How to add code inside the event of a component?
lazarus / 8-Other

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cyber_python · 2008-04-23 14:16

Hello,
I am trying to create a descendant of TTreeview that behaves like
this:

When a node expands (TTreeview.OnExpanding event) a certain procedure
is called (the procedure adds children nodes to the child nodes).

Does anyone know how can I do this?

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Leledumbo · 2008-04-24 03:16

Can't you just assign a method to it?
Code: [Select]
type
  TMyTreeView = class(TTreeView)
  private
    procedure AddChildrenNodesToChildNodes(Sender: TObject;
      Node: TTreeNode; var AllowExpansion: Boolean);
  end;
...
// for instance, assign it in constructor
constructor TMyTreeView.Create;
begin
  FOnExpanding:=@AddChildrenNodesToChildNodes;
end;

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cyber_python · 2008-04-24 07:59

Thank you very much Leledumbo! It worked fine.
I thought that doing it this way, if a user tried to add some code to
the event at design time, his code would replace my code instead of it
being executed after mine.
Anoter question : How can I add a TStrings property to my component ?

I have added a private variable:

  a : TStrings;

a procedure to set it :

  procedure SetA(const newA: TStrings);

  procedure TMyTreeview.SetA(const newA: TStrings);
  begin
        if (newA <> nil) then
        begin
              a.Assign(newA);          
        end;
  end;

and a published property :

   property myProperty: TStrings read a write SetA;

in the constructor :

  a := TStringList.Create;

and in the destructor :

  FreeThenNil(a);
  inherited Destroy;

but when I try to add some strings(by clicking the button with the
dots) at design time I get an error ('Access violation').
I checked the implementation of TCustomMemo and it was pretty similar
to my code. Any ideas?

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Marc · 2008-04-24 14:44

Usually there is a virtual method just calling the event. Something
like DoOnExpanding; You can override that to add your own code.
To find out you can set a breakpoint in your handler and  check where
it got called.

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cyber_python · 2008-04-25 12:35

Thanks Marc! I used the DoPaintNode procedure and it works fine!
I also solved the problem with the access violation (I had forgotten
to erase a line that was reffering to a TStringList that I do not
create anymore....)

But, now I have a new question : When I assign a StringList to my
Component's TStrings property it works fine (e.g. MyTreeview.a := str;
 where str is a TStringList), but when I edit the property at design
time (with the Lazarus Strings editor) the procedure I have specified
for writing the property does not seem to  get executed. I say it
seems because I added the following code to the form's onCreate event
and now the treeview items got updated (str is a TStringList) :

 str := TStringList.create;
 str.assign(MyTreeview.a);
 MyTreeview.a:=str;
 str.free;

Thank you in advance, cyber_python.


***Update : I just noticed that some integer properties (that have
their own procedures to set the value) that I have added do not work
at design time as well, so it must not have to do with TStrings.