Add API model for Reaction, implement REST methods for reactions

src/Discord.Net.Core/API/Common/Reaction.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Newtonsoft.Json;
+
+namespace Discord.API.Common
+{
+    public class Reaction
+    {
+        [JsonProperty("user_id")]
+        public ulong UserId { get; set; }
+        [JsonProperty("message_id")]
+        public ulong MessageId { get; set; }
+        [JsonProperty("emoji")]
+        public Emoji Emoji { get; set; }
+        [JsonProperty("channel_id")]
+        public ulong ChannelId { get; set; }
+    }
+}

src/Discord.Net.Core/API/DiscordRestApiClient.cs

@@ -512,6 +512,48 @@ namespace Discord.API
             var ids = new BucketIds(channelId: channelId);
             return await SendJsonAsync<Message>("PATCH", () => $"channels/{channelId}/messages/{messageId}", args, ids, clientBucketId: ClientBucket.SendEditId, options: options).ConfigureAwait(false);
         }
+        public async Task AddReactionAsync(ulong channelId, ulong messageId, string emoji, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(channelId, 0, nameof(channelId));
+            Preconditions.NotEqual(messageId, 0, nameof(messageId));
+            Preconditions.NotNullOrWhitespace(emoji, nameof(emoji));
+
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(channelId: channelId);
+
+            await SendAsync("PUT", () => $"channels/{channelId}/messages/{messageId}/reactions/{emoji}/@me", ids, options: options).ConfigureAwait(false);
+        }
+        public async Task RemoveReactionAsync(ulong channelId, ulong messageId, ulong userId, string emoji, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(channelId, 0, nameof(channelId));
+            Preconditions.NotEqual(messageId, 0, nameof(messageId));
+            Preconditions.NotNullOrWhitespace(emoji, nameof(emoji));
+
+            options = RequestOptions.CreateOrClone(options);
+
+            var ids = new BucketIds(channelId: channelId);
+
+            await SendAsync("DELETE", () => $"channels/{channelId}/messages/{messageId}/reactions/{emoji}/{userId}", ids, options: options).ConfigureAwait(false);
+        }
+        public async Task<IReadOnlyCollection<User>> GetReactionUsersAsync(ulong channelId, ulong messageId, string emoji, GetReactionUsersParams args, RequestOptions options = null)
+        {
+            Preconditions.NotEqual(channelId, 0, nameof(channelId));
+            Preconditions.NotEqual(messageId, 0, nameof(messageId));
+            Preconditions.NotNullOrWhitespace(emoji, nameof(emoji));
+            Preconditions.NotNull(args, nameof(args));
+            Preconditions.GreaterThan(args.Limit, 0, nameof(args.Limit));
+            Preconditions.AtMost(args.Limit, DiscordConfig.MaxUsersPerBatch, nameof(args.Limit));
+            Preconditions.GreaterThan(args.AfterUserId, 0, nameof(args.AfterUserId));
+            options = RequestOptions.CreateOrClone(options);
+
+            int limit = args.Limit.GetValueOrDefault(int.MaxValue);
+            ulong afterUserId = args.AfterUserId.GetValueOrDefault(0);
+
+            var ids = new BucketIds(channelId: channelId);
+            Expression<Func<string>> endpoint = () => $"channels/{channelId}/messages/{messageId}/reactions/{emoji}";
+            return await SendAsync<IReadOnlyCollection<User>>("GET", endpoint, ids, options: options).ConfigureAwait(false);
+        }
         public async Task AckMessageAsync(ulong channelId, ulong messageId, RequestOptions options = null)
         {
             Preconditions.NotEqual(channelId, 0, nameof(channelId));

src/Discord.Net.Core/API/Rest/GetReactionUsersParams.cs

@@ -0,0 +1,8 @@
+namespace Discord.API.Rest
+{
+    public class GetReactionUsersParams
+    {
+        public Optional<int> Limit { get; set; }
+        public Optional<ulong> AfterUserId { get; set; }
+    }
+}

src/Discord.Net.Core/Entities/Messages/IUserMessage.cs

@@ -1,5 +1,6 @@
 using Discord.API.Rest;
 using System;
+using System.Collections.Generic;
 using System.Threading.Tasks;
 
 namespace Discord
@@ -12,7 +13,17 @@ namespace Discord
         Task PinAsync(RequestOptions options = null);
         /// <summary> Removes this message from its channel's pinned messages. </summary>
         Task UnpinAsync(RequestOptions options = null);
-        
+
+        /// <summary> Adds a reaction to this message. </summary>
+        Task AddReactionAsync(Emoji emoji, RequestOptions options = null);
+        /// <summary> Adds a reaction to this message. </summary>
+        Task AddReactionAsync(string emoji, RequestOptions options = null);
+        /// <summary> Removes a reaction from message. </summary>
+        Task RemoveReactionAsync(Emoji emoji, IUser user, RequestOptions options = null);
+        /// <summary> Removes a reaction from this message. </summary>
+        Task RemoveReactionAsync(string emoji, IUser user, RequestOptions options = null);
+        Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(string emoji, Action<GetReactionUsersParams> func, RequestOptions options = null);
+
         /// <summary> Transforms this message's text into a human readable form by resolving its tags. </summary>
         string Resolve(
             TagHandling userHandling = TagHandling.Name,

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -45,6 +45,7 @@ namespace Discord
                     throw new ArgumentException("Argument cannot be blank.", name);
             }
         }
+        
 
         //Numerics
         public static void NotEqual(sbyte obj, sbyte value, string name) { if (obj == value) throw new ArgumentOutOfRangeException(name); }

src/Discord.Net.Rest/Entities/Messages/MessageHelper.cs

@@ -23,6 +23,29 @@ namespace Discord.Rest
             await client.ApiClient.DeleteMessageAsync(msg.Channel.Id, msg.Id, options).ConfigureAwait(false);
         }
 
+        public static async Task AddReactionAsync(IMessage msg, Emoji emoji, BaseDiscordClient client, RequestOptions options)
+            => await AddReactionAsync(msg, $"{emoji.Name}:{emoji.Id}", client, options).ConfigureAwait(false);
+        public static async Task AddReactionAsync(IMessage msg, string emoji, BaseDiscordClient client, RequestOptions options)
+        {
+            await client.ApiClient.AddReactionAsync(msg.Channel.Id, msg.Id, emoji, options).ConfigureAwait(false);
+        }
+
+        public static async Task RemoveReactionAsync(IMessage msg, IUser user, Emoji emoji, BaseDiscordClient client, RequestOptions options)
+            => await RemoveReactionAsync(msg, user, $"{emoji.Name}:{emoji.Id}", client, options).ConfigureAwait(false);
+        public static async Task RemoveReactionAsync(IMessage msg, IUser user, string emoji, BaseDiscordClient client,
+            RequestOptions options)
+        {
+            await client.ApiClient.RemoveReactionAsync(msg.Channel.Id, msg.Id, user.Id, emoji, options);
+        }
+
+        public static async Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(IMessage msg, string emoji,
+            Action<GetReactionUsersParams> func, BaseDiscordClient client, RequestOptions options)
+        {
+            var args = new GetReactionUsersParams();
+            func(args);
+            return (await client.ApiClient.GetReactionUsersAsync(msg.Channel.Id, msg.Id, emoji, args, options).ConfigureAwait(false)).Select(u => u as IUser).Where(u => u != null).ToImmutableArray();
+        }
+
         public static async Task PinAsync(IMessage msg, BaseDiscordClient client,
             RequestOptions options)
         {

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -117,6 +117,20 @@ namespace Discord.Rest
             Update(model);
         }
 
+        public Task AddReactionAsync(Emoji emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+        public Task AddReactionAsync(string emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+
+        public Task RemoveReactionAsync(Emoji emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        public Task RemoveReactionAsync(string emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        
+        public Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(string emoji, Action<GetReactionUsersParams> func, RequestOptions options)
+            => MessageHelper.GetReactionUsersAsync(this, emoji, func, Discord, options);
+        
+
         public Task PinAsync(RequestOptions options)
             => MessageHelper.PinAsync(this, Discord, options);
         public Task UnpinAsync(RequestOptions options)

src/Discord.Net.Rpc/Entities/Messages/RpcUserMessage.cs

@@ -101,6 +101,19 @@ namespace Discord.Rpc
         public Task ModifyAsync(Action<ModifyMessageParams> func, RequestOptions options)
             => MessageHelper.ModifyAsync(this, Discord, func, options);
 
+        public Task AddReactionAsync(Emoji emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+        public Task AddReactionAsync(string emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+
+        public Task RemoveReactionAsync(Emoji emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        public Task RemoveReactionAsync(string emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        
+        public Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(string emoji, Action<GetReactionUsersParams> func, RequestOptions options)
+            => MessageHelper.GetReactionUsersAsync(this, emoji, func, Discord, options);               
+
         public Task PinAsync(RequestOptions options)
             => MessageHelper.PinAsync(this, Discord, options);
         public Task UnpinAsync(RequestOptions options)

src/Discord.Net.WebSocket/Entities/Messages/SocketUserMessage.cs

@@ -113,6 +113,19 @@ namespace Discord.WebSocket
         public Task ModifyAsync(Action<ModifyMessageParams> func, RequestOptions options = null)
             => MessageHelper.ModifyAsync(this, Discord, func, options);
 
+        public Task AddReactionAsync(Emoji emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+        public Task AddReactionAsync(string emoji, RequestOptions options)
+            => MessageHelper.AddReactionAsync(this, emoji, Discord, options);
+
+        public Task RemoveReactionAsync(Emoji emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        public Task RemoveReactionAsync(string emoji, IUser user, RequestOptions options)
+            => MessageHelper.RemoveReactionAsync(this, user, emoji, Discord, options);
+        
+        public Task<IReadOnlyCollection<IUser>> GetReactionUsersAsync(string emoji, Action<GetReactionUsersParams> func, RequestOptions options)
+            => MessageHelper.GetReactionUsersAsync(this, emoji, func, Discord, options);
+
         public Task PinAsync(RequestOptions options = null)
             => MessageHelper.PinAsync(this, Discord, options);
         public Task UnpinAsync(RequestOptions options = null)