public BooleanProperty alertProperty() {
        if (null == alert) {
            alert = new BooleanPropertyBase(_alert) {
                @Override protected void invalidated() { fireTileEvent(ALERT_EVENT); }
                @Override public Object getBean() { return Tile.this; }
                @Override public String getName() { return "alert"; }
            };
        }
        return alert;
    }
Defines if the alert property should be set. This
property can be used to visualize an alert situation
in the skin.
@param ALERT
src/main/java/eu/hansolo/tilesfx/Tile.java