// makeScreenGoBlue, makeScreenGoWhite and displayToast are lambdas
// The code below describes the rules for ProximityObserver
// that will scan the beacons around you and invoke your actions
// when given conditions are met.
val rule = proximityObserver.ruleBuilder()
                .forAttachmentKey("shop")
                .withDesiredMeanTriggerDistance(2.0)
                .withOnEnterAction(makeScreenGoBlue)
                .withOnExitAction(makeScreenGoWhite)
                .withOnChangeAction(displayToast)
                .create()