当前最上层的视图控制器vc 和 当前最上层的导航控制器nav

在处理 URL Router 跳转的时候，我们经常需要得到 当前最上层的视图控制器 和 当前最上层的导航控制器 来进行视图跳转或者方法调用。

- (UIViewController *)currentViewController
{
    UIWindow *keyWindow  = [UIApplication sharedApplication].keyWindow;
    UIViewController *vc = keyWindow.rootViewController;
    while (vc.presentedViewController)
    {
        vc = vc.presentedViewController;

        if ([vc isKindOfClass:[UINavigationController class]])
        {
            vc = [(UINavigationController *)vc visibleViewController];
        }
        else if ([vc isKindOfClass:[UITabBarController class]])
        {
            vc = [(UITabBarController *)vc selectedViewController];
        }
    }
    return vc;
}

- (UINavigationController *)currentNavigationController
{
    return [self currentViewController].navigationController;
}

/////////////////////////////////////////////////////////////////////////
2个只读属性：
presentedViewController：The view controller that is presented by this view controlller(read-only)，被本视图控制器present出来的的视图控制器

presentingViewController：The view controller that presented this view controller. (read-only)，present出来本视图控制器的视图控制器

如A-->弹出B, 则A.presentedViewController = B
B.presentingViewController = A

dismissViewControllerAnimated:YES
Dismisses the view controller that was presented modally by the view controller.
也就是在在A上调该方法，dismiss掉A弹出的vc
如果在B上调，会调用presenting view的该方法，即A的该方法，很绕啊。

UINavigationController 中有visibleViewController和topViewController

• visibleViewController 当前显示的控制器
• topViewController 是某个导航栈的栈顶视图
• visibleViewController跟导航栈没有关系，只是当前显示的控制器，也就是说任意一个导航的visibleViewController所返回的值应该是一样的,
• topViewController 是某个导航栈的栈顶视图，和导航控制器相关
  换句话说如果在仅有一个导航栈上，visibleViewController和topViewController应该是没有区别得。

方法一 : 获取当前显示的控制器 UIWindow (Visible)

 - (UIViewController *)visibleViewController {
    UIViewController *rootViewController =[[[[UIApplication sharedApplication] delegate] window] rootViewController];
    return [UIWindow getVisibleViewControllerFrom:rootViewController];
}

+ (UIViewController *) getVisibleViewControllerFrom:(UIViewController *) vc {
    if ([vc isKindOfClass:[UINavigationController class]]) {
        return [UIWindow getVisibleViewControllerFrom:[((UINavigationController *) vc) visibleViewController]];
    } else if ([vc isKindOfClass:[UITabBarController class]]) {
        return [UIWindow getVisibleViewControllerFrom:[((UITabBarController *) vc) selectedViewController]];
    } else {
        if (vc.presentedViewController) {
            return [UIWindow getVisibleViewControllerFrom:vc.presentedViewController];
        } else {
            return vc;
        }
    }

}

方法二 :

- (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;
    }

}

值得注意的是

• [[[UIApplication sharedApplication] keyWindow] rootViewController]有时为nil 比如当页面有菊花在转的时候，这个rootViewController就为nil;

• 所以使用[[[[UIApplication sharedApplication] delegate] window] rootViewController]
  或者[[[[UIApplication sharedApplication] windows] objectAtIndex:0] rootViewController]

• presentedViewController 和presentingViewController
  当A弹出B
  A.presentedViewController=B
B.presentingViewController=A