[Lazarus] TFrame improvements

[Martin Frb]

On 30/11/2021 00:27, Juha Manninen via lazarus wrote:
> On Mon, Nov 29, 2021 at 11:14 PM Ondrej Pokorny via lazarus 
> <lazarus at lists.lazarus-ide.org> wrote:
>
>     On 29.11.2021 17:18, Juha Manninen via lazarus wrote:
>>     The commit message is not perfect but the committed code is, now
>>     that I fully understand the issue.
>
>     That is nonsense. I reverted your change. The code user code is
>     just plain wrong and your change in TFrame doesn't change anything
>     about it.
>
> No, the code is valid although not recommended. Accessing Canvas 
> outside Paint works with some widgetsets and then an exception is wrong.
>

There are 2 (or 3) question.

1) Is the fix correct: Yes (if 2 or 3), otherwise "not applicable"

2) Is it a bug in first? (and therefore is a fix needed): No.
No (at least if it is at runtime) it is not a bug, because it is by 
design that a frame needs a form as parent.

3) Before the patch, ignoring the design time issues => did it work at 
runtime? (And is it indented to?)
When (without the patch) the stream is read at runtime, and the frame is 
embedded in a Form, does the example code then work?
Basically, will at runtime the parent be set, before any properties are 
loaded?
(Or, Is that documented that it should?)

=> *If* this works at runtime (and if this is by intention), then there 
is a bug in the designer, in that the designer fails to set the parent 
(parent = designer) in time.

I have not tested the example from the bug. But I did a quick debug of a 
TFrame being loaded, at runtime:
- The TFrame is loading twice from lfm (the frames own lfm, and the 
changeds stored in the form)
-- the first loading, there is no parent set => so it would/could fail.
-- the 2nd loading, there is a parent

The exact "expected" (not tested) behaviour for the app from the bug 
would be:
- If the offending property has a default, and is not changed from the 
default => all good, property is not set during TReader
- If the default is changed in the Frame's lfm => crash, because the 
property is set, when there is no parent.
- if the default is only changed for the frame's instance on the form 
(stored in the form's lfm) => ok

So IMHO it is save to say, that this is not supported at runtime either.
Should it be? IMHO not, see below.

-----------------------
This leaves improving the designer, to add a better error message, and 
handle it more gracefully.


-----------------------

Otherwise Ondrej is right that code accessing the handle (and that 
includes canvas) must do checks.
And TFrame is not the only place were it this is needed.

   b := TPanel.Create(Form1);
   a := TGraphicControl.Create(b);

   //a.Parent := b;
   //b.Parent := Form1;

   a.Canvas.Line(1,1,2,2);

You must set both parents, or it crashes.
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