The development server returned

react-native start

提示：

 ERROR  Metro Bundler can't listen on port8081

杀死进程

On a mac, run the following command to find id of the process which is using port 8081

sudo lsof -i :8081

Then run the following to terminate process:

kill -9 23583（PID）

react-native start  后

Metro Bundler ready.  成功