Sorting a TListView

Copyright © 2000 Ernesto De Spirito

Sorting by the first column

Sorting a TListView by the first column is easy:

ListView1.SortType := stText;

Setting SortType to stText is more or less like setting Sorted to True in a TListBox object. The list will be sorted and will remain sorted after additions and modifications, until SortType is set back to stNone:

ListView1.SortType := stNone;

It's like setting Sorted to False in a TListBox object. It won't undo the sorting, but future additions and modifications to the items list won't be sorted.

Sorting with an OnCompare event

To have a TListView sorted on another column (or arbitrary data stored or referenced in TListItem objects), we should either write an OnCompare event or an ordering function to be used with the CustomSort method.

If you want to sort keep a list sorted while adding and modifying items, then you should use an OnCompare event.

procedure(Sender: TObject; Item1, Item2: TListItem;
  Data: Integer; var Compare: Integer) of object;

The parameter Compare which is passed by reference should be set to 1, -1 or 0 depending on whether the first item is greater than (or should be placed after) the second item, the first item is lower than (or should be placed before) the second item, or if the two items are equal, respectively.

In the following example we are sorting a TListView by its fourth column (wich represents integer values) in descending order:

procedure TForm1.ListView1Compare(Sender: TObject; Item1,
  Item2: TListItem; Data: Integer; var Compare: Integer);
var
  n1, n2: integer;
begin
  n1 := StrToInt(Item1.SubItems[2]);
  n2 := StrToInt(Item2.SubItems[2]);
  if n1 > n2 then
    Compare := -1
  else if n1 < n2 then
    Compare := 1
  else
    Compare := 0;
end;

ListView1.SortType := stBoth;

If you just want to perform a temporal sort, you can do the following:

ListView1.SortType := stBoth;
ListView1.SortType := stNone;

or else:

ListView1.CustomSort(nil, 0);

Sorting with an ordering function

If you need a faster sort, then you should write an ordering function. This function should return 1, -1 or 0 (like the Compare parameter of the OnCompare event discussed above). For example:

function ByFourth(Item1, Item2: TListItem; Data: integer):
  integer; stdcall;
var
  n1, n2: cardinal;
begin
  n1 := StrToInt(Item1.SubItems[2]);
  n2 := StrToInt(Item2.SubItems[2]);
  if n1 > n2 then
    Result := -1
  else if n1 < n2 then
    Result := 1
  else
    Result := 0;
end;

Then, every time you want to sort the list, you call CustomSort passing the address of the ordering function. For example:

ListView1.CustomSort(@ByFourth, 0);

The Data parameter of the OnCompare event is 0 if the event is called automatically when SortType is stData or stBoth, but if it is generated because of a call to CustomSort, then its value is the second parameter to this method. The same happens with the Data parameter of the ordering function, so the Data parameter is normally used to specify a column to sort (we didn't use it in our example to make it simple).