live ListGrid and updating

[sunnyl]

I've been working on getting the ListGrid to fulfill a particular scenario:
1) Behave as a "live" grid, possibly loading several thousand rows.
2) Update its cells/rows without completely refreshing via a timer every several seconds. In order to help performance it should only update the rows in the viewport.

The grid is backed by a GWT-RPC datasource (backed by hibernate+mssql) which observes the startRow/endRow requests of the grid. This works wonderfully and I've loaded up to 50,000 records before, so I can do #1 already.

#2 however I can only partially do. From what I've gathered there are two ways that the grid is being used:
1) It loads its data using the datasource and does its own thing caching using a ResultSet. Using a ResultSet will allow the grid to correctly fetch/not-fetch rows in/outside the current page.
2) I need to mess with the data returned in order to directly update the grid without refreshing it completely.

Here is my logic so far:

private void refreshGrid(boolean forceRedraw) {
    // ...create criteria

    // force redraw will force the grid to completely reload
    if (forceRedraw) {
        grid.invalidateCache();
        grid.fetchData(criteria, loadCallback);
    } else {
      // set the viewport for quick refreshing, returns -1 if no rows visible
      int startRow = grid.getVisibleRows()[0];
      int endRow = grid.getVisibleRows()[1];
      if (startRow >= 0) {
          reqProp.setStartRow(startRow);
          reqProp.setEndRow(endRow);
      } else {
          reqProp.setStartRow(0);
          reqProp.setEndRow(LiveGrid.DEFAULT_PAGE_SIZE);
      }
      System.out.println("quick refresh for "+startRow+"-"+endRow);
      grid.getDataSource().fetchData(criteria, refreshCallback, reqProp);
    }
}


private class LoadCallback implements DSCallback {
    @Override
    public void execute(DSResponse response, Object rawData, DSRequest request) {			
        
        for (int x=0; x<recordList.getLength(); x++)
            recordList.removeAt(x);
	
        recordList.addList(response.getData());
    }
}


private class RefreshCallback implements DSCallback {
      @Override
      public void execute(DSResponse response, Object rawData, DSRequest request) {

            Record[] newData = response.getData();
            RecordList recordList = grid.getRecordList();

            // populate from nothing to something
            if (recordList.getLength() == 0 && newData.length != 0) {
                  // using an RS instead of the recordList will make the grid 
                  // behave correctly wrt leaving space for rows that haven't 
                  // been loaded yet
                  ResultSet rs = new ResultSet();
                  rs.setDataSource(StatusUpdateDS.getInstance());
                  rs.setInitialData(newData);
                  rs.setInitialLength(response.getTotalRows());
                  grid.setData(rs);
				
            // populate from something to nothing
            } else if (recordList.getLength() != 0 && newData.length == 0) {
                  grid.setData(new RecordList());
				
            } else if (recordList.getLength() == 0 && newData.length == 0) {
                  // do nothing
				
            } else {
                  // using this will cause the grid to completely refresh
                  // ResultSet rs = new ResultSet();
                  // rs.setDataSource(StatusUpdateDS.getInstance());
                  // rs.setInitialData(newData);
                  // rs.setInitialLength(response.getTotalRows());
                  // grid.setData(rs);
				
                  // the code below will update the rows to reflect the new data,
                  // however, I cannot find a way to add/remove new rows such that
                  // the new rows are in the correct order
                  RecordList rl = grid.getDataAsRecordList();
                  int viewStart = grid.getVisibleRows()[0];
                  int viewEnd = grid.getVisibleRows()[1];
                  Record[] visibleRecords = rl.getRange(viewStart, viewEnd);				
                  System.out.println("there are "+visibleRecords.length+" visible records");

                  // more complex logic would go here to update the rows
                  for (Record record : visibleRecords) {
                        record.setAttribute(SomeField, "some new value");
                  }
				
                  // try removing existing rows, this fails
                  for (int x=0; x<10; x+=2) {
                        // grid.remove
                        // rl.removeAt(x);
                  }

            }
            grid.redraw();
	}
}

What I cannot figure out how to do it to add/remove rows from the ResultSet (addAt and removeAt methods cause "is not a function" errors) in the hopes of dynamically adding/removing rows without refreshing.

I've also tried another approach of working only with the grid.add/remove/update methods, but I was unable to have those methods add records in the grid in the correct sort order.

I'm also guessing dynamically adding/removing rows like this may cause issues with the ResultSet's cache (ie, the viewport gets refreshed with rows added/missing)

Is there another approach I can try?