推荐订阅源

In 3.x you look for the popups hide event

https://developers.arcgis.com/javascript/3/jsapi/popup-amd.html#event-hide 

In 4.x

 watchUtils.whenTrue(view.popup,'visible', function(){
          watchUtils.whenFalseOnce(view.popup,'visible', function(){
            console.info('popup has been closed');
          })
        });‍‍‍‍‍