使用的插件：
1.IOS端：ReactNative API中的CameraRoll
2.Android端：react-native-fs

引入方法：

import {CameraRoll} from "react-native"
import RNFS from 'react-native-fs';

用法

<TouchableOpacity
       onPress={() => {
                if (Platform.OS === 'ios') {
                    CameraRoll.saveToCameraRoll(this.state.modalImage)
                        .then(res => {
                            Toast("圖片保存成功", 2000)
                            this.closeModal()
                        }).catch(error => {
                        console.log(error.response)
                    })
                } else {
                    const downloadDest = `${RNFS.DocumentDirectoryPath}/${((Math.random() * 1000) | 0)}.jpg`;
                    const ret = RNFS.downloadFile({
                        fromUrl: this.state.modalImage,
                        toFile: downloadDest
                    })
                    ret.promise.then(res => {
                        console.log(res, "11111111")
                        if (res && res.statusCode === 200) {
                            CameraRoll.saveToCameraRoll("file://" + downloadDest)
                                .then(res => {
                                    Toast("圖片保存成功", 2000)
                                    this.closeModal()
                                }).catch((error) => {
                                console.log('error:', error);
                            })
                        }
                    }).catch(error => {
                        console.log(error)
                    })


                }
            }}> 
       <Text></Text>
       </TouchableOpacity>