Add Spotify track support (#970)

* Initial Spotify support

* Remove GameAsset#ToEntity

- appId doesn't seem to be necessary, and Spotify Game doesn't return appId either.

* Implement SpotifyGame details

* Implement song Duration prop

* Add album art CDN

* Fix ActivityType

* Remove payload debug

* Add changes according to review

+ Make `ApplicationId` nullable
+ Move ctor after props

src/Discord.Net.Core/Entities/Activities/Game.cs

@@ -1,4 +1,4 @@
-using System.Diagnostics;
+using System.Diagnostics;
 
 namespace Discord
 {

src/Discord.Net.Core/Entities/Activities/GameAsset.cs

@@ -1,15 +1,15 @@
-namespace Discord
+namespace Discord
 {
     public class GameAsset
     {
         internal GameAsset() { }
 
-        internal ulong ApplicationId { get; set; }
+        internal ulong? ApplicationId { get; set; }
         
         public string Text { get; internal set; }
         public string ImageId { get; internal set; }
         
         public string GetImageUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
-            => CDN.GetRichAssetUrl(ApplicationId, ImageId, size, format);
+            => ApplicationId.HasValue ? CDN.GetRichAssetUrl(ApplicationId.Value, ImageId, size, format) : null;
     }
 }

src/Discord.Net.Core/Entities/Activities/RichGame.cs

@@ -1,4 +1,4 @@
-using System.Diagnostics;
+using System.Diagnostics;
 
 namespace Discord
 {

src/Discord.Net.Core/Entities/Activities/SpotifyGame.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace Discord
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public class SpotifyGame : Game
+    {
+        public string[] Artists { get; internal set; }
+        public string AlbumArt { get; internal set; }
+        public string AlbumTitle { get; internal set; }
+        public string TrackTitle { get; internal set; }
+        public string SyncId { get; internal set; }
+        public string SessionId { get; internal set; }
+        public TimeSpan? Duration { get; internal set; }
+
+        internal SpotifyGame() { }
+
+        public override string ToString() => Name;
+        private string DebuggerDisplay => $"{Name} (Spotify)";
+    }
+}

src/Discord.Net.Rest/API/Common/Game.cs

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 using Newtonsoft.Json.Serialization;
 using System.Runtime.Serialization;
@@ -29,6 +29,10 @@ namespace Discord.API
         public Optional<API.GameTimestamps> Timestamps { get; set; }
         [JsonProperty("instance")]
         public Optional<bool> Instance { get; set; }
+        [JsonProperty("sync_id")]
+        public Optional<string> SyncId { get; set; }
+        [JsonProperty("session_id")]
+        public Optional<string> SessionId { get; set; }
 
         [OnError]
         internal void OnError(StreamingContext context, ErrorContext errorContext)

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -4,6 +4,27 @@ namespace Discord.WebSocket
     {
         public static IActivity ToEntity(this API.Game model)
         {
+            // Spotify Game
+            if (model.SyncId.IsSpecified)
+            {
+                var assets = model.Assets.GetValueOrDefault()?.ToEntity();
+                string albumText = assets?[1]?.Text;
+                string albumArtId = assets?[1]?.ImageId?.Replace("spotify:","");
+                var timestamps = model.Timestamps.IsSpecified ? model.Timestamps.Value.ToEntity() : null;
+                return new SpotifyGame
+                {
+                    Name = model.Name,
+                    SessionId = model.SessionId.GetValueOrDefault(),
+                    SyncId = model.SyncId.Value,
+                    AlbumTitle = albumText,
+                    TrackTitle = model.Details.GetValueOrDefault(),
+                    Artists = model.State.GetValueOrDefault()?.Split(';'),
+                    Duration = timestamps?.End - timestamps?.Start,
+                    AlbumArt = albumArtId != null ? $"https://i.scdn.co/image/{albumArtId}" : null,
+                    Type = ActivityType.Listening
+                };
+            }
+
             // Rich Game
             if (model.ApplicationId.IsSpecified)
             {
@@ -34,7 +55,7 @@ namespace Discord.WebSocket
         }
 
         // (Small, Large)
-        public static GameAsset[] ToEntity(this API.GameAssets model, ulong appId)
+        public static GameAsset[] ToEntity(this API.GameAssets model, ulong? appId = null)
         {
             return new GameAsset[]
             {