Hello, you may recall my previous email that I stated that I get a lot of
access violation errors in a TListView when a pagecontrol's multiline
property is changed. Well here's a test to prove the bug and the solution to
work around it.
The bug
Add a TPageControl on a form and insert a few pages. Add a TListView inside
one of the pages. Add 3 buttons to the form. Add a OnDeletion event to the
TListView. Use this code for the test.
TtestObj = class(TObject)
end;
Implementation
procedure someOnClickProcedure1(Sender: TObject);
var
i: integer;
begin
for i:= 1 to 10 do
ListView1.AddItem(Format('test%d', [i]), TtestObj.Create);
end;
procedure someOnClickProcedure2(Sender: TObject);
begin
PageControl1.Multiline:= not PageControl1.Multiline;
end;
procedure someOnClickProcedure3(Sender: TObject);
begin
ListView1.Clear;
end;
procedure ListView1Deletion(Sender: TObject; Item: TListItem);
begin
TtestObj(Item.Data).Free;
end;
Ok by looking at this code, there is no problem right? The OnDeletion code
clears up the memory that was allocated with the AddItem function. No memory
leaks, every thing is fine. However, setting the Multiline property, when
the TListView is inside the TPageControl, some how activates the OnDeletion
event. This is the bug, it shouldn't do that, it's not logical. Now to fix
this problem, we simply need a flag to prevent an unlawful deletion. Add a
global or local variable called isDeleting of type boolean. Now we simply
adjust the code like this
procedure someOnClickProcedure3(Sender: TObject);
begin
isDeleting:= True;
ListView1.Clear;
isDeleting:= False;
end;
procedure ListView1Deletion(Sender: TObject; Item: TListItem);
begin
if isDeleting then
TtestObj(Item.Data).Free;
end;
voila! problem solved.
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