EasyUI DataGrid 添加 Footer

做后台管理界面时，EasyUI 的 DataGrid 经常会被用到，有时候一些总的统计数据不合适放在数据表格里，需要单独显示，这时候就可以放在Footer中显示而不必另外布局。

该怎么给 DataGrid 添加 Footer，Document 里面说的不是很详细，Demo 里面有这个例子，看 datagrid_data2.json 数据格式就明白了。

{"total":28,"rows":[
	{"productid":"FI-SW-01","unitcost":10.00,"status":"P","listprice":36.50,"attr1":"Large","itemid":"EST-1"},
	{"productid":"K9-DL-01","unitcost":12.00,"status":"P","listprice":18.50,"attr1":"Spotted Adult Female","itemid":"EST-10"},
	{"productid":"RP-SN-01","unitcost":12.00,"status":"P","listprice":28.50,"attr1":"Venomless","itemid":"EST-11"},
	{"productid":"RP-SN-01","unitcost":12.00,"status":"P","listprice":26.50,"attr1":"Rattleless","itemid":"EST-12"},
	{"productid":"RP-LI-02","unitcost":12.00,"status":"P","listprice":35.50,"attr1":"Green Adult","itemid":"EST-13"},
	{"productid":"FL-DSH-01","unitcost":12.00,"status":"P","listprice":158.50,"attr1":"Tailless","itemid":"EST-14"},
	{"productid":"FL-DSH-01","unitcost":12.00,"status":"P","listprice":83.50,"attr1":"With tail","itemid":"EST-15"},
	{"productid":"FL-DLH-02","unitcost":12.00,"status":"P","listprice":63.50,"attr1":"Adult Female","itemid":"EST-16"},
	{"productid":"FL-DLH-02","unitcost":12.00,"status":"P","listprice":89.50,"attr1":"Adult Male","itemid":"EST-17"},
	{"productid":"AV-CB-01","unitcost":92.00,"status":"P","listprice":63.50,"attr1":"Adult Male","itemid":"EST-18"}
],"footer":[
	{"unitcost":19.80,"listprice":60.40,"productid":"Average:"},
	{"unitcost":198.00,"listprice":604.00,"productid":"Total:"}
]}

还有一种更直观的方法是，在data-options中用“footer”属性指定一个ID选择器，但是这个属性在Document中没有说明，至少我没找到。

<table class="easyui-datagrid" data-options="toolbar:'#tb',footer:'#ft'">

这个在官方例子 DataGrid Complex Toolbar 中可以看到