Warning: Attempt to present UINavigationController on NavigationController which is already presenti

在iOS8中，弹出ActionSheet后选择其中一项，然后在ActionSheet的代理方法actionSheet(actionSheet:clickedButtonAtIndex:)中使用了performSegueWithIdentifier方法来跳转到下一个ViewController时，发现ViewController不但没有跳转，控制台却出现了以下警告：

Warning: Attempt to present <UINavigationController: 0x13c57ecd0> on <XXViewController: 0x13c580d10> which is already presenting (null)

然而在iOS7中一切都正常，也不会出现任何警告。出现这种问题的原因是:

Actionsheets and AlertViews were refactored to AlertControllers (iOS8 only), which are both being presented. So, basically, you’re trying to present a controller where you already have a controller presented (ActionSheet), but by the time the debugger gets it, it’s deallocated and it becomes null, producing the warning you see.

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以下是解决办法：

• 1.跳转ViewController这种类型的代码最好写到actionSheet(actionSheet:, didDismissWithButtonIndex)方法中，保证ActionSheet消失后才跳转。

• 2.如果确实需要在actionSheet(actionSheet: clickedButtonAtIndex: )就跳转，那么可以做个延时，比如0.1秒之后再调用performSegueWithIdentifier方法。