EasyUI datagird 排序 按數字類型的問題

easyui datagird 默認顯示的數據都是字符， 對要數字列進行排序規則，需要自定義排序規則
如果按字符排序 27竟然小於4 這不是我們想要的。
解決方案

<table id='grid'class='easyui-datagrid' style='width:1100px;height:430px' 
       url='Ajax-index.php?module=<{$module_name}>&action=Ajax_ReportView_Agent_Call_Report_Select&assigned_user_id=<{$assigned_user_id}>'  
      title='列表' iconCls='icon-table'  rownumbers='true' fitColumns='true' singleSelect='true' toolbar='#toolbar'  remoteSort='false'>  
</table>

<script type='text/javascript'>    
//按數字類型排序 自定義排序
function sort_int(a,b){  
    if(a.length > b.length) return 1;
        else if(a.length < b.length) return -1;
        else if(a > b) return 1;
        else return -1;
}  

//顯示列
$('#grid').datagrid({
    columns: [[

        {field:'user_name',title:'坐席工號',width:20,align:'center'},
        {field:'first_name',title:'坐席員',width:20,align:'center'},
        {field:'call_count',title:'電話總量',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'callout_count',title:'呼出數量',width:20,align:'center' , sortable:true,sorter:sort_int},
        {field:'callin_count',title:'呼入數量',width:20,align:'center', sortable:true,sorter:sort_int },
        {field:'callout_time_len',title:'呼出總時長(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'callin_time_len',title:'呼入總時長(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_time_len',title:'通話總時長(秒',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'avg_talk_len',title:'平均通話時長(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_count',title:'接通總量',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_ratio',title:'接通率',width:20,align:'center'}
    ]]
});

</script>