title: ExtJS 4 - Refreshing Summary Feature of Grid Panel Without Refreshing Entire View
body: <p>I'm using ExtJS's GridPanel library to render a list of rows that have a CheckboxModel for selections. There is also a summary row that adds up all of the selected data and displays it in a row at the bottom of the GridPanel. The code for this is:</p>
  
  <pre><code>var sm = Ext.create('Ext.selection.CheckboxModel', {
  
      /////////
      // With large number of rows ... this takes forever
      /////////
      grid.getView().refresh();
      /////////
      /////////
  
  
      listeners:{
          selectionchange: function(selectionModel, selectedRecords, options){
            // Do stuff
  
          }
      }
  });
  
  var selSumFn = function(column, selModel){
      return function(){
          var records = selModel.getSelection(),
          result  = 0;
          //console.log("records:" + records.length);
          Ext.each(records, function(record){
              result += record.get(column) * 1;
          });
  
          return result;
      };
  };
  
  var grid = Ext.create('Ext.grid.Panel', {
      autoScroll:true,
      features: [{
          ftype: 'summary'
      }],
      store: store,
      defaults: {
          sortable:true
      },
      selModel: sm,
      columns: [
          {header: 'Column 1', width: 100, dataIndex: 'col1', summaryType: selSumFn('col1', sm)},
          {header: 'Column 2', width: 100, dataIndex: 'col2', summaryType: selSumFn('col2', sm)}
      ],
      width: 730,
      height: 400 ,
      title: 'Data',
      renderTo: 'data-div',
      viewConfig: {
          stripeRows: true
      },
      listeners: {'beforerender' : {fn:function(){this.store.load();}}}
  });
  </code></pre>
  
  <p>Is there any way to only refresh the summaryrow feature and not the entire view? Refreshing the view was the only way I could find to refresh the summary row when updates were made to checkbox selections of the GridPanel.</p>