I want to ask about Delphi deign and object linking so I understand

1

A TWinControl descendant (TEdit) is always going to want to sit on a canvas parent
Or should I say this the other way around - A TWinControl descendant (TEdit) how can you use a TWinControl descendant object without a canvas containing parent?
I’m saying this TWinControl descendant object is effectively only a screen object to the user so it needs a parent with a canvas to use it - no other use.
TWinControl does not have a canvas but it has to sit over a canvas or why are you uising a TWinControl descendant object? is it possible to use it for something else?

A form is to hold a list of TComponets - example dialog box’s so it puts every component in it’s Tlist that is dropped on its form and TWinControl is a descendant of TComponet like every other TComponet descendant on the form.
So we have a create(AOwner: TComponet); with everything dropped on a form

If its a TWinControl descendant object you cannot drop a TWinControl descendant object onto a TComponet - impossible task and a task without any purpose because the user needs to see the TWinControl descendant object to see it any way!!!

So why doesn’t a TWinControl descendant object use its create(AOwner: TComponet); as it currently is - and in the create code put

If  AOwner is TWinControl then parent := TWinControl(AOwner);

Then parent := TForm; is not required programiclly in the code
old code can run the update but its not required any more

Another way is in the aftercreation to then ask
If AOwner is TWinControl then parent := TWinControl(AOwner); or
search though AOwner and AOwner’s parents to find aTCustomForm object then parent := TWinControl(AOwner);

I’m saying what is the point to put in code parent := AOwner; ?
can I be understood by this question

procedure TLBImage.AfterConstruction;
begin
  inherited;
  Parent := TWinControl(Owner);
end;

this code works but do I need any safety margins for any reason
example If Owner is TWinControl then Parent := TWinControl(Owner);
I cannot see the need of a safety margin ?

2

You asked why doesn’t a TWinControl automatically set it’s Parent to match its Owner?

It’s because you don’t always want them to be one and the same (though it sounds like you do in your component, which is perfectly fine).

For example take “auto-created” forms, that is forms created in your DPR source code with a call to Application.CreateForm().

TForm is ultimately derived from TWinControl but when a form is “auto-created”, it is created with an Owner of the Vcl.Forms.Application singleton instance of TApplication (a TComponent descendant).

Also if I remember correctly every control you place at design time on a form is created with an Owner set to the TForm it sits on, but a Parent of the containing TWinControl.

If I could recommend a great book for all these fundamental inner workings of the VCL and components, try and get your hands on a copy of Delphi Component Design by Danny Thorpe.

Danny Thorpe was one of the original engineers working on the Delphi development team and this book is universally regarded as the best source for understanding the building blocks of the VCL and RTL. Looks like it’s quite affordable now but 10 years or so ago you had to pay a lot of money to buy a copy on the second hand market.

3

So I’m pulling out the archives of Danny Thorpe

I totally agree with this
Also if I remember correctly every control you place at design time on a form is created with an Owner set to the TForm it sits on, but a Parent of the containing TWinControl.
The TWinControl descendant to make TCustomForm has TList listing all the forms Component’s on the TForm. Even though a example TEdit.create(AOwner: TComponet); is asking for a TComponet to link into the TCustomForm’s TList to be recorded.
Its always got to be a TWinControl descendant to be in the example TEdit.create(AOwner: TComponet); Never in Delphi can it link into a TComponet.
So why is it not writtern example TEdit.create(AOwner: TWinControl); and TCustomForm’s TList see it as a TComponet and the example TEdit has installed

procedure TLBImage.AfterConstruction;
begin
  inherited;
  Parent := WinControl(Owner);
end;

and TEdit.create(AOwner: TWinControl);

TEdit.create(AOwner: TWinControl);
Begin
  inherited create(TComponet(AOwner));
4

I think you’re asking, why does TForm.Components exist when you have TForm.Controls?

So TForm inherits its Components list from TComponent.Components and its Controls list from TWinControl.Controls.

It’s true that TForm.Controls is useful more often than TForm.Components. But that doesn’t mean that TForm.Components is useless.

What if you want to iterate across all TComponents on your form to find non-visual components such as database components or even semi-visual components like TMenu? You won’t find either of those in TForm.Controls.

Now think of a TDataModule. Nothing on that design surface is visual, it doesn’t even descend from TWinControl. On a datamodule the TDataModule.Components list is your only way of iterating through the components on that form.

So the two different lists exist side by side for the flexibility of being able to support both visual and non-visual components.

5

This general approach is fine but I’d suggest the following for the case that somebody passes in a non-TWinControl as the Owner to your TLBImage.

procedure TLBImage.AfterConstruction;
begin
  inherited;
  if Owner is TWinControl then
    Parent := TWinControl(Owner);
end;

I don’t have a good answer as to why it isn’t done this way. The original Delphi design team probably just thought it was unnecessary.

Incidentally you don’t need to do that cast when calling the inherited constructor. The following would compile just fine.

constructor TMyEdit.Create(AOwner: TWinControl);
begin
  inherited Create(AOwner);
6

I have always understood what you have just said
I’m more talking the installing of a TWinControl to a TForm and simplifying it
Its just come to mind I wonder if AfterConstruction was working correctly on Delphi 7?

7

any way I have a number of screen components I can create programmicly
they only need the create to make and place on a the form
They center them selves and come to the fount
They disappear and replaced with a move object for the user to move in the form
I have a few more objects to make yet - but is looking good