Refactor event system to support returnable events

Added support for validating and managing return values in events. Implemented `validate` and `clone` methods in the event base class for event mutation and return handling. Additionally, enhanced error handling, introduced "once" listeners, and improved async execution management in the `EventManager`.

app/src/core/events/Event.ts

@@ -1,17 +1,31 @@
-export abstract class Event<Params = any> {
+export abstract class Event<Params = any, Returning = void> {
+   _returning!: Returning;
+
    /**
     * Unique event slug
     * Must be static, because registering events is done by class
     */
    static slug: string = "untitled-event";
    params: Params;
+   returned: boolean = false;
+
+   validate(value: Returning): Event<Params, Returning> | void {}
+
+   protected clone<This extends Event<Params, Returning> = Event<Params, Returning>>(
+      this: This,
+      params: Params
+   ): This {
+      const cloned = new (this.constructor as any)(params);
+      cloned.returned = true;
+      return cloned as This;
+   }
 
    constructor(params: Params) {
       this.params = params;
    }
 }
 
-// @todo: current workaround: potentially there is none and that's the way
+// @todo: current workaround: potentially there is "none" and that's the way
 export class NoParamEvent extends Event<null> {
    static override slug: string = "noparam-event";
 
@@ -19,3 +33,9 @@ export class NoParamEvent extends Event<null> {
       super(null);
    }
 }
+
+export class InvalidEventReturn extends Error {
+   constructor(expected: string, given: string) {
+      super(`Expected "${expected}", got "${given}"`);
+   }
+}

app/src/core/events/EventListener.ts

@@ -4,15 +4,16 @@ import type { EventClass } from "./EventManager";
 export const ListenerModes = ["sync", "async"] as const;
 export type ListenerMode = (typeof ListenerModes)[number];
 
-export type ListenerHandler<E extends Event = Event> = (
+export type ListenerHandler<E extends Event<any, any>> = (
    event: E,
-   slug: string,
-) => Promise<void> | void;
+   slug: string
+) => E extends Event<any, infer R> ? R | Promise<R | void> : never;
 
 export class EventListener<E extends Event = Event> {
    mode: ListenerMode = "async";
    event: EventClass;
    handler: ListenerHandler<E>;
+   once: boolean = false;
 
    constructor(event: EventClass, handler: ListenerHandler<E>, mode: ListenerMode = "async") {
       this.event = event;

app/src/core/events/EventManager.ts

@@ -1,4 +1,4 @@
-import type { Event } from "./Event";
+import { type Event, InvalidEventReturn } from "./Event";
 import { EventListener, type ListenerHandler, type ListenerMode } from "./EventListener";
 
 export interface EmitsEvents {
@@ -6,7 +6,7 @@ export interface EmitsEvents {
 }
 
 export type EventClass = {
-   new (params: any): Event;
+   new (params: any): Event<any, any>;
    slug: string;
 };
 
@@ -17,16 +17,20 @@ export class EventManager<
    protected listeners: EventListener[] = [];
    enabled: boolean = true;
 
-   constructor(events?: RegisteredEvents, listeners?: EventListener[]) {
+   constructor(
+      events?: RegisteredEvents,
+      private options?: {
+         listeners?: EventListener[];
+         onError?: (event: Event, e: unknown) => void;
+         onInvalidReturn?: (event: Event, e: InvalidEventReturn) => void;
+         asyncExecutor?: typeof Promise.all;
+      }
+   ) {
       if (events) {
          this.registerEvents(events);
       }
 
-      if (listeners) {
-         for (const listener of listeners) {
-            this.addListener(listener);
-         }
-      }
+      options?.listeners?.forEach((l) => this.addListener(l));
    }
 
    enable() {
@@ -128,6 +132,18 @@ export class EventManager<
       this.addListener(listener as any);
    }
 
+   onEventOnce<ActualEvent extends EventClass, Instance extends InstanceType<ActualEvent>>(
+      event: ActualEvent,
+      handler: ListenerHandler<Instance>,
+      mode: ListenerMode = "async"
+   ) {
+      this.throwIfEventNotRegistered(event);
+
+      const listener = new EventListener(event, handler, mode);
+      listener.once = true;
+      this.addListener(listener as any);
+   }
+
    on<Params = any>(
       slug: string,
       handler: ListenerHandler<Event<Params>>,
@@ -145,27 +161,73 @@ export class EventManager<
       this.events.forEach((event) => this.onEvent(event, handler, mode));
    }
 
-   async emit(event: Event) {
+   protected executeAsyncs(promises: (() => Promise<void>)[]) {
+      const executor = this.options?.asyncExecutor ?? ((e) => Promise.all(e));
+      executor(promises.map((p) => p())).then(() => void 0);
+   }
+
+   async emit<Actual extends Event<any, any>>(event: Actual): Promise<Actual> {
       // @ts-expect-error slug is static
       const slug = event.constructor.slug;
       if (!this.enabled) {
          console.log("EventManager disabled, not emitting", slug);
-         return;
+         return event;
       }
 
       if (!this.eventExists(event)) {
          throw new Error(`Event "${slug}" not registered`);
       }
 
-      const listeners = this.listeners.filter((listener) => listener.event.slug === slug);
-      //console.log("---!-- emitting", slug, listeners.length);
+      const syncs: EventListener[] = [];
+      const asyncs: (() => Promise<void>)[] = [];
+
+      this.listeners = this.listeners.filter((listener) => {
+         // if no match, keep and ignore
+         if (listener.event.slug !== slug) return true;
 
-      for (const listener of listeners) {
          if (listener.mode === "sync") {
-            await listener.handler(event, listener.event.slug);
+            syncs.push(listener);
          } else {
-            listener.handler(event, listener.event.slug);
+            asyncs.push(async () => await listener.handler(event, listener.event.slug));
+         }
+         // Remove if `once` is true, otherwise keep
+         return !listener.once;
+      });
+
+      // execute asyncs
+      this.executeAsyncs(asyncs);
+
+      // execute syncs
+      let _event: Actual = event;
+      for (const listener of syncs) {
+         try {
+            const return_value = (await listener.handler(_event, listener.event.slug)) as any;
+
+            if (typeof return_value !== "undefined") {
+               const newEvent = _event.validate(return_value);
+               // @ts-expect-error slug is static
+               if (newEvent && newEvent.constructor.slug === slug) {
+                  if (!newEvent.returned) {
+                     throw new Error(
+                        // @ts-expect-error slug is static
+                        `Returned event ${newEvent.constructor.slug} must be marked as returned.`
+                     );
+                  }
+                  _event = newEvent as Actual;
+               }
+            }
+         } catch (e) {
+            if (e instanceof InvalidEventReturn) {
+               this.options?.onInvalidReturn?.(_event, e);
+               console.warn(`Invalid return of event listener for "${slug}": ${e.message}`);
+            } else if (this.options?.onError) {
+               this.options.onError(_event, e);
+            } else {
+               throw e;
+            }
          }
       }
+
+      return _event;
    }
 }

app/src/core/events/index.ts

@@ -1,8 +1,8 @@
-export { Event, NoParamEvent } from "./Event";
+export { Event, NoParamEvent, InvalidEventReturn } from "./Event";
 export {
    EventListener,
    ListenerModes,
    type ListenerMode,
-   type ListenerHandler,
+   type ListenerHandler
 } from "./EventListener";
 export { EventManager, type EmitsEvents, type EventClass } from "./EventManager";