ReactNative报错Error watching file

RN项目之前一直运行OK，换了新的Mac，操作系统10.13.6，再次运行项目时报错。

该错误同样出现在升级Mac系统时。

原因：watchman需要重装
直接install watchman提示watchman已经安装，那就使用升级命令：brew upgrade watchman

➜  mywork brew install watchman
Error: watchman 4.9.0 is already installed
To upgrade to 4.9.0_2, run `brew upgrade watchman`
➜  mywork brew upgrade watchman

安装后，项目运行正常。