Using FV's TListBox, Memory Leak,

[gasensor]

I am Using FV's TListBox. But find it Memory Leaked. It has been bothering me for a long time.

sechshelme 's "FPC FreeVision Tutorial"  do not have a  TListBox DEMO...

Can anyone give me an example of how to use TLISTBOX properly?

TKS

[Mathias]

After trying back and forth for a long time, this unpleasant memory corpse remains with TList.
I have described all the findings in my GitHub Issues channel.
https://github.com/sechshelme/Lazarus-FreeVision/issues/3

I tried out several components, where I created them and then released them again with Dispose/Done.

Code: Pascal  [Select][+][-]

1. constructor TMyDialog.Init;
2. var
3.   Rect: TRect;
4.   ScrollBar: PScrollBar;
5.   i: Sw_Integer;
6.   rad: PRadioButtons;
7.   bp:PButton;
8. const
9.   Tage: array [0..6] of shortstring = (
10.     'Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag');
11.  
12. begin
13.   Rect.Assign(10, 5, 67, 17);
14.   inherited Init(Rect, 'ListBox Demo');
15.  
16.   rad := New(PRadioButtons, Init(Rect, NewSItem('~G~ross', NewSItem('~M~ittel', NewSItem('~K~lein', nil)))));
17.   Dispose(rad, Done); // io.
18.  
19.   bp := New(PButton, Init(Rect, '~T~ag', cmTag, bfNormal));
20.   Dispose(bp, Done); // io.
21.   exit;

But if I do the same with PListBox, there are memory corpses.

Code: Pascal  [Select][+][-]

1.   ListBox := new(PListBox, Init(Rect, 1, nil));
2.   Dispose(ListBox, Done);  // Memory Leak !
3.   exit;

Could there be a bug in PListBox?

[Mathias]

I took a closer look at the TFileListBox used by the TFileDialog. Then I came across the following:

Code: Pascal  [Select][+][-]

1. destructor TFileList.Done;
2. begin
3.   if List <> nil then Dispose(List, Done);
4.   TListBox.Done;
5. end;
6.  

They added their own destructor there.

Now I also built a new ListBox and added a destructor and used this ListBox in my dialog.
As it seems, this works. the memory leak is gone.

I'll make a few more tries.
In my opinion, this is a bug in TListBox, which lacks the destructor.

Code: Pascal  [Select][+][-]

1. type
2.   PNewListBox = ^TNewListBox;
3.   TNewListBox = object(TListBox)
4.     destructor Done; virtual;
5.   end;
6. ...
7. destructor TNewListBox.Done;
8. begin
9.   if List <> nil then begin
10.     Dispose(List, Done);
11.   end;
12.   TListBox.Done;
13. end;
14. constructor TMyDialog.Init;
15. var
16.   Rect: TRect;
17.   ScrollBar: PScrollBar;
18.   i: Sw_Integer;
19. const
20.   Tage: array [0..6] of shortstring = (
21.     'Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag', 'Sonntag');
22.  
23. begin
24.   Rect.Assign(10, 5, 67, 17);
25.   inherited Init(Rect, 'ListBox Demo');
26.  
27.   // StringCollection
28.   StringCollection := new(PStringCollection, Init(5, 5));
29.   for i := 0 to Length(Tage) - 1 do begin
30.     StringCollection^.Insert(NewStr(Tage[i]));
31.   end;
32.  
33.   // ScrollBar für ListBox
34.   Rect.Assign(31, 2, 32, 7);
35.   ScrollBar := new(PScrollBar, Init(Rect));
36.   Insert(ScrollBar);
37.  
38.   // ListBox
39.   Rect.Assign(5, 2, 31, 7);
40.   ListBox := new(PNewListBox, Init(Rect, 1, ScrollBar));
41.   ListBox^.NewList(StringCollection);
42.   Insert(ListBox);
43.  
44.   // Cancel-Button
45.   Rect.Assign(19, 9, 32, 10);
46.   Insert(new(PButton, Init(Rect, '~T~ag', cmTag, bfNormal)));
47.  
48.   // Ok-Button
49.   Rect.Assign(7, 9, 17, 10);
50.   Insert(new(PButton, Init(Rect, '~O~K', cmOK, bfDefault)));
51. end;

[Mathias]

So, now a tutorial for ListBoxes has been added.
With the ListBoxes, you can manually remove the loaded list from memory. This is described in the tutorial.