Added docs for IPlayableItem, fixes #454

SpotifyAPI.Docs/docs/iplayableitem.md

@@ -0,0 +1,39 @@
+---
+id: iplayableitem
+title: IPlayableItem
+---
+
+When working with playlists or the current playing context, you will encounter a type `IPlayableItem`, which only contains a `Type` property. Spotify recently introduced shows/episodes to the API, and thus had to adapt API endpoints which previously just returned track objects. Now, playlists and the current playing context can include two types, tracks and episodes. To reflect this in our models, we introduced `IPlayableItem`.
+
+```csharp
+var spotify = new SpotifyClient("YourAccessToken");
+
+var playlist = await spotify.Playlists.Get("37i9dQZEVXbMDoHDwVN2tF");
+foreach (PlaylistTrack<IPlayableItem> item in playlist.Tracks.Items)
+{
+  // When was it added
+  Console.WriteLine(item.AddedAt);
+  // The only propety on item is item.Type, it's a IPlayableItem
+  Console.WriteLine(item.Track.Type);
+}
+```
+
+Now, this type per se is probably useless to you. You're interested in the name, uri or artist of the episode/track. To get that info, you have to type cast the `IPlayableItem` to the respective type:
+
+```csharp
+foreach (PlaylistTrack<IPlayableItem> item in playlist.Tracks.Items)
+{
+  if (item.Track is FullTrack track)
+  {
+    // All FullTrack properties are available
+    Console.WriteLine(track.Name);
+  }
+  if (item.Track is FullEpisode episode)
+  {
+    // All FullTrack properties are available
+    Console.WriteLine(episode.Name);
+  }
+}
+```
+
+To this day, `IPlayableItem` can only be `FullTrack` or `FullEpisode`.

SpotifyAPI.Docs/sidebars.js

@@ -13,6 +13,7 @@ module.exports = {
           'proxy',
           'pagination',
           'retry_handling',
+          'iplayableitem',
           'unit_testing'
         ]
       },

SpotifyAPI.Web/Models/Response/CurrentlyPlaying.cs

@@ -9,6 +9,10 @@ namespace SpotifyAPI.Web
     public string CurrentlyPlayingType { get; set; } = default!;
     public bool IsPlaying { get; set; }
 
+    /// <summary>
+    /// Can be a FullTrack or FullEpisode
+    /// </summary>
+    /// <value></value>
     [JsonConverter(typeof(PlayableItemConverter))]
     public IPlayableItem Item { get; set; } = default!;
     public int? ProgressMs { get; set; }

SpotifyAPI.Web/Models/Response/CurrentlyPlayingContext.cs

@@ -12,6 +12,10 @@ namespace SpotifyAPI.Web
     public int ProgressMs { get; set; }
     public bool IsPlaying { get; set; }
 
+    /// <summary>
+    /// Can be a FullTrack or FullEpisode
+    /// </summary>
+    /// <value></value>
     [JsonConverter(typeof(PlayableItemConverter))]
     public IPlayableItem Item { get; set; } = default!;
 

SpotifyAPI.Web/Models/Response/FullPlaylist.cs

@@ -12,6 +12,11 @@ namespace SpotifyAPI.Web
     public PublicUser Owner { get; set; } = default!;
     public bool Public { get; set; }
     public string SnapshotId { get; set; } = default!;
+
+    /// <summary>
+    /// A list of PlaylistTracks, which items can be a FullTrack or FullEpisode
+    /// </summary>
+    /// <value></value>
     public Paging<PlaylistTrack<IPlayableItem>> Tracks { get; set; } = default!;
     public string Type { get; set; } = default!;
     public string Uri { get; set; } = default!;

SpotifyAPI.Web/Models/Response/SimplePlaylist.cs

@@ -16,6 +16,11 @@ namespace SpotifyAPI.Web
     public PublicUser Owner { get; set; } = default!;
     public bool? Public { get; set; }
     public string SnapshotId { get; set; } = default!;
+
+    /// <summary>
+    /// A list of PlaylistTracks, which items can be a FullTrack or FullEpisode
+    /// </summary>
+    /// <value></value>
     public Paging<PlaylistTrack<IPlayableItem>> Tracks { get; set; } = default!;
     public string Type { get; set; } = default!;
     public string Uri { get; set; } = default!;