执行 pod lib lint 时，报错：
ERROR | [iOS] unknown: Encountered an unknown error (Could not find a `ios` simulator (valid values: com.apple.coresimulator.simruntime.ios-10-3, com.apple.coresimulator.simruntime.ios-12-1, com.apple.coresimulator.simruntime.ios-8-1, com.apple.coresimulator.simruntime.tvos-12-1, com.apple.coresimulator.simruntime.watchos-5-1). Ensure that Xcode -> Window -> Devices has at least one `ios` simulator listed or otherwise add one.
原因rubygem源发生变化
https://gems.ruby-china.org/ 域名不可用 要换成 https://gems.ruby-china.com/
移除旧源添加新的ruby源
gem sources --add https://gems.ruby-china.com/ --remove https://gems.ruby-china.org/
查看ruby源是否替换成功
gem sources -l
如果替换成功，重新初始化cocoapods
sudo gem install cocoapods