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