add UseBeforeEvent

src/decorators.ts

 import {
+  BeforeEventName,
   CommandConfigExtended,
   CommandDefinitionFun,
   CommandPutConfig,
@@ -39,6 +40,12 @@ export const UseMiddleware = (prepend?: boolean): MethodDecorator =>
 export const UseEvent = (name: EventName, prepend?: boolean): MethodDecorator =>
   DoRegister(GenerateMappingStruct('onevent', { name, prepend }));
 
+export const UseBeforeEvent = (
+  name: BeforeEventName,
+  prepend?: boolean,
+): MethodDecorator =>
+  DoRegister(GenerateMappingStruct('beforeEvent', { name, prepend }));
+
 export const UsePlugin = (): MethodDecorator =>
   DoRegister(GenerateMappingStruct('plugin'));
 

src/def/interfaces.ts

 import {
   App,
   Argv,
+  BeforeEventMap,
   Channel,
   Command,
   Context,
@@ -76,17 +77,24 @@ export function PluginDef<T extends keyof Modules | Plugin>(
   return { plugin, options, select };
 }
 
-export type EventName = keyof EventMap;
-export interface EventNameAndPrepend {
-  name: EventName;
+export interface CommonEventNameAndPrepend<T extends keyof any> {
+  name: T;
   prepend?: boolean;
 }
 
+export type EventName = keyof EventMap;
+export type EventNameAndPrepend = CommonEventNameAndPrepend<EventName>;
+
+export type BeforeEventName = keyof BeforeEventMap;
+export type BeforeEventNameAndPrepend =
+  CommonEventNameAndPrepend<BeforeEventName>;
+
 export type ContextFunction<T> = (ctx: Context) => T;
 export type OnContextFunction = ContextFunction<Context>;
 export interface DoRegisterConfigDataMap {
   middleware: boolean; // prepend
   onevent: EventNameAndPrepend;
+  beforeEvent: BeforeEventNameAndPrepend;
   plugin: never;
   command: CommandRegisterConfig;
   route: KoishiRouteDef;

src/register.ts

@@ -247,14 +247,16 @@ export function KoishiPlugin<T = any>(
               (...args: any[]) => this[methodKey](...args),
               eventData.prepend,
             );
-            // special events
-            /*if (eventName === 'service') {
-              const serviceName = eventName.slice(8);
-              if (baseContext[serviceName] != null) {
-                this[methodKey]();
-              }
-            }*/
             break;
+          case 'beforeEvent':
+            const { data: beforeEventData } =
+              regData as DoRegisterConfig<'beforeEvent'>;
+            const beforeEventName = beforeEventData.name;
+            baseContext.before(
+              beforeEventName,
+              (...args: any[]) => this[methodKey](...args),
+              beforeEventData.prepend,
+            );
           case 'plugin':
             this._applyInnerPlugin(baseContext, methodKey);
             break;