android：显示进度对话框
问题描述：
我已经在*上查看了Android API和其他帖子，但无法弄清楚这一点。
I have looked at the Android API and other posts here on *, but have not been able to figure this out.
我的应用程序将文件下载到SD卡。我想在文件下载时弹出一个加载...对话框，然后在下载完成后消失。这是我想到的使用android API：
My app downloads files to the sd card. I would like to pop a "loading..." dialog while the file is downloading and then have it disappear when the download is finished. This is what i came up with using the android API:
ProgressDialog pd = ProgressDialog.show(this,"","Loading. Please wait...",true);
//download file
pd.cancel();
但是对话框实际上并不显示。当我调试它，它声称它显示，但显然不在屏幕上。
however the dialog doesn't actually show. when i debug it, it claims that it is showing, but it is obviously not on the screen.
我可以做什么？