Complete Preconditions implementation

.gitignore

@@ -200,3 +200,4 @@ project.lock.json
 /test/Discord.Net.Tests/config.json
 /docs/_build
 *.pyc
+/.editorconfig

src/Discord.Net.Commands/Attributes/PreconditionAttribute.cs

@@ -7,6 +7,6 @@ namespace Discord.Commands
 {
     public abstract class PreconditionAttribute : Attribute
     {
-        public abstract void CheckPermissions(PreconditionContext context);
+        public abstract Task<PreconditionResult> CheckPermissions(IMessage context);
     }
 }

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

@@ -7,10 +7,12 @@ namespace Discord.Commands
 {
     public class RequireDMAttribute : PreconditionAttribute
     {
-        public override void CheckPermissions(PreconditionContext context)
+        public override Task<PreconditionResult> CheckPermissions(IMessage context)
         {
-            if (context.Message.Channel is IGuildChannel)
-                context.Handled = true;
+            if (context.Channel is IGuildChannel)
+                return Task.FromResult(PreconditionResult.FromError("Command must be used in a DM"));
+
+            return Task.FromResult(PreconditionResult.FromSuccess());
         }
     }
 }

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

@@ -7,10 +7,12 @@ namespace Discord.Commands
 {
     public class RequireGuildAttribute : PreconditionAttribute
     {
-        public override void CheckPermissions(PreconditionContext context)
+        public override Task<PreconditionResult> CheckPermissions(IMessage context)
         {
-            if (!(context.Message.Channel is IGuildChannel))
-                context.Handled = true;
+            if (!(context.Channel is IGuildChannel))
+                return Task.FromResult(PreconditionResult.FromError("Command must be used in a guild"));
+
+            return Task.FromResult(PreconditionResult.FromSuccess());
         }
     }
 }

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

@@ -0,0 +1,45 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace Discord.Commands
+{
+    public class RequireRoleAttribute : RequireGuildAttribute
+    {
+        public string Role { get; set; }
+        public StringComparer Comparer { get; set; }
+
+        public RequireRoleAttribute(string roleName)
+        {
+            Role = roleName;
+            Comparer = StringComparer.Ordinal;
+        }
+
+        public RequireRoleAttribute(string roleName, StringComparer comparer)
+        {
+            Role = roleName;
+            Comparer = comparer;
+        }
+
+        public override async Task<PreconditionResult> CheckPermissions(IMessage context)
+        {
+            var result = await base.CheckPermissions(context).ConfigureAwait(false);
+
+            if (!result.IsSuccess)
+                return result;
+
+            var author = (context.Author as IGuildUser);
+
+            if (author != null)
+            {
+                var hasRole = author.Roles.Any(x => Comparer.Compare(x.Name, Role) == 0);
+
+                if (!hasRole)
+                    return PreconditionResult.FromError($"User does not have the '{Role}' role.");
+            }
+
+            return PreconditionResult.FromSuccess();
+        }
+    }
+}

src/Discord.Net.Commands/Command.cs

@@ -19,7 +19,7 @@ namespace Discord.Commands
         public string Text { get; }
         public Module Module { get; }
         public IReadOnlyList<CommandParameter> Parameters { get; }
-        public IReadOnlyList<PreconditionAttribute> Permissions { get; }
+        public IReadOnlyList<PreconditionAttribute> Preconditions { get; }
 
         internal Command(Module module, object instance, CommandAttribute attribute, MethodInfo methodInfo, string groupPrefix)
         {
@@ -38,22 +38,20 @@ namespace Discord.Commands
                 Synopsis = synopsis.Text;
 
             Parameters = BuildParameters(methodInfo);
-            Permissions = BuildPermissions(methodInfo);
+            Preconditions = BuildPreconditions(methodInfo);
             _action = BuildAction(methodInfo);
         }
 
-        public bool MeetsPreconditions(IMessage message)
+        public async Task<PreconditionResult> CheckPreconditions(IMessage context)
         {
-            var context = new PreconditionContext(this, message);
-
-            foreach (PreconditionAttribute permission in Permissions)
+            foreach (PreconditionAttribute permission in Preconditions)
             {
-                permission.CheckPermissions(context);
-                if (context.Handled)
-                    return false;
+                var result = await permission.CheckPermissions(context).ConfigureAwait(false);
+                if (!result.IsSuccess)
+                    return result;
             }
 
-            return true;
+            return PreconditionResult.FromSuccess();
         }
 
         public async Task<ParseResult> Parse(IMessage msg, SearchResult searchResult)
@@ -68,8 +66,9 @@ namespace Discord.Commands
             if (!parseResult.IsSuccess)
                 return ExecuteResult.FromError(parseResult);
 
-            if (!MeetsPreconditions(msg)) // TODO: should we have to check this here, or leave it entirely to the bot dev?
-                return ExecuteResult.FromError(CommandError.UnmetPrecondition, "Permissions check failed");
+            var precondition = await CheckPreconditions(msg).ConfigureAwait(false);
+            if (!precondition.IsSuccess) // TODO: should we have to check this here, or leave it entirely to the bot dev?
+                return ExecuteResult.FromError(precondition);
 
             try
             {
@@ -82,7 +81,7 @@ namespace Discord.Commands
             }
         }
 
-        private IReadOnlyList<PreconditionAttribute> BuildPermissions(MethodInfo methodInfo)
+        private IReadOnlyList<PreconditionAttribute> BuildPreconditions(MethodInfo methodInfo)
         {
             return methodInfo.GetCustomAttributes<PreconditionAttribute>().ToImmutableArray();
         }

src/Discord.Net.Commands/CommandService.cs

@@ -208,16 +208,19 @@ namespace Discord.Commands
             if (!searchResult.IsSuccess)
                 return searchResult;
 
-            // TODO: this logic is for users who don't manually search/execute: should we keep it?
-            
-            IReadOnlyList<Command> commands = searchResult.Commands
-                .Where(x => x.MeetsPreconditions(message)).ToImmutableArray();
-
-            if (commands.Count == 0 && searchResult.Commands.Count > 0)
-                return ParseResult.FromError(CommandError.UnmetPrecondition, "Unmet precondition");
+            var commands = searchResult.Commands;
 
             for (int i = commands.Count - 1; i >= 0; i--)
             {
+                var preconditionResult = await commands[i].CheckPreconditions(message);
+                if (!preconditionResult.IsSuccess)
+                {
+                    if (commands.Count == 1)
+                        return preconditionResult;
+                    else
+                        continue;
+                }
+
                 var parseResult = await commands[i].Parse(message, searchResult);
                 if (!parseResult.IsSuccess)
                 {

src/Discord.Net.Commands/Context/PreconditionContext.cs

@@ -1,23 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
-
-namespace Discord.Commands
-{
-    public class PreconditionContext
-    {
-        public Command Command { get; internal set; }
-        public IMessage Message { get; internal set; }
-
-        public bool Handled { get; set; }
-
-        internal PreconditionContext(Command command, IMessage message)
-        {
-            Command = command;
-            Message = message;
-
-            Handled = false;
-        }
-    }
-}

src/Discord.Net.Commands/Results/ExecuteResult.cs

@@ -28,6 +28,8 @@ namespace Discord.Commands
             => new ExecuteResult(ex, CommandError.Exception, ex.Message);
         internal static ExecuteResult FromError(ParseResult result)
             => new ExecuteResult(null, result.Error, result.ErrorReason);
+        internal static ExecuteResult FromError(PreconditionResult result)
+            => new ExecuteResult(null, result.Error, result.ErrorReason);
         
         public override string ToString() => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
         private string DebuggerDisplay => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";

src/Discord.Net.Commands/Results/PreconditionResult.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics;
+
+namespace Discord.Commands
+{
+    [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
+    public struct PreconditionResult : IResult
+    {
+        public CommandError? Error { get; }
+        public string ErrorReason { get; }
+
+        public bool IsSuccess => !Error.HasValue;
+
+        private PreconditionResult(CommandError? error, string errorReason)
+        {
+            Error = error;
+            ErrorReason = errorReason;
+        }
+
+        internal static PreconditionResult FromSuccess()
+            => new PreconditionResult(null, null);
+        internal static PreconditionResult FromError(string reason)
+            => new PreconditionResult(CommandError.UnmetPrecondition, reason);
+
+        public override string ToString() => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
+        private string DebuggerDisplay => IsSuccess ? "Success" : $"{Error}: {ErrorReason}";
+    }
+}