cmdo/valkyr

import type { EventRecord } from "./Event/mod.js";
import { Queue } from "./Queue.js";

export class Validator<Record extends EventRecord> {
  #listeners: Listeners<any> = {};
  #queue: Queue<Record>;

  constructor() {
    this.validate = this.validate.bind(this);
    this.#queue = new Queue(async (event) => {
      return Promise.all(Array.from(this.#listeners[event.type] || []).map((fn) => fn(event)));
    });
  }

  /**
   * Validate a event before its committed to the event store. Throwing an error results
   * in invalidation, otherwise the event is committed.
   *
   * @param record - Event record to validate.
   */
  async validate(record: Record) {
    return new Promise<boolean>((resolve, reject) => {
      this.#queue.push(record, resolve, reject);
    });
  }

  /**
   * Register a validation handler for a specific event type used to validate the event
   * before its committed to the event store. Throwing an error results in invalidation,
   * otherwise the event is committed.
   *
   * @param type    - Event type to register the validation handler for.
   * @param handler - Validation handler to register.
   *
   * @returns function to unregister the validation handler.
   */
  on<T extends Record["type"], R extends Record = Extract<Record, { type: T }>>(
    type: T,
    handler: ValidationHandler<R>
  ): () => void {
    const listeners = this.#listeners[type];
    if (listeners) {
      listeners.add(handler);
    } else {
      this.#listeners[type] = new Set([handler]);
    }
    return () => {
      this.off(type, handler);
    };
  }

  /**
   * Unregister a validation handler for a specific event type.
   *
   * @param type    - Event type to unregister the validation handler for.
   * @param handler - Validation handler to unregister.
   */
  off<T extends Record["type"], R extends Record = Extract<Record, { type: T }>>(
    type: T,
    handler: ValidationHandler<R>
  ) {
    this.#listeners[type]?.delete(handler);
  }
}

/*
 |--------------------------------------------------------------------------------
 | Types
 |--------------------------------------------------------------------------------
 */

type Listeners<R extends EventRecord> = Record<string, Set<ValidationHandler<R>>>;

type ValidationHandler<Record extends EventRecord> = (record: Record) => Promise<void>;