App.js

  render() {
    return (
      <View >
        <View >
          <FlatList
            data = {this.state.busLists}
            renderItem = {({item}) => (
                <View>
                    <Text>{item}</Text>
                </View>   
            )}
          />
        </View>
      </View>
    );
  }
}

fetch.js

    constructor(props) {
        super(props);

        this.state = ({
            busList: '',
            busLists: []
        })
    }

    componentDidMount() {
        firebase.database().ref().child('busList/').once('value', snapshot => {
            const data = snapshot.val()
            if(snapshot.val()) {
                const initMessages = [];
                Object.keys(data).forEach(busList => initMessages.push(data[busList]));
                this.setState({
                    busLists: initMessages
                })
            }
        })

        firebase.database().ref().child('busList/').on("child_added", snapshot =>{
            const data = snapshot.val();
            if(data) {
                this.setState(prevState => ({
                    busLists: [data, ...prevState.busLists]
                }))
            }
        }) 
    }