[Lazarus] A change in LCL require your TCustomForm descendants to have a resource or to use a CreateNew constructor

[Vincent Snijders]

2011/2/2 Paul Ishenin <ip at kmiac.ru>:
> Hello, Lazarus mailing list.
>
> I've changed the behavior of TCustomForm.Create constructor. Now it raises
> an exception if your TCustomForm descendant has no resource.
>
> 1. Why the change is needed.
>
> Historically TCustomForm has 2 constructors: Create() - for creating forms
> from resources and CreateNew() - for resourceless forms. When IDE creates a
> new form it creates a new unit (.pas file) and a new resource (.lfm file).
> IDE also adds {$R *.lfm} directive into the form unit file to attach the
> form resource into the executable. During the runtime TCustomForm.Create
> searches for resource and construct itself based on information from that
> resource. But what can happen if it can't find an appopriate resource? You
> will see an empty form. I think most of you will spend plenty of time trying
> to understand why do you see an empty form if you or someone else
> accidentally remove the {$R *.lfm} directive. So absence of the form
> resource is an exceptional situation and LCL must report an exception. LCL
> already report an exception if you have no resource for TFrame and
> TDataModule descendants.
>
> On the other hand there are some situations when you want to construct a
> form in code without any resources. How to be in this case? You must use
> CreateNew() constructor - the difference between Create and CreateNew only
> in resource search. Moreover Create constructor also calls CreateNew inside.
>
> 2. How this change can affect your applications.
>
> This change is applied only for new projects created by the Lazarus IDE. It
> does not affect your old projects.
>
> 3. Should you change anything in your application.
>
> Yes, please do. If you are using Create() constructor to construct your
> resourceless forms - you are doing a mistake. Please replace it with
> CreateNew(). This will also speedup your form construction because no
> resource will be searched and no resource will be accidentally loaded. To
> simplify the search of accidental Create() constructor calls set
> "RequireDerivedFormResource" variable to True and test how your application
> works. If you see no exceptions - then you are doing things right.
>
> 4. How to change the behavior back.
>
> The behavior is controlled by a global variable *RequireDerivedFormResource*
> which is set to True for all new applications created by the IDE and is set
> to False by default. If you for some reason want the old behavior just
> remove "RequireDerivedFormResource := True" line from .lpr file of your new
> application.

What about adding TApplication.CreateNewFrom?:
http://bugs.freepascal.org/bug_view_advanced_page.php?bug_id=19678

Is that a necessary extension?

Vincent