[Feature] Follow news channels (#2590)

* initial commit

* Apply suggestions from code review

---------

Co-authored-by: Casmir <68127614+csmir@users.noreply.github.com>

src/Discord.Net.Core/Entities/Channels/INewsChannel.cs

@@ -1,3 +1,5 @@
+using System.Threading.Tasks;
+
 namespace Discord
 {
     /// <summary>
@@ -5,5 +7,12 @@ namespace Discord
     /// </summary>
     public interface INewsChannel : ITextChannel
     {
+        /// <summary>
+        ///     Follow this channel to send messages to a target channel.
+        /// </summary>
+        /// <returns>
+        ///     The Id of the created webhook.
+        /// </returns>
+        Task<ulong> FollowAnnouncementChannelAsync(ulong channelId, RequestOptions options);
     }
 }

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

@@ -0,0 +1,12 @@
+using Newtonsoft.Json;
+
+namespace Discord.API;
+
+internal class FollowedChannel
+{
+    [JsonProperty("channel_id")]
+    public ulong ChannelId { get; set; }
+
+    [JsonProperty("webhook_id")]
+    public ulong WebhookId { get; set; }
+}

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -1146,6 +1146,16 @@ namespace Discord.API
             var ids = new BucketIds(channelId: channelId);
             await SendAsync("POST", () => $"channels/{channelId}/messages/{messageId}/crosspost", ids, options: options).ConfigureAwait(false);
         }
+
+        public async Task<FollowedChannel> FollowChannelAsync(ulong newsChannelId, ulong followingChannelId, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(newsChannelId, 0, nameof(newsChannelId));
+            Preconditions.NotEqual(followingChannelId, 0, nameof(followingChannelId));
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(channelId: newsChannelId);
+            return await SendJsonAsync<FollowedChannel>("POST", () => $"channels/{newsChannelId}/followers", new { webhook_channel_id = followingChannelId}, ids, options: options).ConfigureAwait(false);
+        }
         #endregion
 
         #region Channel Permissions

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -590,6 +590,12 @@ namespace Discord.Rest
             return models.Select(x => RestWebhook.Create(client, channel, x))
                 .ToImmutableArray();
         }
+
+        public static async Task<ulong> FollowAnnouncementChannelAsync(INewsChannel newsChannel, ulong channelId, BaseDiscordClient client, RequestOptions options)
+        {
+            var model = await client.ApiClient.FollowChannelAsync(newsChannel.Id, channelId, options);
+            return model.WebhookId;
+        }
         #endregion
 
         #region Categories

src/Discord.Net.Rest/Entities/Channels/RestNewsChannel.cs

@@ -4,6 +4,7 @@ using System.Diagnostics;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+
 using Model = Discord.API.Channel;
 
 namespace Discord.Rest
@@ -25,5 +26,9 @@ namespace Discord.Rest
             return entity;
         }
         public override int SlowModeInterval => throw new NotSupportedException("News channels do not support Slow Mode.");
+
+        /// <inheritdoc />
+        public Task<ulong> FollowAnnouncementChannelAsync(ulong channelId, RequestOptions options = null)
+            =>  ChannelHelper.FollowAnnouncementChannelAsync(this, channelId, Discord, options);
     }
 }

src/Discord.Net.WebSocket/Entities/Channels/SocketNewsChannel.cs

@@ -1,3 +1,4 @@
+using Discord.Rest;
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -36,5 +37,13 @@ namespace Discord.WebSocket
         public override int SlowModeInterval
             => throw new NotSupportedException("News channels do not support Slow Mode.");
 
+        /// <inheritdoc cref="INewsChannel.FollowAnnouncementChannelAsync"/>
+        public Task<ulong> FollowAnnouncementChannelAsync(ITextChannel channel, RequestOptions options = null)
+            => FollowAnnouncementChannelAsync(channel.Id, options);
+
+        /// <inheritdoc />
+        public Task<ulong> FollowAnnouncementChannelAsync(ulong channelId, RequestOptions options = null)
+            => ChannelHelper.FollowAnnouncementChannelAsync(this, channelId, Discord, options);
+
     }
 }