Setting TDBGrid visible rows

I want to adjust a TDBGrid height given a VisibleRows parameter. the grid may or may not have titles.

Suppose I select 100 records from the db, but I want the grid height to adjust to show the first 10 rows (make them visible). the dataset will still hold 100 records.

i.e.

procedure SetVisibleRows(DBGrid: TCustomDBGrid; VisibleRows: Integer);
begin
  ...
  DBGrid.Height := ???
end;

I know how to get the visible rows:

type
  TCustomGridHack = class(TCustomGrid);

function GetVisibleRows(DBGrid: TCustomDBGrid): Integer;
begin
   Result := TCustomGridHack(DBGrid).VisibleRowCount;
end;

But, is there a way to set the VisibleRowCount?

Solution

This seems to work (maybe could be optimized more):

type
  TCustomDBGridHack = class(TCustomDBGrid);

procedure SetVisibleRows(DBGrid: TCustomDBGrid; VisibleRows: Integer);
var
  TitleHeight, RowHeight: Integer;
begin
  with TCustomDBGridHack(DBGrid) do
  begin
    if dgTitles in Options then
    begin
      TitleHeight := RowHeights[0] + GridLineWidth;
      RowHeight := RowHeights[1] + GridLineWidth;
    end
    else
    begin
      TitleHeight := 0;
      RowHeight := RowHeights[0] + GridLineWidth;
    end;
  end;
  DBGrid.ClientHeight := TitleHeight + (RowHeight * VisibleRows) + 1;
end;

Or by using TGridDrawInfo as suggested by @nil. It produces more accurate results (I modified it a bit):

procedure SetVisibleRows(DBGrid: TCustomDBGrid; VisibleRows: Integer);
var
  DrawInfo: TGridDrawInfo;
  TitleHeight, RowHeight: Integer;
begin
  with TCustomDBGridHack(DBGrid) do
  begin
    CalcDrawInfo(DrawInfo);
    TitleHeight := DrawInfo.Vert.FixedBoundary;
    RowHeight := RowHeights[DrawInfo.Vert.FirstGridCell] + DrawInfo.Vert.EffectiveLineWidth;
  end;
  DBGrid.ClientHeight := TitleHeight + (RowHeight * VisibleRows) + 1;
end;

As mentioned by @nil, RowHeight could be also calculated with:

RowHeight := DrawInfo.Vert.GetExtent(DrawInfo.Vert.FirstGridCell) + DrawInfo.Vert.EffectiveLineWidth;

I haven't noticed any difference though. (should be further investigated).

The above could be further improved to make the TDBGrid scroll-bars adjust better:

procedure SetVisibleRows(DBGrid: TCustomDBGrid; VisibleRows: Integer);
var
  DrawInfo: TGridDrawInfo;
  TitleHeight, RowHeight: Integer;
  HasActiveDataSet: Boolean;
begin
  if VisibleRows < 0 then VisibleRows := 0;
  HasActiveDataSet := Assigned(DBGrid.DataSource) and
    Assigned(DBGrid.DataSource.DataSet) and
    DBGrid.DataSource.DataSet.Active;
  if HasActiveDataSet then
    DBGrid.DataSource.DataSet.DisableControls;
  try
    with TCustomDBGridHack(DBGrid) do
    begin
      CalcDrawInfo(DrawInfo);
      TitleHeight :=  DrawInfo.Vert.FixedBoundary;
      RowHeight := RowHeights[DrawInfo.Vert.FirstGridCell] + DrawInfo.Vert.EffectiveLineWidth;      
    end;
    DBGrid.ClientHeight := TitleHeight + (RowHeight * VisibleRows) + 1;
  finally
    if HasActiveDataSet then
      DBGrid.DataSource.DataSet.EnableControls;
  end;
end;