How can check user is login or not
_bootstrapAsync =异步（）=> { const userToken =等待AsyncStorage.getItem（'userToken'）;
// This will switch to the App screen or Auth screen and this loading
// screen will be unmounted and thrown away.
_bootstrapAsync = async () => { const userToken = await
AsyncStorage.getItem('userToken');
if (userToken!=true) {
alert("Session Expired!!");
this.props.navigation.navigate('Home');
}
console.log("+++++++++++++++++++++++++++++++++++++++Working++++++++++++++++++++++++++++++");
};