react-navigation 如何判断用户是否登录跳转到登录页

import React, {Component} from 'react'; 
import {AppRegistry, Text, View, Button,Image,StyleSheet,BackHandler,ToastAndroid} from 'react-native'; 
import { StackNavigator,TabNavigator,NavigationActions } from 'react-navigation'; 
 
 
 
//全局存储 
import stroage from './StorageUtil'; 
import './Global' 
 
import IndexScreen from './Index' 
import MeScreen from './Me' 
import Login from './Login' 
 
 
//底部菜单栏设置 
const MainScreenNavigator = TabNavigator({ 
    IndexScreen: { 
      screen: IndexScreen, 
      navigationOptions: { 
        tabBarLabel: '首页', 
        headerLeft:null,//去除返回按钮 
        tabBarIcon: ({ tintColor }) => ( 
          <Image 
            source={require('./img/ic_image.png')} 
            style={[styles.icon, {tintColor: tintColor}]} 
          /> 
        ), 
        onNavigationStateChange:(()=> alert("首页")) 
        // initialRouteName:'IndexScreen' 
      }, 
    }, 
     
    MeScreen: { 
      screen: MeScreen, 
      navigationOptions: { 
        tabBarLabel:'我的', 
        tabBarIcon: ({ tintColor }) => ( 
          <Image 
            source={require('./img/ic_me.png')} 
            style={[styles.icon, {tintColor: tintColor}]} 
          /> 
        ), 
        // initialRouteName:'MeScreen' 
      } 
    } 
  }, 
  { 
    // trueinitialRouteName:'MeScreen',//设置默认的页面组件 
    // initialRouteName:'MeScreen', 
    lazy:true,//是否根据需要懒惰呈现标签，而不是提前，意思是在app打开的时候将底部标签栏全部加载，默认false,推荐为true 
    // order: ['IndexScreen','FindScreen','ListNewScreen','MeScreen'], //order 来定义tab显示的顺序，数组形式 
    animationEnabled: false, // 切换页面时是否有动画效果 
    tabBarPosition: 'bottom', // 显示在底端，android 默认是显示在页面顶端的 
    swipeEnabled: false, // 是否可以左右滑动切换tab 
    // backBehavior: 'none', // 按 back 键是否跳转到第一个Tab(首页)， none 为不跳转 
    tabBarOptions: { 
      activeTintColor: '#2196f3', // 文字和图片选中颜色 
      inactiveTintColor: '#999', // 文字和图片未选中颜色 
      showIcon: true, // android 默认不显示 icon, 需要设置为 true 才会显示 
      indicatorStyle: { 
        height: 0 // 如TabBar下面显示有一条线，可以设高度为0后隐藏 
      }, 
      style: { 
        backgroundColor: '#fff', // TabBar 背景色 
        height: 60 
      }, 
      labelStyle: { 
        fontSize: 14, // 文字大小 
        marginTop:2 
        // lineHeight:44 
      }, 
    } 
  }); 
 
//跳转路由设置 
const FirstApp = StackNavigator({ 
  IndexScreen: { 
    screen: MainScreenNavigator, 
    // initialRouteName: 'IndexScreen' 
  }, 
  MeScreen: {screen: MeScreen}, 
  Login:{screen: Login}, 
   
}, { 
  initialRouteName: 'IndexScreen', // 默认显示界面 
  navigationOptions: { // 屏幕导航的默认选项, 也可以在组件内用 static navigationOptions 设置(会覆盖此处的设置) 
    headerStyle:{elevation: 0,shadowOpacity: 0,height:48,backgroundColor:"#2196f3"}, 
    headerTitleStyle:{color:'#fff',fontSize:16}, //alignSelf:'center' 文字居中 
    headerBackTitleStyle:{color:'#fff',fontSize:12}, 
    // headerTintColor:{}, 
    gesturesEnabled:true,//是否支持滑动返回收拾，iOS默认支持，安卓默认关闭 
  }, 
  mode: 'card', // 页面切换模式, 左右是card(相当于iOS中的push效果), 上下是modal(相当于iOS中的modal效果) 
  headerMode: 'screen', // 导航栏的显示模式, screen: 有渐变透明效果, float: 无透明效果, none: 隐藏导航栏 
  onTransitionStart: (Start)=>{console.log('导航栏切换开始');}, // 回调 
  onTransitionEnd: ()=>{ console.log('导航栏切换结束'); } // 回调 
}); 
// 
const defaultGetStateForAction = FirstApp.router.getStateForAction; 
 
FirstApp.router.getStateForAction = (action, state) => { 
  //页面是MeScreen并且 global.user.loginState = false || ''（未登录） 
  if (action.routeName ==='MeScreen'&& !global.user.loginState) { 
    this.routes = [ 
      ...state.routes, 
      {key: 'id-'+Date.now(), routeName: 'Login', params: { name: 'name1'}}, 
    ]; 
    return { 
      ...state, 
      routes, 
      index: this.routes.length - 1, 
    }; 
  } 
  return defaultGetStateForAction(action, state); 
}; 
 
 
export default class FirstAppDemo extends Component { 
  render() { 
    return ( 
      <FirstApp /> 
    ); 
  } 
} 
 
AppRegistry.registerComponent('FirstApp', () => FirstAppDemo); 
 
 
const styles = StyleSheet.create({ 
  icon: { 
    width: 26, 
    height: 26, 
  }, 
});