Sencha Touch grouping list on button click

I'm new to Sencha Touch and the Ext JS logic. Actually, I'm displaying a list of items containing two fields (title, type) grouped by default alphabetically by first character of the title and I added a 开发者_Go百科button to a toolbar that I want it to switch the grouping to be by type. I tried to programmatically set the getGroupString function in the buton handler in this way :

var toolbar = new Ext.Toolbar({
  dock: 'top',
  title: 'Toolbar',
  items: [{
    text: 'By type',
    handler: function() {
      MyApp.MyStore.getGroupString = function(record) {
        return record.get('type');
      };
      MyApp.itemsList.update();

    }
  }]
});

But that seems just to not work. Any help ?

Thanks

Just wanted to close the question as I have already answered it MyApp.itemsList.refresh();