How can Idisplay a meeage in a background service?

I have succeed to trigger the background service with the phone message in simulator but I can not display it in the tridderd function. The codes are as below

public function onPhoneAppMessage(msg as Communications.PhoneAppMessage){

//WatchUi.requestUpdate();

//WatchUi.showToast("Lost GPS",null);

var b ;

var a =0;

a=1;

}

The both WatchUI function will cause the following error
Error: Permission Required Details: Module 'Toybox.WatchUi' not available to 'Background'

I know it must be that the WatchUI related functions are not surpported in background service. So, how can I display the message from the phone ?
The other similar question is : How to implement the notifincaton function in the watch ? You know if the wathc receives a message from the connectIQ

it will be displayed ASAP.

Thanks a lot in advance.