install the official website  https://reactnative.cn/docs/g., and build the react-native development environment on mac. After startup, you don"t see 
 but see the interface of iphone 6 simulator 
  
?
version of the environment I use:
 xcode: 9.0 
 react-native-cli: 2.0.1 
 react-native: 0.44.3 
