Add method decorator for applying middleware to command actions

If used alongside `Command#use()`, the decorator-specified middleware will be run after those passed via `Command#use()`

src/index.ts

@@ -5,6 +5,7 @@ export { Command } from './lib/command/Command';
 export { CommandLoader } from './lib/command/CommandLoader';
 export { CommandRegistry } from './lib/command/CommandRegistry';
 export { CommandDispatcher } from './lib/command/CommandDispatcher';
+export { CommandDecorators } from './lib/command/CommandDecorators';
 export { GuildStorage } from './lib/storage/GuildStorage';
 export { GuildStorageLoader } from './lib/storage/GuildStorageLoader';
 export { GuildStorageRegistry } from './lib/storage/GuildStorageRegistry';

src/lib/command/CommandDecorators.ts

@@ -0,0 +1,44 @@
+import { MiddlewareFunction } from '../types/MiddlewareFunction';
+import { Message } from '../types/Message';
+import { Command } from './Command';
+
+export class CommandDecorators
+{
+	/**
+	 * Apply a middleware function to the action method of a command.
+	 * Identical to `Command#use()` but used as a method decorator
+	 */
+	public static using(func: MiddlewareFunction): MethodDecorator
+	{
+		return function(target: Command<any>, key: string, descriptor: PropertyDescriptor): PropertyDescriptor
+		{
+			if (!target) throw new Error('@using must be used as a method decorator for a Command action method.');
+			if (key !== 'action') throw new Error(`"${target.constructor.name}#${key}" is not a valid method target for @using.`);
+			if (!descriptor) descriptor = Object.getOwnPropertyDescriptor(target, key);
+			const original: any = descriptor.value;
+			descriptor.value = async function(message: Message, args: any[]): Promise<any>
+			{
+				let middlewarePassed: boolean = true;
+				try
+				{
+					let result: Promise<[Message, any[]]> | [Message, any[]] =
+						func.call(this, message, args);
+					if (result instanceof Promise) result = await result;
+					if (!(result instanceof Array))
+					{
+						if (typeof result === 'string') message.channel.send(result);
+						middlewarePassed = false;
+					}
+					if (middlewarePassed) [message, args] = result;
+				}
+				catch (err)
+				{
+					middlewarePassed = false;
+					message.channel.send(err.toString());
+				}
+				if (middlewarePassed) return await original.apply(this, [message, args]);
+			};
+			return descriptor;
+		};
+	}
+}