[Solved] Access Violation When Clicking In TListView
lazarus / 21-General

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Wilko500 · 2021-03-30 22:40

I have a project with three ListView controls.  If there is clear
space below the last item in the ListView and I click (accidentally)
on that blank space 2 of the 3 give access violation error.

I have simulated this in the attached project. The error is caused by
the line
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
s:=ListView1.Selected.Caption; in the following
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.ListView1Click(Sender: TObject);Var  s:    String;begin   
s:=ListView1.Selected.Caption;    ShowMessage(s);end;
I would have expected that with the onclick event nothing would occur
unless an item is clicked.  Evidently not the case. The solution I
have come up with is
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  procedure
TForm1.ListView1Click(Sender: TObject);Var  s:    String;  i:  
 Integer;begin    i:=ListView1.ItemIndex;    if i >-1 then     
 Begin       s:=ListView1.Selected.Caption;       ShowMessage(s)     
 End;end;  While this works it seems a bit clumsy.  Is there a better
way of getting the caption for the selected item?
Thanks

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trev · 2021-03-30 23:42

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.ListView1Click(Sender: TObject);begin    if(ListView1.ItemIndex
> -1) then       ShowMessage(ListView1.Selected.Caption)end;

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Sieben · 2021-03-31 00:04

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i    
procedure TForm1.ListView1Click(Sender: TObject);    begin      if
Assigned(ListView1.Selected) then       
ShowMessage(ListView1.Selected.Caption)    end;

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Wilko500 · 2021-04-01 21:35

I'm not sure I understand the reason for the error. I have another
ListView that does not error when blank space is clicked but in that
case I do not access a list view item.  Seems to me that the OnClick
event is fired when the listView control is clicked whereas I would
have expected that OnClick should fire when an item is clicked and be
ignored when space is clicked.

Nevertheless both suggestions work fine thank you.

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Gustavo 'Gus' Carreno · 2021-04-01 22:38

Hey Wilko,

Quote from: Wilko500 on April 01, 2021, 11:35:11 pmI'm not sure I
understand the reason for the error. I have another ListView that does
not error when blank space is clicked but in that case I do not access
a list view item.  Seems to me that the OnClick event is fired when
the listView control is clicked whereas I would have expected that
OnClick should fire when an item is clicked and be ignored when space
is clicked.

When you make the TListview not have any selected item, in the case of
clicking in a white space, it then assigns the value -1 to the
TListview.ItemIndex and, most probably, will assign nil to
TListview.Selected.

So if any event is triggered after the TListview as done that, you
will always have an error when you access TListview.Selected, since
it's now invalid.

Get it now?

Cheers,
Gus

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Wilko500 · 2021-04-01 22:47

Thanks Gus, yes, you’ve supplied the missing link.   Actually, it
might be that I’m the missing link!!!

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Gustavo 'Gus' Carreno · 2021-04-02 00:44

Hey Wilko,

Quote from: Wilko500 on April 02, 2021, 12:47:23 amThanks Gus, yes,
you’ve supplied the missing link.   Actually, it might be that I’m the
missing link!!!

Hey, hey, don't say that!! Don't say bad things about you like that!!

We all have to learn. I, also, had to learn until I got to the level I
got.
And I got to this level by learning and being taught.
Learning by myself and being taught by other patient ones.

Sometimes on these forums, you get less patient people that won't have
the patience to explain to the ones that are just initiating the
language.
I still remember being in that stage and I always strive to ease that
pain of having no clue what you're doing.

So I'm glad you learned something today, but please don't pull
yourself down like that!!

Cheers,
Gus