Warning: Attempt to present * on * whose view is not in the window hierarchy!

Warning: Attempt to present  * on *  whose view is not in the window hierarchy!

出现这样的警告的原因是：ios 不允许在某个viewController的viewDidLoad方法中去呈现其他的viewController，你如果想要的是XXviewController显示之后马上呈现YY的话，可以在viewDidAppear中试试。

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