我试图在我的应用程序的屏幕之间传递数据。目前我正在使用

"react-native": "0.46.0",
"react-navigation": "^1.0.0-beta.11"

我有我的index.js

 import React, { Component } from 'react';
    import {
      AppRegistry,
    } from 'react-native';
    import App from './src/App'
    import { StackNavigator } from 'react-navigation';
    import SecondScreen from './src/SecondScreen'

    class med extends Component {
      static navigationOptions = {
        title: 'Home Screen',
      };

      render(){
        const { navigation } = this.props;

        return (
          <App navigation={ navigation }/>
        );
      }
    }

    const SimpleApp = StackNavigator({
      Home: { screen: med },
      SecondScreen: { screen: SecondScreen, title: 'ss' },    
    });

    AppRegistry.registerComponent('med', () => SimpleApp);

应用为

    import React, { Component } from 'react';
    import {
      StyleSheet,
      Text,
      Button,
      View
    } from 'react-native';
    import { StackNavigator } from 'react-navigation';

    const App = (props)  => {
      const { navigate } = props.navigation;

      return (
        <View>
          <Text>
            Welcome to React Native Navigation Sample!
          </Text>
          <Button
              onPress={() => navigate('SecondScreen', { user: 'Lucy' })}
              title="Go to Second Screen"
            />
        </View>
      );
    }

    export default App

然后在secondscreen.js中，我们将从前一个屏幕获取的数据作为

    import React, { Component } from 'react';
    import {
      StyleSheet,
      Text,
      View,
      Button
    } from 'react-native';

    import { StackNavigator } from 'react-navigation';


    const SecondScreen = (props)  => {
      const { state} = props.navigation;
      console.log("PROPS" + state.params);


      return (
        <View>
          <Text>
            HI
          </Text>

        </View>
      );
    }

    SecondScreen.navigationOptions = {
      title: 'Second Screen Title',
    };

    export default SecondScreen

每当我console.log我得到未定义。
https://reactnavigation.org/docs/navigators/navigation-
prop
文档说每个屏幕都应该具有这些值，我在做什么错呢？