Extjs 设置垂直滚动条的位置

   var gridview = grid.getView();
   var firstRow = Ext.get(gridview.getRow(0));
   var row = Ext.get(gridview.getRow(store.data.length - 1));
   var distance = row.getOffsetsTo(firstRow)[1];
   gridview.scroller.dom.scrollTop = distance;