错误：Warning: Attempt to present <UIAlertController: 0x7fd192806e20> on <ViewController: 0x7fd1928048d0> whose view is not in the window hierarchy!

系统：mac OS  10.12 (16A323)

Xcod:8.3.3

错误：Warning: Attempt to present <UIAlertController: 0x7fd192806e20> on <ViewController: 0x7fd1928048d0> whose view is not in the window hierarchy!

原因：今天写UIalerCortoller 出现此错误，查找得资料这是个经典的 错误，在往后日子里面都会遇到类似问题，这是因为弹窗视图必须在当前根控制器初始化现实后才可以执行，在viewDidLoad 方法里面还未初始化完毕导致。

p.p1 { margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px Menlo; color: #ffffff }
span.s1 { }

p.p1 { margin: 0.0px 0.0px 0.0px 0.0px; font: 14.0px "PingFang SC"; color: #4dbf56 }
span.s1 { }
span.s2 { font: 14.0px Menlo }