Author: Tomas Rutkauskas

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

Answer:

Add this to your form:

1   uses commctrl;
2   
3   procedure TForm1.SetColumnImage(List: TListView; Column, Image: Integer; ShowImage:
4     Boolean);
5   var
6     Align, hHeader: integer;
7     HD: HD_ITEM;
8   begin
9     hHeader := SendMessage(List.Handle, LVM_GETHEADER, 0, 0);
10    with HD do
11    begin
12      case List.Columns[Column].Alignment of
13        taLeftJustify: Align := HDF_LEFT;
14        taCenter: Align := HDF_CENTER;
15        taRightJustify: Align := HDF_RIGHT;
16      else
17        Align := HDF_LEFT;
18      end;
19  
20      mask := HDI_IMAGE or HDI_FORMAT;
21  
22      pszText := PChar(List.Columns[Column].Caption);
23  
24      if ShowImage then
25        fmt := HDF_STRING or HDF_IMAGE or HDF_BITMAP_ON_RIGHT
26      else
27        fmt := HDF_STRING or Align;
28  
29      iImage := Image
30  
31    end;
32    SendMessage(hHeader, HDM_SETITEM, Column, Integer(@HD));
33  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:


34  var
35    Ascending: boolean;
36  
37  procedure TForm1.ListViewColumnClick(Sender: TObject; Column: TListColumn);
38  var
39    i: integer;
40  begin
41    // Toggle column Tag
42    Column.Tag := 1 - Column.Tag; // 0 -> 1  ;  1 -> 0
43    // Determine sort order based on the value of the Tag
44    Ascending := Column.Tag = 1;
45  
46    // This loop displays the icon in the selected column.
47    for i := 0 to ListView.Columns.Count - 1 do
48      SetColumnImage(ListView, i, Column.Tag, i = Column.Index);
49  
50    // The CustomSort function is not covered in this
51    // article but is explained elsewhere in Delphi Knowledge Base
52    TListView(Sender).CustomSort(@SortByColumn, Column.Index);
53  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.