是否可以捕获取消订阅事件？这是我用来订阅用户通知的代码if('serviceWorker'innavigator){navigator.serviceWorker.ready.then(function(reg){reg.pushManager.subscribe({userVisibleOnly:true}).then(function(sub){add_endpoint_to_db(sub);//imaddingtheuserdeatilstomydb}).catch(function(e){if(Notification.permission==='denied'){}else{