Implemented resume_gateway_url.

src/Discord.Net.WebSocket/API/Gateway/ReadyEvent.cs

@@ -20,6 +20,8 @@ namespace Discord.API.Gateway
         public User User { get; set; }
         [JsonProperty("session_id")]
         public string SessionId { get; set; }
+        [JsonProperty("resume_gateway_url")]
+        public string ResumeGatewayUrl { get; set; }
         [JsonProperty("read_state")]
         public ReadState[] ReadStates { get; set; }
         [JsonProperty("guilds")]

src/Discord.Net.WebSocket/DiscordSocketApiClient.cs

@@ -28,6 +28,7 @@ namespace Discord.API
         private readonly bool _isExplicitUrl;
         private CancellationTokenSource _connectCancelToken;
         private string _gatewayUrl;
+        private string _resumeGatewayUrl;
 
         //Store our decompression streams for zlib shared state
         private MemoryStream _compressed;
@@ -37,6 +38,14 @@ namespace Discord.API
 
         public ConnectionState ConnectionState { get; private set; }
 
+        /// <summary>
+        ///     Sets the gateway URL used for resumes.
+        /// </summary>
+        public string ResumeGatewayUrl
+        {
+            set => _resumeGatewayUrl = FormatGatewayUrl(value);
+        }
+
         public DiscordSocketApiClient(RestClientProvider restClientProvider, WebSocketProvider webSocketProvider, string userAgent,
             string url = null, RetryMode defaultRetryMode = RetryMode.AlwaysRetry, JsonSerializer serializer = null,
 			bool useSystemClock = true, Func<IRateLimitInfo, Task> defaultRatelimitCallback = null)
@@ -157,6 +166,17 @@ namespace Discord.API
 #endif
         }
 
+        /// <summary>
+        ///     Appends necessary query parameters to the specified gateway URL.
+        /// </summary>
+        private static string FormatGatewayUrl(string gatewayUrl)
+        {
+            if (gatewayUrl == null)
+                return null;
+
+            return $"{gatewayUrl}?v={DiscordConfig.APIVersion}&encoding={DiscordSocketConfig.GatewayEncoding}&compress=zlib-stream";
+        }
+
         public async Task ConnectAsync()
         {
             await _stateLock.WaitAsync().ConfigureAwait(false);
@@ -191,24 +211,41 @@ namespace Discord.API
                 if (WebSocketClient != null)
                     WebSocketClient.SetCancelToken(_connectCancelToken.Token);
 
-                if (!_isExplicitUrl)
+                string gatewayUrl;
+                if (_resumeGatewayUrl == null)
+                {
+                    if (!_isExplicitUrl)
+                    {
+                        // TODO: 'GetGatewayAsync' -> 'GetBotGatewayAsync', but it could just be hardcoded to 'wss://gateway.discord.gg/'
+                        var gatewayResponse = await GetGatewayAsync().ConfigureAwait(false);
+                        gatewayUrl = _gatewayUrl = FormatGatewayUrl(gatewayResponse.Url);
+                    }
+                    else
+                    {
+                        gatewayUrl = _gatewayUrl;
+                    }
+                }
+                else
                 {
-                    var gatewayResponse = await GetGatewayAsync().ConfigureAwait(false);
-                    _gatewayUrl = $"{gatewayResponse.Url}?v={DiscordConfig.APIVersion}&encoding={DiscordSocketConfig.GatewayEncoding}&compress=zlib-stream";
+                    gatewayUrl = _resumeGatewayUrl;
                 }
 
 #if DEBUG_PACKETS
-                Console.WriteLine("Connecting to gateway: " + _gatewayUrl);
+                Console.WriteLine("Connecting to gateway: " + gatewayUrl);
 #endif
 
-                await WebSocketClient.ConnectAsync(_gatewayUrl).ConfigureAwait(false);
+                await WebSocketClient.ConnectAsync(gatewayUrl).ConfigureAwait(false);
 
                 ConnectionState = ConnectionState.Connected;
             }
             catch
             {
                 if (!_isExplicitUrl)
+                {
+                    // TODO: '_gatewayUrl = null' doesn't do anything, it's never null checked
                     _gatewayUrl = null; //Uncache in case the gateway url changed
+                }
+
                 await DisconnectInternalAsync().ConfigureAwait(false);
                 throw;
             }

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -322,7 +322,6 @@ namespace Discord.WebSocket
         }
         private async Task OnDisconnectingAsync(Exception ex)
         {
-
             await _gatewayLogger.DebugAsync("Disconnecting ApiClient").ConfigureAwait(false);
             await ApiClient.DisconnectAsync(ex).ConfigureAwait(false);
 
@@ -353,6 +352,10 @@ namespace Discord.WebSocket
                 if (guild.IsAvailable)
                     await GuildUnavailableAsync(guild).ConfigureAwait(false);
             }
+
+            _sessionId = null;
+            _lastSeq = 0;
+            ApiClient.ResumeGatewayUrl = null;
         }
 
         /// <inheritdoc />
@@ -832,6 +835,7 @@ namespace Discord.WebSocket
 
                             _sessionId = null;
                             _lastSeq = 0;
+                            ApiClient.ResumeGatewayUrl = null;
 
                             if (_shardedClient != null)
                             {
@@ -889,6 +893,7 @@ namespace Discord.WebSocket
                                             AddPrivateChannel(data.PrivateChannels[i], state);
 
                                         _sessionId = data.SessionId;
+                                        ApiClient.ResumeGatewayUrl = data.ResumeGatewayUrl;
                                         _unavailableGuildCount = unavailableGuilds;
                                         CurrentUser = currentUser;
                                         _previousSessionUser = CurrentUser;