一次给了下面两条警告（精彩吧）：
Presenting view controllers on detached view controllers is discouraged
Unbalanced calls to begin/end appearance transitions for <UITabBarController: 0x7fc046131d70>.
解决办法，dispatch_after 延迟present就好了。
UITabBarController未呈现时present另一个ViewController会发生什么？
原文：http://acalism.blog.51cto.com/9587059/1834795