// 获取当前处于activity状态的view controller
- (UIViewController *)activityViewController
{
    UIViewController* activityViewController = 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;
            }
        }
    }
    
    NSArray *viewsArray = [window subviews];
    if([viewsArray count] > 0)
    {
        UIView *frontView = [viewsArray objectAtIndex:0];
        
        id nextResponder = [frontView nextResponder];
        
        if([nextResponder isKindOfClass:[UIViewController class]])
        {
            activityViewController = nextResponder;
        }
        else
        {
            activityViewController = window.rootViewController;
        }
    }
    
    return activityViewController;
}

获取iOS应用中当前处于Activity状态的ViewController