The chrome.dial API is backed by a service that multicasts discovery requests on the local network to discover DIAL-capable devices and maintains a list of devices that have responded. Adding an onDeviceList listener causes the service to periodically issue discovery requests to maintain the device list. (No polling is done when there are no onDeviceList listeners.)

The onDeviceList event is fired when discovery respnses are received and in other circumstances; see the documentation for onDeviceList.

The client can request that network discovery can be done immediately by invoking discoverNow() which is useful for presenting the user with an updated list of devices.

On-demand use (updates when discoverNow() is called):

chrome.dial.onDeviceList.addListener(function (list) { updateMenu(list); });
chrome.dial.discoverNow();

Background use (updates only when periodic polling happens):

var myList;
chrome.dial.onDeviceList.addListener(function (list) { myList = list; });

These can be combined to poll for devices to prime the device menu, then refresh the menu when it is displayed.