export default class Emitter {
  // Add an event listener for given event
  on(event, fn) {
    this._callbacks = this._callbacks || {}
    // Create namespace for this event
    if (!this._callbacks[event]) {
      this._callbacks[event] = []
    }
    this._callbacks[event].push(fn)
    return this
  }

  emit(event, ...args) {
    this._callbacks = this._callbacks || {}
    const callbacks = this._callbacks[event]

    if (callbacks) {
      callbacks.forEach(callback => callback.apply(this, args))
    }

    return this
  }

  // Remove event listener for given event.
  // If fn is not provided, all event listeners for that event will be removed.
  // If neither is provided, all event listeners will be removed.
  off(event, fn) {

    if (!arguments.length) {
      this._callbacks = {}
    } else {
      // event listeners for the given event
      const callbacks = this._callbacks ? this._callbacks[event] : null
      if (callbacks) {
        if (fn) {
          this._callbacks[event] = callbacks.filter(cb => cb !== fn) // remove specific handler
        } else {
          delete this._callbacks[event] // remove all handlers
        }
      }
    }

    return this
  }
}