To: wakita at is.titech.ac.jp
Cc: lablgtk at kaba.or.jp
Subject: Re: reusing button class
In-Reply-To: <20010104162842A.wakita at is.titech.ac.jp>
References: <20010104162842A.wakita at is.titech.ac.jp>
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Message-Id: <20010105115319P.garrigue at kurims.kyoto-u.ac.jp>
Date: Fri, 05 Jan 2001 11:53:19 +0900
From: Jacques Garrigue <garrigue at kurims.kyoto-u.ac.jp>
Lines: 89

From: Ken Wakita <wakita at is.titech.ac.jp>

> I tried to define [image_button] class that contain an icon and an
> optional label.  It was easy to implement a function that make a
> button with an icon.
> 
> Then I tried to make a lablgtk style object-oriented class definition
> and I was in a trouble.  Lablgtk style seems to require defining both
> the class and constructor function.  However, initialization code by
> using low-level interface makes this difficult to do in a handy
> manner.  So I pushed the iniitalization code in the initializer of the
> class definition and obtained attached below.

> class button ?label ?border_width ?width ?height ?packing ?show () =
>   let obj = GtkButton.Button.create ?label () in
>   object (self)
>     inherit GButton.button_skel (GtkButton.Button.coerce obj)
>     method connect = new GButton.button_signals obj
>     method event = new GObj.event_ops obj
>     initializer
>       GtkBase.Container.set obj ?border_width ?width ?height;
>       ignore (GObj.pack_return self ?packing ?show)
>   end

Class and constructors are not distinguished so much for good style,
but rather for technical reasons. LablGTK classes are just wrappers to
raw GTK widgets, and they must have a constructor taking the raw widget
as argument, since you do not only wrap newly created widgets, but
also those returned by methods, or manually casted ones.
Then the problem is that ocaml objects have only one constructor.
So in the old beta1 version of lablGTK, there were actually two
classes for each widget: one acting just as a wrapper, called
[button_wrapper], and another one defined very much like you are doing
here, called [button]. However this meant a bit of gymnastic to hide
the [button_wrapper] type, and have [new button_wrapper w] return an
object of type [button]. Also this appeared to be a cause of code
bloat, which is not so nice when what you are doing is just
interfacing to the real library.
Still there was something nice about it: according to olabl optional
semantics, [new button] would return a button, without need to write
[new button ()] like with a function. But optional semantics changed
with ocaml 3, and I went back to a simpler approach, using a normal
function as constructor.
Another point was of course that you could inherit from the [button]
class more naturally, as you state here. However I have grown more and
more convinced than inheritance is generally not what you want. That
is, inheritance is not a way to organize a library (that should be
subtyping, at best), but just to throw around your code.

So here is how I would write you image_button example in the new
lablgtk style.
class image_button ~file ?(label = "")
    ?border_width ?width ?height ?packing ?show () =
  let button = GButton.button ?border_width ?width ?height ?packing ?show () in
  let box = GPack.vbox ~packing:button#add () in
  let pm = GMisc.pixmap (GDraw.pixmap_from_xpm ~window: box ~file ())
      ~packing:box#add in
  let label = GMisc.label ~text: label ~packing () in
  object
    inherit GButton.button (GtkButton.Button.cast button#as_widget)
    method label = label
    method pixmap = pm
  end

In this specific case, it is natural to inherit from button, but in
most cases I would even avoid it, and write

class image_button ... =
  ...
  object
    inherit GObj.widget button#coerce
    method connect = new GButton.button_signals obj
    method event = new GObj.event_ops obj
    method button = button
    method pixmap = pm
    method label = label
  end

I don't try to separate class and constructor here, since this is not
really relevant. If you want to add it, you still don't need to
beta-expand
   let image_button = new image_button

Cheers, and Happy New Year,

        Jacques
---------------------------------------------------------------------------
Jacques Garrigue      Kyoto University     garrigue at kurims.kyoto-u.ac.jp
		<A HREF=http://wwwfun.kurims.kyoto-u.ac.jp/~garrigue/>JG</A>