iOS开发当中时常会遇到需要获取当前控制器的需求（如：302重定向跳转，登录跳转等），本例已在项目中实践，小小分享，供参考。stackoverflow上的相关问答：

1、Get reference to top viewcontroller in iOS；
2、iPhone — How to find topmost view controller；

//获取当前最上层的控制器
-(UIViewController *) getTopMostController
{
    UIWindow *topWindow = [UIApplication sharedApplication].keyWindow;
    if (topWindow.windowLevel != UIWindowLevelNormal)
    {
        topWindow = [self returnWindowWithWindowLevelNormal];
    }
    
    UIViewController *topController = topWindow.rootViewController;
    if(topController == nil)
    {
        topWindow = [UIApplication sharedApplication].delegate.window;
        if (topWindow.windowLevel != UIWindowLevelNormal)
        {
            topWindow = [self returnWindowWithWindowLevelNormal];
        }
        topController = topWindow.rootViewController;
    }
    
    while(topController.presentedViewController)
    {
        topController = topController.presentedViewController;
    }
    
    if([topController isKindOfClass:[UINavigationController class]])
    {
        UINavigationController *nav = (UINavigationController*)topController;
        topController = [nav.viewControllers lastObject];
        
        while(topController.presentedViewController)
        {
            topController = topController.presentedViewController;
        }
    }
    
    return topController;
}

-(UIWindow *) returnWindowWithWindowLevelNormal
{
    NSArray *windows = [UIApplication sharedApplication].windows;
    for(UIWindow *topWindow in windows)
    {
        if (topWindow.windowLevel == UIWindowLevelNormal)
            return topWindow;
    }
    return [UIApplication sharedApplication].keyWindow;
}