def setItemStyle(self, itemStyle):
        """
        Sets the item style that will be used for this widget.  If you are
        trying to set a style on an item that has children, make sure to turn 
        off the useGroupStyleWithChildren option, or it will always display as
        a group.
        
        :param      itemStyle | <XGanttWidgetItem.ItemStyle>
        """
        self._itemStyle = itemStyle
        
        # initialize the group icon for group style
        if itemStyle == XGanttWidgetItem.ItemStyle.Group and \
           self.icon(0).isNull():
            ico = projexui.resources.find('img/folder_close.png')
            expand_ico = projexui.resources.find('img/folder_open.png')
            self.setIcon(0, QIcon(ico))
            self.setExpandedIcon(0, QIcon(expand_ico))