reactjs – React Native StackNavigator initialRouteName

我如何通过AsyncStorage设置StackNavigator initialRouteName？

function getInitialScreen() {
    AsyncStorage.getItem('initialScreen')
        .then(screenName => {
            return (screenName)
                ? screenName
                : 'Login';
        })
        .catch(err => {});
}

const Navigator = StackNavigator({
    Splash: { screen: Splash },
    Login: { screen: Login },
    WebPage: { screen: WebPage }
}, {
    initialRouteName: getInitialScreen()
});

const RootNavLogged = StackNavigator({
     ...
  },{
     initialRouteName : 'Home'
  });

  const RootNav = StackNavigator({
     ...
  },{
     initialRouteName : 'Login'
  });

  class App extends Component {
     render(){
         if (this.props.userLogged == true ){
            return (
               <RootNavLogged/>
            ) 
          } else {
             return(
                <RootNav/>
              ) 
          }
     }
 }