Re: Glib / GObject newbie

[muppet <scott asofyet org>]

this list is for development *of* gtk+ and gobject, not *with* them.   
your question is more appropriate for gtk-list or gtk-app-devel-list.

that said...


On Apr 25, 2005, at 6:36 AM, Laurent JALLET wrote:

> that's what I want to implement using GObjects :
> . a Camera containing a position ( => Vector3 : a GObject containing 3  
> gfloats) and other members


> and I'd like to write
> static void
> camera_get_property (GObject *object,  guint property_id,  const  
> GValue *value, GParamSpec   *pspec)
> {
>       Camera *self = (Camera *) object;
>
>        switch (property_id) {
>        case CAMERA_FOV: {
>                g_value_set_float (value, self->priv->mFov);
>        }
>                break;
>
>        case CAMERA_POSITION :
>        // There is my problem
>        // I don't know what to do
> How can I transform a GValue into a GObject ?
> And how do I use it ?

   g_value_set_object (value, self->priv->mPosition);

This takes a reference on the GObject and places the pointer into the  
GValue.  When the GValue is unset, the reference will be released.   
Code that fetches the value with g_object_get (camera, "position",  
&position, NULL); should g_object_ref() the position for as long as it  
wants to keep it.

http://developer.gnome.org/doc/API/2.0/gobject/gobject-Standard- 
Parameter-and-Value-Types.html#g-value-set-object


> Of course there's the same problem with the camera_set_property  
> function.

http://developer.gnome.org/doc/API/2.0/gobject/gobject-Standard- 
Parameter-and-Value-Types.html#g-value-get-object

    /* if the new position is not the same as the old one, take it. */
    Vector3 * new_position = g_value_get_object (value);
    if (new_position != camera->priv->mPosition) {
        if (camera->priv->mPosition) {
            /* disconnect from old */
            g_object_unref (camera->priv->mPosition);
        }
        if (new_position) {
            /* connect to new */
            g_object_ref (new_position);
        }
        camera->priv->mPosition = new_position;
    }

it's usually best to add accessor methods like camera_get_position()  
and camera_set_position(), and have your get_property()/set_property()  
implementations call the accessors.


> I don't really understand what GValue is and how I can use it.

there are plenty of examples in the gtk+ source.  you don't need to be  
using or implementing gtk+ in order to learn from its source.


--
The door is locked.  I tried to open it, but the lock is harder to pick  
than a broken nose.
  -- Sensei, on 'I, Ninja'