EasyUI datagird 排序 按数字类型的问题

easyui datagird 默认显示的数据都是字符， 对要数字列进行排序规则，需要自定义排序规则
如果按字符排序 27竟然小于4 这不是我们想要的。
解决方案

<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'>
//按数字类型排序 自定义排序
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;
}

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

        {field:'user_name',title:'坐席工号',width:20,align:'center'},
        {field:'first_name',title:'坐席员',width:20,align:'center'},
        {field:'call_count',title:'电话总量',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'callout_count',title:'呼出数量',width:20,align:'center' , sortable:true,sorter:sort_int},
        {field:'callin_count',title:'呼入数量',width:20,align:'center', sortable:true,sorter:sort_int },
        {field:'callout_time_len',title:'呼出总时长(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'callin_time_len',title:'呼入总时长(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_time_len',title:'通话总时长(秒',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'avg_talk_len',title:'平均通话时长(秒)',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_count',title:'接通总量',width:20,align:'center', sortable:true,sorter:sort_int},
        {field:'talk_ratio',title:'接通率',width:20,align:'center'}
    ]]
});

</script>