let options = {
fromUrl: url,
toFile: ${path}/${newName}${extension},
background: true
};
RNFS.downloadFile(options).promise.then(res => {
cb(null, res)
var tempURl = path+'/'+newName+extension;
try{
var promise = CameraRoll.saveToCameraRoll(tempURl,'photo');
promise.then(succ=>{
alert('succ：'+JSON.stringify(succ));
}).catch(err=>{
alert('err：'+JSON.stringify(err));
})
}catch(err){
alert(err);
}
}).catch(err => {
cb(err)
})