PangoAttribute is not a GType type and I seem to need to reinvent GType here...

Basic way to summarize the missing feature is: pango hardcodes some attribute types in code, basically all the predefined attribute types.  A user-define attribute never has a chance to be treated similar to PANGO_ATTR_SHAPE for example.  I want to fix that.

Concrete usecase for this is: currently PANGO_ATTR_SHAPE is very hard to use, much harder than it needs to be.  I added pango_cairo_context_set_shape_renderer() which greatly simplifies that, so one doesn't have to implement a PangoRenderer to use shape attributes meaningfully anymore (ok, one didn't have to, see GtkTextView for example), but the biggest caveat still remains: It's hard for custom renders in libraries (like gdkpango) to add useful shape renderers (like rendering a GdkPixbuf) without taking full control of ATTR_SHAPE out of user's hand.



Instead of bandaiding that around ATTR_SHAPE I want to fix it properly (to the extent possible by compatibility).  Here is my proposal:


  - Currently PangoAttrType's are assigned to predefined attributes from 0 (in the PangoAttrType enum) and allocated from 1000 up for custom attributes, in pango_attr_type_register().  We logically divide that integer "type" value into two parts: lowest byte as base type, three other bytes as instance type.


  - New API for creating new instances of other attribute types.  We can only support shallow one-level subtyping here but that's all we care about.  Proposed API:

PangoAttrType    pango_attr_type_register_similar (const gchar   *name,
                                                   PangoAttrType  similar_type);

Not sure about the name... What this does is to set the lower byte of the returned attr type to the lower byte of the base_type.

And a getter:

PangoAttrType    pango_attr_type_get_base_type (PangoAttrType  attr_type);

Base types are defined as those defined by Pango.  An attribute registered using pango_attr_type_register() has base type PANGO_ATTR_INVALID.


  - New API for attribute type similarity checking.  This checks that the lower byte of attribute types for equality, but this is just the implementation detail and private, publicly just a function:

gboolean        pango_attr_type_similar (PangoAttrType first_type,
                                         PangoAttrType second_type);

And a convenience one for actual attributes:

gboolean        pango_attr_similar_to (PangoAttribute *attr,
                                       PangoAttrType   other_type);

Note that this test only means that the type of attr and other_type have the same base type.  attr is not necessarily of type other_type.  As I said, shallow typing only...


  - Change Pango internals to always check for attribute base type instead of exact type.  For example, in pango-renderer.c, instead of this:

      switch (attr->klass->type)
        {
        case PANGO_ATTR_SHAPE:
          if (shape_attr)
            *shape_attr = (PangoAttrShape *)attr;
          break;

        ...
        }

We'll have:

      switch (pango_attr_type_get_base_type (attr->klass->type))
        {
        case PANGO_ATTR_SHAPE:
          if (shape_attr)
            *shape_attr = (PangoAttrShape *)attr;
          break;

        ...
        }


  - pango_cairo_context_set_shape_renderer() only sets shape renderer for ATTR_SHAPE.  Add new API for setting shape renderer for custom shape attributes:

void
pango_cairo_context_set_shape_renderer_for_attr_type
                                       (PangoContext                *context,
                                        PangoAttrType                attr_type,
                                        PangoCairoShapeRendererFunc  func,
                                        gpointer                     data,
                                        GDestroyNotify               dnotify)
{
  g_return_if_fail (pango_attr_type_get_base_type (attr_type) != PANGO_ATTR_SHAPE);

  ...
}


That's all.  With all that, we can then add the following gdkpango API:

PangoAttribute *
gdk_pango_attribute_pixbuf_new (GdkPixbuf  *pixbuf,
                                int width, int height, ...);

Then in gdk_pango_context_get(), register shape renderer for the type GDK_PANGO_ATTR_PIXBUF on the context being returned.

This way they user is free to use ATTR_SHAPE as they wish.
The only problem with this being that all shape renderers need to be set on all PangoContext's.  That's not totally infeasible, but is a bit of a burden.  To fix that, we can provide PangoCairoRenderer API for that too, when we subclass PangoRenderer in gdkpango.


Owen, Matthias, comments?