How to put an image (e.g sort arrow) on a listview column header

Problem/Question/Abstract:

When sorting TListViews it is good practise to show which column is sorted and in which direction.

Answer:

Add this to your form:

uses commctrl;

procedure TForm1.SetColumnImage(List: TListView; Column, Image: Integer; ShowImage:
  Boolean);
var
  Align, hHeader: integer;
  HD: HD_ITEM;
begin
  hHeader := SendMessage(List.Handle, LVM_GETHEADER, 0, 0);
  with HD do
  begin
    case List.Columns[Column].Alignment of
      taLeftJustify: Align := HDF_LEFT;
      taCenter: Align := HDF_CENTER;
      taRightJustify: Align := HDF_RIGHT;
    else
      Align := HDF_LEFT;
    end;

    mask := HDI_IMAGE or HDI_FORMAT;

    pszText := PChar(List.Columns[Column].Caption);

    if ShowImage then
      fmt := HDF_STRING or HDF_IMAGE or HDF_BITMAP_ON_RIGHT
    else
      fmt := HDF_STRING or Align;

    iImage := Image

  end;
  SendMessage(hHeader, HDM_SETITEM, Column, Integer(@HD));
end;


Images are taken from the SmallImages list. You should call this function for each column, and set the ShowImage to TRUE for the column you sorted. You can do this in the OnColumnClick() function:


var
  Ascending: boolean;

procedure TForm1.ListViewColumnClick(Sender: TObject; Column: TListColumn);
var
  i: integer;
begin
  // Toggle column Tag
  Column.Tag := 1 - Column.Tag; // 0 -> 1  ;  1 -> 0
  // Determine sort order based on the value of the Tag
  Ascending := Column.Tag = 1;

  // This loop displays the icon in the selected column.
  for i := 0 to ListView.Columns.Count - 1 do
    SetColumnImage(ListView, i, Column.Tag, i = Column.Index);

  // The CustomSort function is not covered in this
  // article but is explained elsewhere in Delphi Knowledge Base
  TListView(Sender).CustomSort(@SortByColumn, Column.Index);
end;

Problem:  Resizing the column header causes a WM_PAINT which will erase the image.
Solution: Override WM_PAINT and call SetColumnImage again from there. I used TApplicationEvents component from delphi 5.
If someone knows a better solution please let me know.