iOS 获取 顶层控制器

1.我们在非视图类中想要随时展示一个view时，需要将被展示的view加到当前view的子视图，或用当前view presentViewController，或pushViewContrller，这些操作都需要获取当前正在显示的ViewController。

    //获取当前屏幕显示的viewcontroller  
    - (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;  
    }  

2.获取当前屏幕中present出来的viewcontroller。

- (UIViewController *)getPresentedViewController  
{  
    UIViewController *appRootVC = [UIApplication sharedApplication].keyWindow.rootViewController;  
    UIViewController *topVC = appRootVC;  
    if (topVC.presentedViewController) {  
        topVC = topVC.presentedViewController;  
    }  
      
    return topVC;  
}