How to make an attribute list?

[Ken Resander <kresander yahoo com>]

I am on Ubuntu 12.04 LTS using GTK 3 and Cairo.
My application outputs graphs, diagrams and text and the user can configure the output by changing style values, a bit like CSS.

The program needs 'text-in-a-box' with line wrapping and optional jusification. I have written the code that collects stylenames and attribute values, but not yet the placement and rendering functions. I noticed that pango provides this in exactly the way I need, so I want to use that instead.

My program uses a style changes array that points into a text array (offset and length used). I have never used pango before, but I believe I have to create a PangoAttrList and I am not sure how to do this. Here is my current guess:

For each stylechange:
-  use the pango_attr_xxxx_new to allocate and set attribute value in PangoAttribute * a
-  a->start_index = myoffset; a->end_index = myoffset+mylength;

The documentation mentions a pango_attribute_init. If I have to use that I would not know how to, for example where do I get the copy, and equal function addresses from and how can I set the klass.type e.g. to PANGO_ATTR_FOREGROUND which is const (compiler complains).

Here is a fragment from the code:

typedef enum
   {
   TXTCOL_TP ,
   STYLE_TP ,
   WEIGHT_TP ,
   FFLIST_TP ,
   FONTSIZE_TP ,
   BGCOL_TP ,
   //... more
   } VALTYPE;

typedef struct
   {
   unsigned short r ;
   unsigned short g ;
   unsigned short b ;
   } COLOUR ;

typedef struct
   {
   int offset;  // where property starts in text array
   int len;     // length (in chars) of property in text
   VALTYPE valtp;
   union
      {
      COLOUR txtcol; // text colour
      int style;     // normal, oblique, italic
      int weight;    // fontweight 100,200,300,.800,900
      int fflist;    // position in text pool of font family list
      int fontsize;  // in pts
      //.... more
      };
   } STYLEVALUE;

static int numchgs ;     // number style changes in text
static STYLEVALUE chgs [ MAXNUMCHGS ] ;
static char text [ MAXSZTEXT ] ;  // text with style changes given by chgs array

PangoAttrList * makeattrlist()
   {
   PangoAttrList * al = pango_attr_list_new();
   PangoAttribute * a ; // common and type-dependent part
   STYLEVALUE * sv ;
   int k = 0 ;
   while ( k < numchgs )
      {
      sv = &chgs [ k ] ;
      switch ( sv->valtp )
         {
         case TXTCOL_TP:
            //pango_attribute_init (???); // do I have to use this? how?
            a = pango_attr_foreground_new(sv->txtcol.r, sv->txtcol.g, sv->txtcol.b );
            a->klass.type = PANGO_ATTR_FOREGROUND ; // compiler complains about this
            a->start_index = sv->offset;
            a->end_index = a->start_index + sv->len;
            pango_attr_list_insert(al, a);
            break;
         case STYLE_TP:
            // same uncertainties...
            a = pango_attr_style_new( (PangoStyle)sv->style );
            a->klass.type =  PANGO_ATTR_STYLE ;  // compiler complains
            a->start_index = sv->offset;
            a->end_index = a->start_index + sv->len;
            pango_attr_list_insert(al, a);
            break;
         //case WEIGHT_TP:
         //case FFLIST_TP:
         //case FONTSIZE_TP:
         }
      k++;
      }
   return al ;
   }

static gboolean on_expose_event (GtkWidget *darea, GdkEventExpose *event, gpointer user_data)
   {
   cairo_t * cr = gdk_cairo_create(gtk_widget_get_window(darea));
   PangoLayout * layout = pango_cairo_create_layout (cr);

   PangoAttrList * attrs = makeattrlist();
   pango_layout_set_attributes(layout, attrs);

   pango_layout_set_alignment(layout, PANGO_ALIGN_LEFT);
   pango_layout_set_width ( layout , 180*PANGO_SCALE ) ;
   pango_layout_set_wrap ( layout , PANGO_WRAP_WORD ) ;

   pango_layout_set_text (layout, text, -1); // or should it be pango_layout_set_markup?

   cairo_move_to (cr, 100, 30);
   pango_cairo_show_layout (cr, layout);

   g_object_unref (layout);

   return FALSE ;
   }


Am I on the right track? Would be grateful for advice.

Ken