无法从 URL Xamarin Form 下载图像

Question

I am developing a Xamarin app which retrives info from DB, take/choose photo and upload them to remote server, display this images from the remote server and the user can delete them by tap on and press a button. The final step is to download the images stored in the server to the local device gallery.

This is my current button click event:

private void button_download_image_Clicked(object sender, EventArgs e)
{
        Uri image_url_format = new Uri(image_url);
        WebClient webClient = new WebClient();
        try
        {              
            webClient.DownloadDataAsync(image_url_format);
            webClient.DownloadDataCompleted += webClient_DownloadDataCompleted;
        }
        catch (Exception ex)
        {
            DisplayAlert("Error", ex.ToString(), "OK");
        }
}

Below the webClient_DownloadDataCompleted method:

private void webClient_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
{
    try
    {
        Uri image_url_format = new Uri(image_url);
        byte[] bytes_image = e.Result;
        Stream image_stream = new MemoryStream(bytes_image);
        string dest_folder= Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads).ToString();
        string file_name= Path.GetFileName(image_url_format.LocalPath);
        string dest_path= Path.Combine(dest_folder, file_name);
        using (var fileStream = new FileStream(dest_path, FileMode.Create, FileAccess.Write))
        {
              image_stream.CopyTo(fileStream);
        }
              DisplayAlert("Alert", "Download completed!", "OK");
    }
    catch (Exception ex)
    {
        DisplayAlert("Error", ex.ToString(), "OK");
    }
}

But it does not work, no error caught, I get the alert which warn me that the download is completed. Also I gave permission for internet , write_external_storage and read_external_storage .

Another thing is that the images after some time, appears in the gallery under Download album which is correct.

Any idea about this behavior?

EDIT

Below my new button download event:

private void button_download_image_Clicked(object sender, EventArgs e)
{

    Uri image_url_format = new Uri(image_url);
    WebClient webClient = new WebClient();
    try
    {
        byte[] bytes_image = webClient.DownloadData(image_url_format);
        Stream image_stream = new MemoryStream(bytes_image);
        string dest_folder = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads).ToString();
        string file_name = Path.GetFileName(image_url_format.LocalPath);
        string dest_path = Path.Combine(dest_folder, file_name);
        using (var fileStream = new FileStream(dest_path, FileMode.Create, FileAccess.Write))
        {
            image_stream.CopyTo(fileStream);
        }
    }
    catch (Exception ex)
    {
        DisplayAlert("Error", ex.ToString(), "OK");
    }
    DisplayAlert("Alert", "File scaricato con successo", "OK");
}

Answer 1

Causes

Your function "fires and forget" about the download and then directly shows you the "Download complete" popup. The reaons are that you are calling asynchronous functions in a synchronous way ( DownloadDataAsync )... This is why it would still appear after a while in the gallery while you still get the pop up.

Solutions

You should first read that: https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/

Then as a starting point, try with declaring your event handler async and use the await keyword at appropriate places:

private async void button_download_image_Clicked(object sender, EventArgs e)
{
    Uri image_url_format = new Uri(image_url);
    WebClient webClient = new WebClient();
    try
    {            
        await webClient.DownloadDataAsync(image_url_format); // This will await the download
        ...
    }
    catch (Exception ex)
    {
        ...
    }
}

Of course it would be better to also refactor the other method using the async/await pattern all the way down, but this gives you a good starting point I guess.

EDIT:

Regarding the new method you edited, try using DownloadDataTaskAsync and CopyToAsync method along with async/await pattern:

 private async void  button_download_image_Clicked(object sender, EventArgs e)
    {

        Uri image_url_format = new Uri("url");
        WebClient webClient = new WebClient();
        try
        {
            byte[] bytes_image = await webClient.DownloadDataTaskAsync(image_url_format);
            Stream image_stream = new MemoryStream(bytes_image);
            string dest_folder = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads).ToString();
            string file_name = Path.GetFileName(image_url_format.LocalPath);
            string dest_path = Path.Combine(dest_folder, file_name);
            using (var fileStream = new FileStream(dest_path, FileMode.Create, FileAccess.Write))
            {
                await image_stream.CopyToAsync(fileStream);
            }
        }
        catch (Exception ex)
        {
           await DisplayAlertAsync("Error", ex.ToString(), "OK");
        }
        await DisplayAlertAsync("Alert", "File scaricato con successo", "OK");
    }

Also, you should create a DisplayAlertAsync method and call it the same way using await.

Happy coding!

Answer 2

The image was downloaded correctly and I can see it in my file explorer.

I resolved my problem with a resfresh of the gallery, with this lines after saving my image, so the method will be:

private void button_download_image_Clicked(object sender, EventArgs e)
{
    Uri image_url_format = new Uri(image_url);
    WebClient webClient = new WebClient();
    try
    {
        byte[] bytes_image = webClient.DownloadData(image_url_format);
        Stream image_stream = new MemoryStream(bytes_image);
        string dest_folder = Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads).ToString();
        string file_name = Path.GetFileName(image_url_format.LocalPath);
        string dest_path = Path.Combine(dest_folder, file_name);
        using (var fileStream = new FileStream(dest_path, FileMode.Create, FileAccess.Write))
        {
            image_stream.CopyTo(fileStream);
        }
        // this 3 lines fix my problem
        var mediaScanIntent = new Intent(Intent.ActionMediaScannerScanFile);
        mediaScanIntent.SetData(Android.Net.Uri.FromFile(new Java.IO.File(dest_path)));
        Android.App.Application.Context.SendBroadcast(mediaScanIntent);
    }
    catch (Exception ex)
    {
        DisplayAlert("Error", ex.ToString(), "OK");
    }
    DisplayAlert("Alert", "File scaricato con successo", "OK");
}