android - Xamarin.Forms - 如何为 android 定义范围存储以及如何实现它以在 Downloads 目录中下载文件？

Question

在我的应用程序中，我提供了以 PDF 格式下载报告的功能。一切都很好，直到我在 Android 10 及更低版本上测试我的应用程序。最近我将设备更新到 Android 11，现在我无法在共享存储中下载任何文件。我收到 Access Denied下载文件时出错。
我已授予读取和写入外部存储的权限

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

我的下载路径是

Android.OS.Environment.GetExternalStoragePublicDirectory(Android.OS.Environment.DirectoryDownloads).AbsolutePath;

我也设置了 android:requestLegacyExternalStorage="true"在 AndroidManifest.xml我下载文件的代码如下所示

public async void DownloadSupplierSample(object sender, EventArgs e)
    {
        try
        {
            string basepath;
            if (Device.RuntimePlatform == Device.Android)
                basepath = DependencyService.Get<IDownloadPath>().GetDownloadsPath();
            else
                basepath = System.IO.Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Personal), "..", "Library");

            PermissionStatus readstatus = await Permissions.CheckStatusAsync<Permissions.StorageRead>();
            PermissionStatus writestatus = await Permissions.CheckStatusAsync<Permissions.StorageWrite>();

            if (readstatus == PermissionStatus.Granted && writestatus == PermissionStatus.Granted)
            {
                DownloadSupplier(basepath);
            }
            else
            {
                var read = await Permissions.RequestAsync<Permissions.StorageRead>();
                var write = await Permissions.RequestAsync<Permissions.StorageWrite>();

                if (read == PermissionStatus.Granted && write == PermissionStatus.Granted)
                {
                    DownloadSupplier(basepath);
                }
            }
            await DisplayAlert("", "Sample is downaloaded at Downloads directory","Ok");
        }
        catch (Exception ex)
        {
        }
    }

    async void DownloadSupplier(string basepath)
    {
        using (var stream = await FileSystem.OpenAppPackageFileAsync("SupplierMaster.xlsx"))
        {
            string filename = $"SupplierMaster_{DateTime.Now.ToString("yyyyMMddHHmmss")}.xlsx";
            string filepath = Path.Combine(basepath, filename);

            byte[] bytes = Utils.Common.StreamToBytes(stream);
            File.WriteAllBytes(filepath, bytes);
            //using (Stream filestream = File.Create(filepath))
            //{
            //    //stream.Seek(0, SeekOrigin.Begin);
            //    stream.CopyTo(filestream);
            //}
        }
    }

我也试过从设备设置标志 link
如何为我的应用程序定义范围存储？
任何更新如何在我的下载目录中下载 PDF 文件？
谢谢你。

Answer 1

几个月前我遇到了同样的问题。
我在 developer.android 页面上找到了这个:

(https://developer.android.com/reference/android/os/Environment#getExternalStorageDirectory())
第一个替代方法 (getExternalFilesDir) 不会按您的意愿工作，文件不会出现在下载目录中，此位置在应用程序内部。
第二种选择是媒体文件。
我最终使用了第三种选择。我在 Android 项目中创建了一个服务类，如下所示:

public class FilesManager : IFilesManager
{
    private const int CREATE_FILE_REQUEST_CODE = 123;
    private static Context _context;
    
    public static void Init(Context context)
    {
        _context = context;
    }

    public async Task SaveFile(string fileName, byte[] content)
    { 
        Intent intent = new Intent(Intent.ActionCreateDocument);
        intent.AddCategory(Intent.CategoryOpenable);
        intent.SetType(your_file_mimeType);
        intent.PutExtra(Intent.ExtraTitle, fileName);

        var activity = (MainActivity)_context;
        var listener = new ActivityResultListener(activity);
        activity.StartActivityForResult(intent, CREATE_FILE_REQUEST_CODE);

        var result = await listener.Task;

        if (result == null)
        {
            // Cancelled by user
            return;
        }

        using (Stream os = _context.ContentResolver.OpenOutputStream(result.Data))
        {
            os?.Write(content);
            os?.Close();
        }
    }

    private class ActivityResultListener
    {
        private readonly TaskCompletionSource<Intent> Complete = new TaskCompletionSource<Intent>();
        public Task<Intent> Task { get { return this.Complete.Task; } }

        public ActivityResultListener(MainActivity activity)
        {
            // subscribe to activity results
            activity.ActivityResult += OnActivityResult;
        }

        private void OnActivityResult(int requestCode, Result resultCode, Intent data)
        {
            if(requestCode != CREATE_FILE_REQUEST_CODE)
            {
                return;
            }

            // unsubscribe from activity results
            var activity = (MainActivity)_context;
            activity.ActivityResult -= OnActivityResult;

            // process result
            if (resultCode == Result.Ok)
            {
                Complete.TrySetResult(data);
            }
            else
            {
                Complete.TrySetResult(null);
            }
        }
    }
}

在 MainActivity.cs 添加:

public event Action<int, Result, Intent> ActivityResult;

protected override void OnActivityResult(int requestCode, Result resultCode, Intent data) 
{
    ActivityResult?.Invoke(requestCode, resultCode, data); 
}

您可以在 Forms.Init 之后调用 Init 方法:

FilesManager.Init(this);

注:此方法需要用户交互。系统将提示用户选择保存文件的位置。