Xamarin for android：无法从外部文件夹播放 mp3

Question

我无法播放放在应用程序外部存储文件夹中的 mp3 文件。

我在 OnCreate() 中请求了权限

    private const int EXTERNAL_READ_WRITE_REQUEST = 1337;

    protected override async void OnCreate(Bundle savedInstanceState)
    {
        ActivityCompat.RequestPermissions(this, new string[]
        { 
            Manifest.Permission.ReadExternalStorage,
            Manifest.Permission.WriteExternalStorage
        },
        EXTERNAL_READ_WRITE_REQUEST);
    }

然后我验证了权限被授予：

    public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
    {
        if (requestCode == EXTERNAL_READ_WRITE_REQUEST)
        {
            if (grantResults.ToArray().ToList().IndexOf(Permission.Denied) == -1)
            {
                ShowMessage("Permissions granted");
            }
            else
                ShowMessage("Permission request denied, unable to continue.");
        }

        Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);

        base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
    }

然后在服务（不是活动）中检查权限，获取文件夹，获取文件，并尝试使用 MediaPlayer class 播放 mp3：

    MediaPlayer player;


    public string GetSongDirectory()
    {
        string result = "";


        List<Java.IO.File> files = GetExternalFilesDirs(null).ToList();

        files = files.OrderBy(x => x.AbsolutePath.IndexOf("emulated")).ToList();

        var f = files.DefaultIfEmpty(null).FirstOrDefault();

        if (f != null)
        {
            result = f.AbsolutePath + "/songs";

            if (Directory.Exists(result) == false)
                Directory.CreateDirectory(result);
        }

        return result;
    }


    [return: GeneratedEnum]
    public override StartCommandResult OnStartCommand(Intent intent, [GeneratedEnum] StartCommandFlags flags, int startId)
    {
        string songFolder = GetSongDirectory();

        var p = ContextCompat.CheckSelfPermission(this.ApplicationContext, Manifest.Permission.ReadExternalStorage);

        if (p == Android.Content.PM.Permission.Granted)
        {
            try
            {
                foreach (string fileName in System.IO.Directory.GetFiles(songFolder, "*.*", SearchOption.AllDirectories))
                {
                    player = new MediaPlayer();

                    JFile f = new JFile(fileName);

                    Java.IO.FileInputStream fs = new FileInputStream(f);

                    player.Reset();
                    //player.SetDataSource(fileName); this didn't work either
                    player.SetDataSource(fs.FD, 0L, f.Length());
                    player.Prepare();
                    player.Start();

                    break;
                }
            }
            catch (Exception x)
            {
                x.ToString();
            }
        }


        Toast.MakeText(this, "Greetings from serivce", ToastLength.Long).Show();

        return StartCommandResult.Sticky;
    }

但是，在调用 Prepare 时，它会引发异常：

{Java.IO.IOException: Prepare failed.: status=0x1
  at Java.Interop.JniEnvironment+InstanceMethods.CallVoidMethod

文件名包含“/storage/1606-3C08/Android/data/com.companyname.mypod/files/songs/IshDARR - Alpha.mp3”

当它属于外部 SD 卡时，如何从我的 xamarin 服务中播放此 mp3 文件？

编辑：我可以使用 BinaryReader 读取文件的内容，因此它不是文件访问权限问题。

Answer 1

我在网上找到了答案，但失去了原作者，所以如何在 xamarain 上从外部 sd 卡播放 mp3：

    public class StreamMediaDataSource : MediaDataSource
    {
        System.IO.Stream data;

        public StreamMediaDataSource(System.IO.Stream Data)
        {
            data = Data;
        }

        public override long Size
        {
            get
            {
                return data.Length;
            }
        }

        public override int ReadAt(long position, byte[] buffer, int offset, int size)
        {
            data.Seek(position, System.IO.SeekOrigin.Begin);
            return data.Read(buffer, offset, size);
        }

        public override void Close()
        {
            if (data != null)
            {
                data.Dispose();
                data = null;
            }
        }

        protected override void Dispose(bool disposing)
        {
            base.Dispose(disposing);

            if (data != null)
            {
                data.Dispose();
                data = null;
            }
        }
    }

这是一个片段：

StreamReader sr = new StreamReader(fileName);
player.SetDataSource(new StreamMediaDataSource(sr.BaseStream));
player.Prepare();
player.Start();

我相信它失败的原因是它用来读取文件的方法。

如果我尝试使用 FileStream 读取文件。 即使它已通过用户提示通过它授予访问权限，它也会引发访问被拒绝异常。

这是 java 日志，以防有人需要通过谷歌搜索：

06-10 11:37:26.048 E/MediaPlayerNative(31944): error (1, -2147483648)
06-10 11:37:26.175 D/Mono    (31944): DllImport searching in: '__Internal' ('(null)').
06-10 11:37:26.175 D/Mono    (31944): Searching for 'java_interop_jnienv_call_object_method'.
06-10 11:37:26.175 D/Mono    (31944): Probing 'java_interop_jnienv_call_object_method'.
06-10 11:37:26.176 D/Mono    (31944): Found as 'java_interop_jnienv_call_object_method'.