A Storage Access Framework compatible download manager for Xamarin Android

First off, you need to configure the download manager. You can do so by creating a DownloadManagerConfiguration and passing that value to the DownloadManager.Instance.Initialize method.

var configuration = DownloadManagerConfiguration.Create(this, this);
DownloadManager.Instance.Initialize(configuration);

The parameters provided to the DownloadManagerConfiguration.Create method are as follows :

• 1st parameter - the Context object
• 2nd parameter - the instance that implements the StoragePermissionsHandler.ICallback callback. Please see the next section for more details.

The rest is pretty easy. Use the following methods to control downloads

• DownloadManager.Instance.Download(url, downloadLocation /* Absolute path to directory*/, fileName, mimeType) returns the DownloadId which can be used on the rest of the methods to control downloads. If a file by the same name exists, will create a new file by a new name.
• DownloadManager.Instance.Pause - Self explanatory.
• DownloadManager.Instance.Resume - If the file exists on the recorded location, attempts to resume. Otherwise will create a new file and restart.
• DownloadManager.Instance.Remove - Self explanatory.
• DownloadManager.Instance.GetAllDownloads returns all the download records available.
• DownloadManager.Instance.OpenFile. Thought I should include this as well, since there is a high chance that you might need this as well.

When dealing with the Storage Access Framework, the developer is expected to prompt the user for approval when accessing external files. As such, we have to do the same. However doing so within the library feels wrong, and since there is no obvious way of retrieving the approval result, we expect the developer implement this part of the task. Perhaps this code may go into the library itself someday, but for the moment you have to do this. The following code segments includes a simple example.

public void OnStoragePermissionsRequested(StoragePermissionsHandler.StoragePermissionsDetail.Request request)
{
    RunOnUiThread(() =>
    {
        var sdCard = new File(request.StorageLocation);
        var storageManager = (StorageManager) GetSystemService(Context.StorageService);
        var storageVolume = storageManager.GetStorageVolume(sdCard);
        var intent = storageVolume.CreateAccessIntent(null);

        StartActivityForResult(intent, 10001);
    });
}

protected override void OnActivityResult(int requestCode, Result resultCode, Intent data)
{
    if (requestCode == 10001)
    {
        if (resultCode == Result.Ok)
        {
            ContentResolver.TakePersistableUriPermission(data.Data, ActivityFlags.GrantWriteUriPermission);

            var documentFile = DocumentFile.FromTreeUri(this, data.Data);
            request.AcceptRequest(documentFile);
        }
        else
        {
            request.RejectRequest();
        }
    }
}