class Plugin {  constructor(element, options) {    this._setup(element, options);    var pluginName = getPluginName(this);    this.uuid = GetYoDigits(6, pluginName);    if(!this.$element.attr(`data-${pluginName}`)){ this.$element.attr(`data-${pluginName}`, this.uuid); }    if(!this.$element.data('zfPlugin')){ this.$element.data('zfPlugin', this); }    /**     * Fires when the plugin has initialized.     * @event Plugin#init     */    this.$element.trigger(`init.zf.${pluginName}`);  }  destroy() {    this._destroy();    var pluginName = getPluginName(this);    this.$element.removeAttr(`data-${pluginName}`).removeData('zfPlugin')        /**         * Fires when the plugin has been destroyed.         * @event Plugin#destroyed         */        .trigger(`destroyed.zf.${pluginName}`);    for(var prop in this){      this[prop] = null;//clean up script to prep for garbage collection.    }  }}這段代碼里面的this.$element.trigger(`init.zf.${pluginName}`);不是很懂，代碼里面沒有聲明任何 "init.zf.xxx" 這類事件，那這個 trigger 觸發(fā)的是什么？