使方法异步

Question

我有一个方法可以列出媒体库中的所有歌曲。该方法有 2 个方面，它要么从 MediaLibrary 返回 SongsCollection，要么返回自定义对象列表。

public static Task<object> GetSongList(bool lib = true, bool albumArt = true)
{
     MediaLibrary mediaLib = new MediaLibrary();
     var songs = mediaLib.Songs;

     if (lib)
     {
          return songs;
     }
     else
     {
          var list = new List<MusicTitle>();
          foreach (var song in songs)
          {
               list.Add(new MusicTitle()
               {
                   Artist = song.Artist.Name,
                   Title = song.Name,
                   Duration = (new DateTime(song.Duration.Ticks)).ToString("mm:ss"),
                   Album = song.Album.Name, 
                   Art = albumArt ? GetAlbumArt(song, 100) : null
               });
          }

          return list;
      }
  }

MusicTitle 是具有一些属性的自定义类。由于这返回 2 种类型的结果，我将返回类型设置为对象并适当地转换结果。这让 UI 有点支撑，所以我需要它是 async。因此，正如方法所示，我在方法签名中添加了Task<object>，当我这样做时，return songs; 和return lists; 给出了以下编译错误。

Cannot implicitly convert type 'Microsoft.Xna.Framework.Media.SongCollection' to 'System.Threading.Tasks.Task<object>'

Cannot implicitly convert type 'System.Collections.Generic.List<KVKWindowsPhoneHelper.Core.MediaLibrary.MusicTitle>' to 'System.Threading.Tasks.Task<object>'

我该怎么办？我尝试将返回类型转换为object，但没有奏效。我怎样才能使这个方法async？

编辑

以下代码在 Page 的 OnNavigatedTo() 方法中 这会抛出 UnauthorizedAccessException

protected async override void OnNavigatedTo(NavigationEventArgs e)
{
    base.OnNavigatedTo(e);
    // Set Song List
    List<AlphaKeyGroup<MusicTitle>> songList = await Task.Run(() => AlphaKeyGroup<MusicTitle>.CreateGroups(ListHelper.GetSongList(false, false) as List<MusicTitle>, Thread.CurrentThread.CurrentUICulture, (MusicTitle s) => { return s.Title; }, true));
    listSongs.ItemsSource = songList;
    // Set Artist List
    List<AlphaKeyGroup<MusicArtist>> artistList = await Task.Run(() => AlphaKeyGroup<MusicArtist>.CreateGroups(ListHelper.GetArtistList(false) as List<MusicArtist>, Thread.CurrentThread.CurrentUICulture, (MusicArtist ar) => { return ar.Artist; }, true));
    listArtist.ItemsSource = artistList;
    // Set Album List
    List<AlphaKeyGroup<MusicAlbum>> albumList = await Task.Run(() => AlphaKeyGroup<MusicAlbum>.CreateGroups(ListHelper.GetAlbumList(false, true) as List<MusicAlbum>, Thread.CurrentThread.CurrentUICulture, (MusicAlbum al) => { return al.Album; }, true));
    listAlbums.ItemsSource = albumList;
    // Set Genre List
    List<AlphaKeyGroup<MusicGenre>> genreList = await Task.Run(() => AlphaKeyGroup<MusicGenre>.CreateGroups(ListHelper.GetGenreList(false) as List<MusicGenre>, Thread.CurrentThread.CurrentUICulture, (MusicGenre g) => { return g.Genre; }, true));
    listGenres.ItemsSource = genreList;
    // Set PlayList
    List<AlphaKeyGroup<MusicPlaylist>> playList = await Task.Run(() => AlphaKeyGroup<MusicPlaylist>.CreateGroups(ListHelper.GetPlayList(false) as List<MusicPlaylist>, Thread.CurrentThread.CurrentUICulture, (MusicPlaylist pl) => { return pl.Playlist; }, true));
    listPlaylist.ItemsSource = playList;

}

编辑 2

在我的手机中有 233 首歌曲，异常是在我编写的 ListHelper 类中的一个方法中引发的，其中包括获取歌曲、专辑、播放列表、流派和艺术家的所有方法。引发异常的方法是我获取专辑的专辑封面的方法。这就是方法。

public static BitmapImage GetAlbumArt(Song song, int size = 100 )
{
    BitmapImage img = new BitmapImage(); // EXCEPTION IS THROWN HERE
    img.DecodePixelHeight = size;
    img.DecodePixelWidth = size;

    if (song.Album.HasArt)
    {               
         img.SetSource(song.Album.GetAlbumArt());
    }
    else
    {
         img.UriSource = new Uri("/Images/cover.png", UriKind.Relative);
    }
    return img;
  }

Answer 1

您的方法没有做任何异步工作，因此没有理由让它返回Task。相反，如果你想让你的 UI 保持响应，请将其包装在 Task.Run 中，这将在 ThreadPool 线程上排队：

public static object GetSongList(bool lib = true, bool albumArt = true)
{
     MediaLibrary mediaLib = new MediaLibrary();
     var songs = mediaLib.Songs;

     if (lib)
     {
          return songs;
     }
     else
     {
          var list = new List<MusicTitle>();
          foreach (var song in songs)
          {
               list.Add(new MusicTitle()
               {
                   Artist = song.Artist.Name,
                   Title = song.Name,
                   Duration = (new DateTime(song.Duration.Ticks)).ToString("mm:ss"),
                   Album = song.Album.Name, 
                   Art = albumArt ? GetAlbumArt(song, 100) : null
               });
          }

          return list;
      }
}

并在 Task.Run 中使用它：

var obj = await Task.Run(() => GetSongList());

作为一个不相关的附注，我建议你看看你是否可以创建一个公共基类/接口作为返回类型而不是 object，以确保类型安全。