Reported by Arjen de Ruijter; checked by Jordan Russell
Description and cause:
When using Subitems.Add en .Delete, the overridden TSubItems.Add and
TSubItems.Delete maintain the Local FImageIndices well, but when using the
Clear method of the parent Class TStrings, then things go wrong because
there is no overriden TSubItems.Clear and the FImageIndices will not be
cleared.
This bug is especially memory-eating when you use
TCustomListView.OwnerDataFetch:
unit ComCtrls, line 12636-12645:
FTempItem.FSubItems.Clear;
....<cut>....
OwnerDataFetch(FTempItem, Request);
and add SubItems in the overridden OwnerDataFetch or the OnData-event
implemented in the native TCustomListView.OwnerDataFetch.
In that case the FImageIndices grows without bound, because the TempItem is
never destroyed during the use of TCustomListView.
Greg Chapman's comment from borland.public.delphi.vcl.components.using (go here to read the whole article/thread on Google):
...There is a
memory leak when using OwnerData mode under D5. The problem comes from the
TSubItems class which holds the subitems of the temporary list item used when
requesting data for the listview to display. If you have the source to
comctrls.pas, you'll see that TSubItems has a TList member called FImageIndices.
An element is added to this list everytime you add an element to TSubItems.
However, when you clear TSubItems, FImageIndices is not cleared. If you look at
TCustomListView.GetItem, you'll see that each time FTempItem is initialized
(which is each time the virtual listview needs to draw a row), FSubItems.Clear
is called, which will not clear FImageIndices. Then, OwnerDataFetch is called
which adds the current row's subitems (assuming there are any) to
FTempItem.SubItems, which will add new items to FImageIndices. So FImageIndices
grows without limit. The fix is to provide an overridden TSubItems.Clear method
(Clear is a virtual method of TStringList):
procedure TSubItems.Clear;
begin
inherited Clear;
FImageIndices.Clear;
end;
Note from checker:
I can confirm that Greg Chapman's fix works. Without it, one of my applications leaks several hundred kilobytes each time I drag the scroll bar on its owner-data list view! |
