在安卓原始 App中使用的加载框 ProgressBar 在React -Native 中也是有相对应的视图，叫做ActivityIndicator,对应ios 中React-Native 提供的是ActivityIndicatorIos

带动画的加载指示 android 使用 ActivityIndicator ios 使用 ActivityIndicatorIos
  size = 'large'
  color ='#6435c9'
  设置大小和颜色

/**
 * Sample React Native App
 * https://github.com/facebook/react-native
 * @flow
 */
'use strict'
import React,{Component} from 'react';
import styles from '../Styles/Main';
import {
    Text,Image,View,ListView,ActivityIndicator,} from 'react-native';

let REQUEST_URL = 'https://api.douban.com/v2/movie/top250';
export default class Day0718 extends Component {
    constructor(props) {
        super(props);
        this.state = {
            dataSource:new ListView.DataSource({rowHasChanged: (r1,r2) => r1 !== r2}),loaded: false,};
    }
    componentDidMount(){
        this._fetchData();
    }
    _fetchData(){
        fetch(REQUEST_URL)
            .then(response => response.json())
            .then(responseData =>{
                this.setState({
                    movies:this.state.dataSource.cloneWithRows(responseData.subjects),// loaded: true,});
            })
            .done();
    }
    render() {
        if(!this.state.loaded){
            return this._renderLoadingView();
        }
        return (
            <View style={styles.Container}>
                <ListView
                    dataSource={this.state.movies}
                    renderRow={this._renderMovieList}
                    style={styles.listView}

                />
            </View>
        );
    }
    _renderMovieList(movie){
        return(
            <View style = {styles.item}>
                <View style = {styles.itemImage}>
                    <Image
                        style ={styles.image}
                        source ={{uri:movie.images.large}}/>
                </View>
                <View style = {styles.itemContent}>
                    <Text style ={styles.itemHeader}>{movie.title}</Text>
                    <Text style ={styles.itemMeta}>{movie.original_title}({movie.year})</Text>
                    <Text style ={styles.redText}>{movie.rating.average}</Text>

                </View>
            </View>
        );
    };
    _renderLoadingView(){
        return (
            <View style ={styles.loading}>
                <ActivityIndicator
                    size = 'large'
                    color ='#6435c9'
                />
            </View>
        );
    };

}

效果图：