Re: Questions about guile-gtk

[Marius Vollmer <mvo@zagadka.ping.de>]

tomas@fabula.de writes:

>   - I use the function  sgtk_build_args(), which, as I understand,
>     tries to convert an SCM keyword list into a GtkArg array to
>     set the properties of a Gtk object.
>     The third argument to it is a `protector', I don't understand
>     exactly its use (but it's usually passed the object, which is
>     annoying if it's the object just being created ;-)
>     What I'm supposed to do with this?

The 'protector' is used to combine the mark/sweep collector of Guile
with the refcounting collector of Gtk+.  Here is a shortish
explanation:

/* GtkObjects.

   GtkObjects are wrapped with a smob.  The smob of a GtkObject is
   called its proxy.  The proxy and its GtkObject are strongly
   connected; that is, the GtkObject will stay around as long as the
   proxy is referenced from Scheme, and the proxy will not be
   collected as long as the GtkObject is used from outside of Scheme.

   The lifetime of GtkObjects is controlled by a reference count,
   while Scheme objects are managed by a tracing garbage collector
   (mark/sweep).  These two techniques are made to cooperate like
   this: the pointer from the proxy to the GtkObject is reflected in
   the reference count of the GtkObject.  All proxies are kept in a
   list and those that point to GtkObjects with a reference count
   greater than the number of `internal' references are marked during
   the marking phase of the tracing collector.  An internal reference
   is one that goes from a GtkObject with a proxy to another GtkObject
   with a proxy.  We can only find a subset of the true internal
   references (because Gtk does not yet cooperate), but this should be
   good enough.

   By using this combination of tracing and reference counting it is
   possible to break the cycle that is formed by the proxy pointing to
   the GtkObject and the GtkObject pointing back.  It is
   straightforward to extend this to other kind of cycles that might
   occur.  For example, when connecting a Scheme procedure as a signal
   handler, the procedure is very likely to have the GtkObject that it
   is connected to in its environment.  This cycle can be broken by
   including the procedure in the set of Scheme objects that get
   marked when we are tracing GtkObjects with a reference count
   greater than the number of internal references.

   Therefore, each proxy contains a list of `protects' that are marked
   when the proxy itself is marked.  In addition to this, there is
   also a global list of `protects' that is used for Scheme objects
   that are somewhere in Gtk land but not clearly associated with a
   particular GtkObject (like timeout callbacks).

  */

Does this help?  Not specifying a protector will not put the SCM
objects on the global list.  This is not optimal, but it is still safe
in the sense that they might be freed too late, but not too early.

>   - How do I handle C functions with in/out parameters, that is,
>     pointers, like:
>     
>       gtk_canvas_get_scroll_region(GtcCanvas c,
>                         double *x1, double *y1, 
>                         double *x2, double *y2) 
> 
>     Again: is there something in guile-gtk to cope with it and didn't
>     I see it, or is it ``roll your own''?

There is a section in the README named "Composite Types and
`call-by-reference'".

> In the mid-term I think it's important to have an ``official'' bindings
> generator tool, just to attract good bindings...

Yes!

-- 
GPG: D5D4E405 - 2F9B BCCC 8527 692A 04E3  331E FAF8 226A D5D4 E405


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