无法从 URL Xamarin 表单下载图像

Question

我正在开发一个 Xamarin 应用程序，它从数据库中检索信息，拍摄/选择照片并将它们上传到远程服务器，从远程服务器显示这些图像，用户可以通过点击并按下按钮来删除它们。最后一步是将存储在服务器中的图像下载到本地设备库。

这是我当前的按钮点击事件：

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");
        }
}

webClient_DownloadDataCompleted 方法下方：

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");
    }
}

但它不起作用，没有发现错误，我收到警告，警告我下载已完成。我还授予了 internet、write_external_storage 和 read_external_storage 的权限。

另一件事是图像在一段时间后出现在下载相册下的图库中，这是正确的。

对这种行为有任何想法吗？

编辑

在我的新按钮下载事件下方：

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

原因

您的功能“触发并忘记”下载，然后直接向您显示“下载完成”弹出窗口。原因是您正在以同步方式调用 异步 函数 (DownloadDataAsync)...这就是为什么它仍然会在一段时间后出现在图库中，而您仍然会弹出窗口。

解决方案

您应该首先阅读：https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/

然后作为起点，尝试将事件处理程序声明为异步并在适当的位置使用 await 关键字：

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)
    {
        ...
    }
}

当然，最好也使用 async/await 模式一直重构其他方法，但我想这会给你一个很好的起点。

编辑：

关于你编辑的新方法，尝试使用DownloadDataTaskAsync和CopyToAsync方法以及async/await模式：

 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");
    }

另外，您应该创建一个 DisplayAlertAsync 方法并使用 await 以相同的方式调用它。

编码愉快！

Answer 2

图片已正确下载，我可以在文件资源管理器中看到它。

我通过重新刷新画廊解决了我的问题，在保存我的图像后使用这条线，所以方法是：

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");
}