解决报错“Ignoring ffi-1.12.2 because its extensions are not built. Try: gem pristine ffi --version 1.12”

执行pod install时报错

Ignoring ffi-1.12.2 because its extensions are not built. Try: gem pristine ffi --version 1.12.2

提示执行

gem pristine ffi --version 1.12.2

执行之后又会报错

ERROR:  While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions for the /Library/Ruby/Gems/2.6.0 directory.

提示没有写入权限。
加上权限，继续执行

sudo gem pristine ffi --version 1.12.2

终端输出日志 

Restoring gems to pristine condition...

Building native extensions. This could take a while...

Restored ffi-1.12.2

成功！