xcode运行报错如下： 

Logging Error: Failed to initialize logging system. Log messages may be missing. If this issue persists, try setting IDEPreferLogStreaming=YES in the active scheme actions environment variables.

解决办法

点击Product > Scheme > Edit Scheme

点击Arguments > 添加一个新的环境变量： 

添加环境变量：

IDEPreferLogStreaming=YES

然后重新运行就好了：