download下载时获取到的totalSize一直是0

dtask = plus.downloader.createDownload(data.downloadUrl, {
filename: "_doc/update/"
}, function(download, status) {
if (status == 200) {
popupObj.change({
progressValue: 100,
progressTip:"正在安装文件...",
progress: false,
buttonNum: 0
});
plus.runtime.install(download.filename, {}, function() {
popupObj.change({
contentText: "应用资源更新完成！",
buttonNum: 1,
progress: false
});
}, function(e) {
popupObj.cancel();
plus.nativeUI.alert("安装文件失败[" + e.code + "]：" + e.message);
});
} else {
popupObj.change({
contentText: "文件下载失败...",
buttonNum: 1,
progress: false
});
}
});
dtask.start();
dtask.addEventListener("statechanged", function(task, status) {
// console.log(task);
switch (task.state) {
case 1: // 开始
popupObj.change({
// progressValue:0,
// progressTip:"准备下载...",
progress: false
});
break;
case 2: // 已连接到服务器
popupObj.change({
// progressValue:0,
// progressTip:"开始下载...",
progress: false
});
break;
case 3:
const progress = parseInt(task.downloadedSize / task.totalSize * 100);
// console.log(progress);
if(progress - lastProgressValue >= 2){
lastProgressValue = progress;
popupObj.change({
// progressValue:progress,
// progressTip: "正在下载，请稍后",
progress: false
});
}
break;
}
});