Diego Ponce de León
Large file downloads on Windows 10 mobile

Photo by Deepesh Pareek on Unsplash

Large file downloads on Windows 10 mobile

Diego Ponce de León's photo
Diego Ponce de León
·

2 min read

To download big files correctly (especially on mobile devices) we should try to achieve the following goals:

  • Separate the download process from the current view in a background task: we don´t want the download to stop if we navigate to another view or the device goes idle

  • The download stream should not write directly to memory: the device may run out of memory very soon, crashing your app. Instead, we should write to the file system directly as soon as the bytes are downloaded.

  • Report progress: users should know how long it will take to finish. Otherwise, it may look like it´s stuck

The same rules apply to any operating system, like iOS and Android (that will come in a future post).

It turns out that doing all this on Windows RT is fairly easy thanks to the class Windows.Networking.BackgroundTransfer.BackgroundDownloader, because it will follow all the rules:

var downloader = new BackgroundDownloader();
var download = downloader.CreateDownload(uri, destinationFile);
download.Priority = BackgroundTransferPriority.High;

var progressCallback = new Progress<DownloadOperation>(operation =>
{
    if (download.Progress.TotalBytesToReceive > 0)
    {
        var percent = download.Progress.BytesReceived * 100 / download.Progress.TotalBytesToReceive;
        // TODO report percent
    }
});

await download.StartAsync().AsTask(progressCallback);
// at this point the download has been completed

That´s it actually, but to implement this in a cross-platform way (thinking about iOS and Android and Xamarin.Forms) I created an event OnDownloadPercentProgress and wrapped it all with a platform-agnostic interface:

using System;
using System.Threading.Tasks;
using PCLStorage;

namespace Core.Helpers
{
    public interface IFileManager
    {
        /// <summary>
        /// Parameter double is the download percent
        /// Parameter string is the uri string currently being downloaded
        /// </summary>
        event Action<double, string> OnDownloadPercentProgress;


        Task<IFile> Download(Uri uri, IFolder downloadFolder, string desiredFileName = null);
    }
}

IFolder comes from the cross-platform storage library called PCLStorage

And here is the implementation on Windows:

using System;
using System.IO;
using System.IO.Compression;
using System.Threading.Tasks;
using Windows.Networking.BackgroundTransfer;
using Windows.Storage;
using Core.Helpers;
using PCLStorage;

namespace UWP.Helpers
{
    public class FileManager : IFileManager
    {
        public event Action<double, string> OnDownloadPercentProgress;

        public async Task<IFile> Download(Uri uri, IFolder downloadFolder, string desiredFileName = null)
        {
            desiredFileName = desiredFileName ?? Path.GetFileName(uri.LocalPath);

            var localFolder = await StorageFolder.GetFolderFromPathAsync(downloadFolder.Path);
            var destinationFile = await localFolder.CreateFileAsync(desiredFileName, Windows.Storage.CreationCollisionOption.GenerateUniqueName);

            var downloader = new BackgroundDownloader();
            var download = downloader.CreateDownload(uri, destinationFile);
            download.Priority = BackgroundTransferPriority.High;

            var progressCallback = new Progress<DownloadOperation>(operation =>
            {
                if (download.Progress.TotalBytesToReceive > 0)
                {
                    var percent = download.Progress.BytesReceived * 100 / download.Progress.TotalBytesToReceive;
                    OnDownloadPercentProgress?.Invoke(percent, uri.ToString());
                }
            });

            await download.StartAsync().AsTask(progressCallback);

            return await downloadFolder.GetFileAsync(desiredFileName);
        }
    }
}

We are done.

UWP