element-call-Github/src/LazyEventEmitter.ts

/*
Copyright 2022-2024 New Vector Ltd.

SPDX-License-Identifier: AGPL-3.0-only
Please see LICENSE in the repository root for full details.
*/

import EventEmitter from "events";

type NonEmptyArray<T> = [T, ...T[]];

/**
 * An event emitter that lets events pile up in a backlog until a listener is
 * present, at which point any events that were missed are re-emitted.
 */
export class LazyEventEmitter extends EventEmitter {
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  private eventBacklogs = new Map<string | symbol, NonEmptyArray<any[]>>();

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  public emit(type: string | symbol, ...args: any[]): boolean {
    const hasListeners = super.emit(type, ...args);

    if (!hasListeners) {
      // The event was missed, so add it to the backlog
      const backlog = this.eventBacklogs.get(type);
      if (backlog) {
        backlog.push(args);
      } else {
        // Start a new backlog
        this.eventBacklogs.set(type, [args]);
      }
    }

    return hasListeners;
  }

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  public on(type: string | symbol, listener: (...args: any[]) => void): this {
    super.on(type, listener);

    const backlog = this.eventBacklogs.get(type);
    if (backlog) {
      // That was the first listener for this type, so let's send it all the
      // events that have piled up
      for (const args of backlog) super.emit(type, ...args);
      // Backlog is now clear
      this.eventBacklogs.delete(type);
    }

    return this;
  }

  public addListener(
    type: string | symbol,
    // eslint-disable-next-line @typescript-eslint/no-explicit-any
    listener: (...args: any[]) => void,
  ): this {
    return this.on(type, listener);
  }

  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  public once(type: string | symbol, listener: (...args: any[]) => void): this {
    super.once(type, listener);

    const backlog = this.eventBacklogs.get(type);
    if (backlog) {
      // That was the first listener for this type, so let's send it the first
      // of the events that have piled up
      super.emit(type, ...backlog[0]);
      // Clear the event from the backlog
      if (backlog.length === 1) {
        this.eventBacklogs.delete(type);
      } else {
        backlog.shift();
      }
    }

    return this;
  }
}