[Lazarus] When do I need a component rather than a plain object?

[Bo Berglund]

On Mon, 15 Nov 2010 16:36:38 -0500, Steve Smith <stevie at collector.org>
wrote:

>Message: 5
>Date: Mon, 15 Nov 2010 16:32:34 +0100
>From: Bo Berglund <bo.berglund at gmail.com>
>Subject: Re: [Lazarus] When do I need a component rather than a plain
>    object?
>To: lazarus at lists.lazarus.freepascal.org
>Message-ID: <ruj2e61613s285b5pj9ieitbf2qld9d56k at 4ax.com>
>Content-Type: text/plain; charset=us-ascii
>
>On, 15 Nov 2010 16:32:34 +0100, Bo Berglund <bo.berglund at gmail.com>
>wrote:
>
>> According to what I have read in the WIKI you only install components
>> into Lazarus if they are visual and you really need them to be part of
>> a visual form.
>
>> In all other cases you should not install a component in the IDE,
>> instead you instantiate the objects from that component in code.
>
>You create and install a component, not because it is visual or 
>non-visual
>but because you wish to use it in the visual forms designer in Lazarus,
>just as in Delphi.  For example TTimer is not a visual component, but
>because you wish to add it to the component palette and add it to your
>form, it has to be a descendent of TComponent.

Yes I understand what you say, but why not create the TTimer in code
instead and not put it into a form where it will not show up in
runtime?
Of course my discussion is not about the built-in IDE components but
rather about self-developed non-visual components. These should NOT be
installed in the IDE and the corresponding objects created in code
instead. Then one gets more control (see below).

>If you are creating a visual component it should be a descendent of
>TControl or one of it's descendants.
>
>TPersistent is the base class that implements support for the streaming
>subsystem used by the forms designer and it's components to save their
>properties in the form file.
>
>If you don't wish any of these facilities, you can use TObject.

What I was getting at is this:
Any component that you want to put visually on a form requires that
component to be installed in the IDE, which also means that its
sources are stored within the Lazarus own folder tree.
But in a software development environment with many developers working
on many projects where these components are used there is a problem of
ensuring that the components are up to date within the projects. We
faced this many times when developing with Delphi because we fell into
the trap of creating "components" instead of ordinary classes for
stuff that had no visuality in teh programs. It was sort of a
convenience that you could drop for instance a logging component onto
your main form and then set its properties in the designer. But the
component had no visual part and should not have been a component in
the first place.

We stopped this and instead create normal classes based on TObject.
These can then be checked out into a project source tree very much
apart from the IDE folder tree. Here we have full control and can be
sure that we can tag all project files so that we can get back to an
old state when we need to fix user bug reports.
And the classes used in a project will be easily updated to the latest
state as part of the general project source update from CVS (yes we
use CVS...).
In the component case we always had to "know" that we also needed to
update the IDE components in a different place to get the latest and
we could not tag the project in one place and be sure that we tagged
all sources...

Now that we are stuck with a lot of component stuff from old times we
have stopped installing them in the IDE. We just use tham as normal
classes and we create them as needed in the project start code and it
works just fine. The sources are checked out into each project that
use them and this means that a tag on the project will hit them too.
So we can recreate the exact situation for a user version that has a
bug and fix it.

Of course this holds for all components tha are non-visual, which most
of ours were. But we do have stuff that are visual too, and here we
have to install in teh IDE but then (in Delphi) we make sure that
there is no global path to the component sources so we can add the
component sources to the project module and check out as usual. Then
we set the project search path to include these local copies. It seems
to work but is a bit of a hassle.

-- 

Bo Berglund
Developer in Sweden