Mega Code Archive
Sorting a TListView
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Title: Sorting a TListView
Question: How can I sort the items in a TListView?
Answer:
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 Compare := 1
else
Compare := 0;
end;
Now that we have an OnCompare event, to sort the list and having
sorted, we should set SortType to stBoth (instead of stText, that
sorts by the first column without using the OnCompare event):
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 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).
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