如何在ReactNative的水平和垂直中心文本?

我在rnplay.org中有一个示例应用程序，其中justifyContent=“中心”和alignItems=“中心”是不工作的: https://rnplay.org/apps/AoxNKQ

文字应该居中。 为什么在顶部的文本(黄色)和父容器之间有一个边距?

代码:

'use strict';

var React = require('react-native');
var {
  AppRegistry,
  StyleSheet,
  Text,
  Image,
  View,
} = React;

var SampleApp = React.createClass({
  render: function() {
    return (
            <View style={styles.container}>
                <View style={styles.topBox}>
                    <Text style={styles.headline}>lorem ipsum{'\n'}ipsum lorem lorem</Text>

                </View>
                <View style={styles.otherContainer}>
                </View>
            </View>
    );
  }
});

var styles = StyleSheet.create({

    container: {
        flex: 1,
        flexDirection: 'column',
        backgroundColor: 'red',
        justifyContent: 'center',
        alignItems: 'center',
    },

    topBox: {
        flex: 1,
        flexDirection: 'row',
        backgroundColor: 'lightgray',
        justifyContent: 'center',
        alignItems: 'center',
    },
    headline: {
        fontWeight: 'bold',
        fontSize: 18,
    marginTop: 0,
        width: 200,
        height: 80,
    backgroundColor: 'yellow',
        justifyContent: 'center',
        alignItems: 'center',
    },

  otherContainer: {
        flex: 4,
        justifyContent: 'center',
        alignItems: 'center',
    backgroundColor: 'green',
    },


});

AppRegistry.registerComponent('SampleApp', () => SampleApp);

module.exports = SampleApp;