How do I pass a message from an object to another object in Free Pascal - lazarus

Question

I have a form and then I have a 'TPageControl' object (named 'MyPages') and a 'TButton' object (named 'MyButton') placed on it at design time.
Then I have a new class called 'TTab' which extends 'TTabSheet'. 'TTab' class has a 'TButton' object as one of its member variables like below.
class TTab = class(TTabSheet)
private
m_btnCloseTab: TButton;
end;
When I click on the 'MyButton', it would create a new 'TTab' object, init the tab (like instantiating the 'm_btnCloseTab') and add it to 'MyPages' at run time.
Procedure TForm1.MyButtonClick(Sender:TObject);
var
newTab: TTab;
newCaption: AnsiString;
begin
newCaption:= 'Tab' + IntToStr(count); //count is a global var
inc(count);
newTab:= TTab.Create(nil);
newTab.Init(newCaption);
newTab.Parent(MyPages);
end;
This is what the TTab.Init(newCaption: AnsiString) Procedure looks like.
Procedure TTab.Init(newCaption: AnsiString);
begin
Self.Caption:= newCaption;
m_btnCloseTab:= TButton.Create(nil);
with m_btnCloseTab do begin
Parent:= Self;
Left:= 10;
Top:= 10;
Caption:= 'Close Tab';
Visible:= True;
OnClick:= #closeTab;
end;
end;
That adds a new tab alright. The close button is also shown on each tab.
How do I click on the 'm_btnCloseTab' on each tab to close that particular tab?
If I define a destructor (by overriding the destructor of the TTabSheet) for TTab like below, I can call it from outside.
Destructor TTab.Destroy;
begin
if m_btnCloseTab <> nil then begin
m_btnCloseTab.Destroy;
m_btnCloseTab:= nil;
end;
inherited;
end;
But I cannot call the Destructor from inside the tab (well, you can). If I do it, I cannot free the m_btnCloseTab object as it would give an exception, because we are still its event handler. If I don't free it, the tab gets closed fine, but the memory gets leaked (because we did not free m_btnCloseTab).
I believe I have to trigger an event so that the destructor can be called from the outside of 'TTab'. I don't know how to do it.
Any help would be appreciated.
Thanks.

Answer 1

You can find Notification methods all over the LCL sources (and in Delphi as well, of course). A simple example is a TLabeledEdit: this is some kind of "TEdit" which contains a TLabel. If the Label is destroyed the LabeledEdit is notified of this because it must set the reference to the label to nil. Otherwise the destructor of TLabeledEdit would attempt to destroy the label again - BOOM. Here the method is like this (pasted from ExtCtrls):
procedure TCustomLabeledEdit.Notification(AComponent: TComponent;
Operation: TOperation);
begin
inherited Notification(AComponent, Operation);
if (AComponent = FEditLabel) and (Operation = opRemove) then
FEditLabel := nil;
end;
And here you can see what you have to do in your case:
procedure TTab.Notification(AComponent: TComponent;
Operation: TOperation);
begin
inherited Notification(AComponent, Operation);
if (AComponent = m_BtnCloseTab) and (Operation = opRemove) then
m_BtnCloseTab := nil;
end;
Please note that Notification is a virtual method and must be declared with the attribute "override" in the protected section of the component.

Answer 2

I would use a single button for this task.
Take m_btnCloseTab declaration out of TTab and put it in private main form.
Then on your main form's FormCreate:
m_btnCloseTab := TButton.Create( MyPages );
(the above assumes MyPages is a component placed on the form, if not it must be created first.)
Give the button a top and left that makes sense for your TTab's.
Now m_btnCloseTab will be freed when MyPages is freed which is freed when the form is closed.
Now all you have to do is create your new tabs as you like and when one is focused simply make that tab the parent of your button. You could do this, for instance, in the MyPages OnChange method or whatever it has like that.
When the button is clicked it does something like TTab( Parent ).Free;
However, you may need to store Parent in a local variable in the button's OnClick, say:
TempTab: TTab
Then simply set TempTab := TTab( Parent ), set Button's Parent to nil, then call TempTab.Free;
I would also give your Tabs an owner. That way if the user closes the form with tabs still open (that is, your button hasn't been clicked) the owner will free them.
So declare your tabs like:
newTab:= TTab.Create( MyPages );
This should solve all your problems and, after a bit of fiddling, is quite easy to manage.
One final recommendation I would use the method .Free and/or FreeAndNil( ) rather than calling .destroy directly.