Announcing guile-gobject

[Martin Baulig <martin at home-of-linux dot org>]

Hi guys,

about a week ago I started some little hacking project because I felt
like I must hack something to feel good.

The result of this hacking week is now in GNOME CVS in the `guile-gobject'
module, right in time before GUADEC ...

Unfortunately, I don't have much time to write what guile-gobject is and
how it is different from guile-gtk since I need to leave to GUADEC soon,
but maybe we can have some little BOF about it there.

It's basically a new set of scheme / guile bindings for GNOME 2.0 which is
based on gobject. The main difference to guile-gtk is that it is using goops
and g-wrap and that it's based on gobject.

Basically, with guile-gobject you don't need any C compiled glue code to
access arbitrary GObject's and other glib/gobject based types such as
GEnum and GFlags. This all works with very little C code, most of the stuff
is done in scheme.

The other big advantage of guile-gobject is that it's a two-way wrapper;
you cannot only access a GObject which is defined in some C library from
Guile, you can also write your own GObjects in Guile and access them from C.

This code is still very experimental and unfinished, I just hacked on it for
about a week or so.

Here's some example code:

====
(use-modules (gobject gobject) (oop goops))

;; Assuming your C code has a GObject `Foo' and there's
;; a C function foo_get_type ().
(define-class <foo> (<gobject>))
(define foo (make <foo>))

;; Assuming there's a GObject `Bar' and a scheme function
;; (bar-get-type) returning Bar's GType.
(define bar (make <gobject> #:type (bar-get-type)))

;; Derive `Bar', creating your own `SuperBar' GObject in scheme
(define super-bar-type (g-type-create-object (bar-get-type) 'SuperBar '()))
(define super-bar-class (g-object-class-from-type super-bar-type))
(define super-bar (make <gobject> #:type super-bar-type))

;; Assuming `Foo' has a boolean `foo property
(g-object-get-property foo 'foo)
(g-object-set-property foo 'foo #t)

;; Assuming `Foo' has a signal 'hello' which takes a boolean and a float
;; argument and returns a long
(define retval (g-object-signal-emit foo 'hello #t 3.45))
(display retval) (newline) ;; this will be a long

;; Now let's connect something to the 'hello' signal:
(g-object-signal-connect foo 'hello
  (lambda (object a b)
    (display (list "HELLO" object a b "END")) (newline)
    305))

;; Add a new boolean `super' property to the `SuperBar' GObject
(g-object-class-install-property super-bar-class
  (make <gparam-spec-wrapper> #:name 'super #:value-type g-type-boolean))
====

So all glue code that's needed is for C functions, but the whole object
management can be done directly in Scheme. The following isn't finished
yet, but basically it'll be possible to do something like this:

====
;; Assume you have a C function `void test_func (GObject *object)' and its
;; scheme wrapper, you can create your own GObject and pass it to this
;; function

(define super-bar-type (g-type-create-object g-type-object 'SuperBar '()))
(define super-bar-class (g-object-class-from-type super-bar-type))
(define super-bar-get-type (lambda () super-bar-type))

;; This will assume there is a `super-bar-get-type' function
(define-class <super-bar> (<gobject>))

;; You can also say
(define-class <super-bar> (<gobject>) #:type super-bar-type)

;; This create a new instance of `SuperBar'.
(define super-bar (make <super-bar>))

;; You can also use this without creating a class:
(define super-bar (make <gobject> #:type super-bar-type))

;; (test-func) takes a GObject, so let's pass it our `SuperBar':
(test-func super-bar)
====

For the C part, I also had a new and much simpler idea how to wrap a
GObject and how to do the reference counting.

For each GObject, there's a GObjectWrapper:

===
struct _GuileGObject {
    int ref_count;
    gboolean floating;

    size_t size;

    void (*finalize) (gpointer);

    SCM scm_class;
};

struct _GObjectWrapper {
    GuileGObject guile;

    GObject *object;
};
===

This wrapper is created on-the-fly when a GObject is about to be passed to
Scheme code and it'll only stay around as long as it's needed:

* when a GObject is passed to Scheme code, a new GObjectWrapper object is
  created and the GObject ref'ed.

* the GObjectWrapper is freeded during garbage collecting which will cause
  the GObject to be unref'ed.

So when we create a new GObject, it'll be "owned" by its GObjectWrapper and
the unref during garbage collecting will cause it to be destroyed.

* when we call a C function from scheme which takes a GObject, we pass the
  GObject itself to C and not its Scheme wrapper; the GObject doesn't know
  anything about its GObjectWrapper so that the wrapper can safely be freed
  during garbage collecting even if the GObject is still in use (since the C
  code must ref the object anyways because it's owned by the caller).

This also works when we create our own GObjects in scheme:

* (g-type-create-object) basically calls g_type_register_static() to register
  a new type for the derived object (with the same class and instance sizes
  as its parent type) and uses g_type_set_qdata() to assign some per-class
  data with the new type. It provides a custom class_init function for the new
  type which will override the `get_property', `set_property' etc. functions.

-- 
Martin Baulig
martin@gnome.org (private)
baulig@suse.de (work)