android 下载管理器何时会通过下载参考 ID 给出下载状态？

Question

我有一个用例，

我开始使用 android 下载管理器下载文件，并在中间关闭了移动设备。当我再次重新启动时，下载继续并完成。我通过提供下载参考 ID 获得了状态。但我想知道即使在使用该参考 ID 10 天后的状态。

所以我的问题是 android 下载管理器何时会通过下载参考 ID 给出下载状态？

我查看了文档并遍历了 stack over flow，但找不到答案。有人帮帮我。

Answer 1

您可以使用 SharedPreferences 来存储您的下载参考 ID。 像这样 -

SharedPreferences settings = getSharedPreferences("DownloadIDS", 0);
SharedPreferences.Editor editor = settings.edit();
editor.putLong("downloadIds", downloadID);
editor.commit();

您可以稍后在使用此

时检索 ID

SharedPreferences downloadids = context.getSharedPreferences("DownloadIDS", 0);
long savedDownloadIds = downloadids.getLong("downloadIds", 0);

Answer 2

Android DownloaderManager is a system service. It is supposed to be 运行ning always. But there are some cases when it can't 运行.

The download happens via the HTTP persistent connection. It means the same established connection is used for successive HTTP request/response. The connection break means error occurs, and thus, you can't track the status by reference id.

You track via Android DownloadManager service, where Android DownloadManager service gets STATUS code from the server.

Android DownloadManager uses the content-length based download from the server. The Content-Length header will not allow streaming (link). The content-length based download has the advantage of resume, pause, partial download -- see the link1 above. So, even when you reboot the system, again it restarts (increments) the download.

The content-length based download is store and forward (link). You should forward the buffered content to the persistent storage because you have limited fixed buffers.

The Android DownloadManager has ERROR_CANNOT_RESUME int flag (link). The ERROR_CANNOT_RESUME is based on the COLUMN_STATUS flag. There are two types of COLUMN STATUS: STATUS_PAUSED or STATUS_FAILED. Before system turns off, the system via the BroadcastReceiver sends Android DownloadManager service about turn off. The Android DownloadManager then activates STATUS_PAUSED flag. And, when next time you restart the device, the system service 运行s automatically, checks if STATUS_PAUSED then starts downloading again.

Answer: so until there happens error (in client side, connection or server-side) or you are not done downloading the file (it means until STATUS_SUCCESSFUL), you keep getting status from the Android DownloadManager. You can't get status when there happens STATUS_FAILED -- it says download will not be retried (link).

How STATUS_FAILED happens? Client's DownloadManager service detects HTTP status code 4XX (Server guesses client is erred) and 5XX (Server detects server is erred) (link), now STATUS_FAILED becomes true.

Some other situations: When Clients keeps turned-off and based on server-logic, the server can terminate the connection with the timeout. So, this control is explicitly based on different HTTP server. We can't ask these many days here. We don't know the server-side logic. The status_codes are based on the server. When server decides client has failed, and the server then does timeout the connection making STATUS_FAILED active at the server-side. clients must be prepared for TCP connections to disappear at arbitrary times, and must be able to re-establish the connection and retry the HTTP request. A prematurely closed connection should not be treated as an error; an error would only be signaled if the attempt to re-establish the connection fails. Your question doesn't have an exact answer.

Note: TCP connections to disappear at arbitrary times (link) is the main logic here that can resume your connection after a certain number of days of your device turned off.

1) On STATUS_FAILED, you can't continue track further data.

2) On If COLUMN_STATUS is neither STATUS_FAILED nor STATUS_PAUSED, this column's value is undefined, here you may not be able to track further data.

-- Anything in other than above two conditions, the download is in progress.