIApplication sharedApplication] windows];
        for(UIWindow * tmpWin in windows){
            if (tmpWin.windowLevel == UIWindowLevelNormal){
                window = tmpWin;
                break;
            }
        }
    }
    UIViewController *result = window.rootViewController;
    while (result.presentedViewController) {
        result = result.presentedViewController;
    }
    if ([result isKindOfClass:[MYTabBarViewController class]]) {
        result = [(MYTabBarViewController *)result selectedViewController];
    }
    if ([result isKindOfClass:[UINavigationController class]]) {
        result = [(UINavigationController *)result topViewController];
    }
    return result;
}

三、参数传入导航试图控制器或者UITabBarController self.window.rootViewController 也可。（这个比较通用）

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