Split up Context class into more extension methods

PluralKit.Bot/CommandSystem/ContextChecksExt.cs

@@ -0,0 +1,52 @@
+using DSharpPlus;
+
+using PluralKit.Core;
+
+namespace PluralKit.Bot
+{
+    public static class ContextChecksExt
+    {
+        public static Context CheckGuildContext(this Context ctx)
+        {
+            if (ctx.Channel.Guild != null) return ctx;
+            throw new PKError("This command can not be run in a DM.");
+        }
+        
+        public static Context CheckSystemPrivacy(this Context ctx, PKSystem target, PrivacyLevel level)
+        {
+            if (level.CanAccess(ctx.LookupContextFor(target))) return ctx;
+            throw new PKError("You do not have permission to access this information.");
+        }
+        
+        public static Context CheckOwnMember(this Context ctx, PKMember member)
+        {
+            if (member.System != ctx.System?.Id)
+                throw Errors.NotOwnMemberError;
+            return ctx;
+        }
+        
+        public static Context CheckSystem(this Context ctx)
+        {
+            if (ctx.System == null)
+                throw Errors.NoSystemError;
+            return ctx;
+        }
+
+        public static Context CheckNoSystem(this Context ctx)
+        {
+            if (ctx.System != null)
+                throw Errors.ExistingSystemError;
+            return ctx;
+        }
+        
+        public static Context CheckAuthorPermission(this Context ctx, Permissions neededPerms, string permissionName)
+        {
+            // TODO: can we always assume Author is a DiscordMember? I would think so, given they always come from a
+            // message received event...
+            var hasPerms = ctx.Channel.PermissionsInSync(ctx.Author);
+            if ((hasPerms & neededPerms) != neededPerms)
+                throw new PKError($"You must have the \"{permissionName}\" permission in this server to use this command.");
+            return ctx;
+        }
+    }
+}