If we wish to download files or resources but do not need fine control over the actual download, we can hand the task over to the DownloadManager instance.
We can use the DownloadManager instance to initiate, cancel, and query downloads:
- As with normal HTTP requests, we need to request permission to access the Internet:
[assembly: UsesPermission(Manifest.Permission.Internet)]
- Then, we get hold of
DownloadManagerso that we can work with it:var manager = DownloadManager.FromContext(this);
- Once we have the manager, we can start downloads using the
Enqueue()method with aDownloadManager.Requestinstance:var request = new DownloadManager.Request(Uri.Parse(uri)); long downloadId = manager.Enqueue(request);
- We can also specify whether we only want the download to progress on Wi-Fi, mobile data, or both by using the
DownloadNetworkflags enumeration:request.SetAllowedNetworkTypes(DownloadNetwork.Wifi);
- If we want to cancel a download, we pass the download ID to the
Remove()method:var removed = manager.Remove(downloadId);
- To obtain the progress or state of a download, we pass an instance of
DownloadManager.Queryto theInvokeQuery()method:var query = new DownloadManager.Query(); query.SetFilterById(downloadId); ICursor cursor = manager.InvokeQuery(query);
- Once we have the cursor, we can query the result just as we would with a content provider:
if (cursor.MoveToFirst()) { var statusIndex = cursor.GetColumnIndex( DownloadManager.ColumnStatus); var soFarIndex = cursor.GetColumnIndex( DownloadManager.ColumnBytesDownloadedSoFar); var totalIndex = cursor.GetColumnIndex( DownloadManager.ColumnTotalSizeBytes); int status = (DownloadStatus)cursor.GetInt(statusIndex); double soFar = cursor.GetDouble(soFarIndex); double total = cursor.GetDouble(toitalIndex); string progress = (soFar / total).ToString("0.00 %"); }
If we want to be notified when the download is completed, is canceled, or fails, we register a BroadcastReceiver instance with the activity or service:
- We need to create an instance of
BroadcastReceivethat will handle the download event:private class DownloadReceiver : BroadcastReceiver { public override void OnReceive( Context context, Intent intent) { long downloadId = intent.GetLongExtra( DownloadManager.ExtraDownloadId, 0); } } - Once we have the download ID, we can query the
DownloadManager instanceas we normally would:var manager = DownloadManager.FromContext(context); var query = new DownloadManager.Query(); query.SetFilterById(downloadId); ICursor cursor = manager.InvokeQuery(query); if (cursor.MoveToFirst()) { // handle the download } - In order to register the receiver with the activity or service, we use the
RegisterReceiver()method:var filter = new IntentFilter( DownloadManager.ActionDownloadComplete); receiver = new DownloadReceiver(); RegisterReceiver(receiver, filter);
- And we must make sure that we unregister it when we no longer need it:
if (receiver != null) { UnregisterReceiver(receiver); }
We can also display the Android Downloads app, if our app downloads files that can be accessed by the user directly:
- If we want to open the native Downloads app, we use an
Intentinstance:var intent = new Intent(); intent.SetAction(DownloadManager.ActionViewDownloads); StartActivity(intent);
If we have an app that downloads large files, we can use the DownloadManager instance. The DownloadManager instance is useful because it will manage our downloads for us, for example by retrying downloads after the network connection is lost or the device is restarted. It will also broadcast an action when the download is completed.
Once we have the DownloadManager instance, we can start queuing up downloads using the DownloadManager.Request type and the Enqueue() method. The request is constructed with a URI, but can have several additional properties set.
We could set the download to be invisible to the user by passing false to the SetVisibleInDownloadsUi() method, or we can specify on which types of network the download can run via the SetAllowedNetworkTypes() method.
When a download is queued, the Enqueue() method will return an ID for the download. We can use the ID to find out the progress or state of the download. If we want to query the status of a download, we construct a DownloadManager.Query instance and set the filter via the SetFilterById() or SetFilterByStatus() methods.
Once we have a query, we can get results by invoking the InvokeQuery() method on the download manager. Similar to querying a ContentProvider() method, we will receive an ICursor instance that we can use. With the cursor, we read the progress, status, any errors, and paths associated with a download.
If we want to be notified when a download is complete, we can create a BroadcastReceiver instance that will respond to the ActionDownloadComplete broadcast. This broadcast will contain the ID of the download in the extras of the intent, which we can use to update our app.