Method on

packages/editor/src/plugin/emitter.ts:56–92

(type: string | number, listener: Listener, prepend = false)

  }

  on(type: string | number, listener: Listener, prepend = false): this {
    let existing = this.events[type]
    if (existing === undefined) {
      // Optimize the case of one listener. Don't need the extra array object.
      this.events[type] = listener
      ++this.eventsCount
    } else {
      if (typeof existing === 'function') {
        // Adding the second element, need to change to array.
        existing = this.events[type] = prepend ? [listener, existing] : [existing, listener]
        // If we've already got an array, just append.
      } else if (prepend) {
        existing.unshift(listener)
      } else {
        existing.push(listener)
      }

      // Check for listener leak
      const maxCount = this.getMaxListeners()
      if (maxCount > 0 && existing.length > maxCount) {
        // No error code for this since it is a Warning
        // eslint-disable-next-line no-restricted-syntax
        const e = new Error(
          'Possible EventEmitter memory leak detected. ' +
            existing.length +
            ' ' +
            String(type) +
            ' listeners ' +
            'added. Use emitter.setMaxListeners() to ' +
            'increase limit',
        )
        console.warn(e)
      }
    }

    return this
  }

  off(type: string | number, listener: Listener): this {
    const list = this.events[type]