Ext GridPanel多选问题

//自动判断是否全选并选中或不选中表头的checkbox
function autoCheckGridHead(){
var hd_checker = grid.getEl().select('div.x-grid3-hd-checker'); //CompositeElementLite/CompositeElement
    var hd = hd_checker.first(); //呵呵，终于搞定了，这句测了好久，才找对对象;
    if(hd != null){ 
    if(grid.getSelectionModel().getSelections().length != grid.getStore().getCount()){    //没有全选的话
                //清空表格头的checkBox  
                if(hd.hasClass('x-grid3-hd-checker-on')){
                hd.removeClass('x-grid3-hd-checker-on');     //x-grid3-hd-checker-on
                //grid.getSelectionModel().clearSelections();
            }
            }else{
            if(grid.getStore().getCount() == 0){ //没有记录的话清空;
            return;
            }
            hd.addClass('x-grid3-hd-checker-on');
                grid.getSelectionModel().selectAll();
            }
        }
}