Add IChannel.Nsfw, RequireNsfw precondition (#633)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

src/Discord.Net.Commands/Attributes/Preconditions/RequireNsfwAttribute.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    /// <summary>
+    /// Require that the command is invoked in a channel marked NSFW
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+    public class RequireNsfwAttribute : PreconditionAttribute
+    {
+        public override Task<PreconditionResult> CheckPermissions(ICommandContext context, CommandInfo command, IDependencyMap map)
+        {
+            if (context.Channel.IsNsfw)
+                return Task.FromResult(PreconditionResult.FromSuccess());
+            else
+                return Task.FromResult(PreconditionResult.FromError("This command may only be invoked in an NSFW channel."));
+        }
+    }
+}

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

@@ -8,6 +8,9 @@ namespace Discord
         /// <summary> Gets the name of this channel. </summary>
         string Name { get; }
 
+        /// <summary> Checks if the channel is NSFW. </summary>
+        bool IsNsfw { get; }
+
         /// <summary> Gets a collection of all users in this channel. </summary>
         IAsyncEnumerable<IReadOnlyCollection<IUser>> GetUsersAsync(CacheMode mode = CacheMode.AllowDownload, RequestOptions options = null);
         

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

@@ -289,5 +289,10 @@ namespace Discord.Rest
                 author = RestUser.Create(client, guild, model, webhookId);
             return author;
         }
+
+        public static bool IsNsfw(IChannel channel) =>
+            IsNsfw(channel.Name);
+        public static bool IsNsfw(string channelName) =>
+            channelName == "nsfw" || channelName.StartsWith("nsfw-");
     }
 }

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

@@ -46,6 +46,7 @@ namespace Discord.Rest
 
         //IChannel
         string IChannel.Name => null;
+        bool IChannel.IsNsfw => ChannelHelper.IsNsfw(this);
 
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IUser>(null); //Overriden

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

@@ -97,6 +97,7 @@ namespace Discord.Rest
 
         //IChannel
         string IChannel.Name { get { throw new NotSupportedException(); } }
+        bool IChannel.IsNsfw { get { throw new NotSupportedException(); } }
         IAsyncEnumerable<IReadOnlyCollection<IUser>> IChannel.GetUsersAsync(CacheMode mode, RequestOptions options)
         {
             throw new NotSupportedException();

src/Discord.Net.Rpc/Entities/Channels/RpcChannel.cs

@@ -1,4 +1,5 @@
-using System;
+using Discord.Rest;
+using System;
 
 using Model = Discord.API.Rpc.Channel;
 
@@ -7,6 +8,7 @@ namespace Discord.Rpc
     public class RpcChannel : RpcEntity<ulong>
     {
         public string Name { get; private set; }
+        public bool IsNsfw => ChannelHelper.IsNsfw(Name);
 
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
 

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

@@ -1,4 +1,5 @@
-using System;
+using Discord.Rest;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -40,6 +41,7 @@ namespace Discord.WebSocket
 
         //IChannel
         string IChannel.Name => null;
+        bool IChannel.IsNsfw => ChannelHelper.IsNsfw(this);
 
         Task<IUser> IChannel.GetUserAsync(ulong id, CacheMode mode, RequestOptions options)
             => Task.FromResult<IUser>(null); //Overridden