Creating New Items

Creating New Items — how to create new canvas items.

How to Create New Canvas Items

Here's an example of how to create a new simple canvas item. (Note that this item doesn't have a model-view split.)

Here's the header file - a fairly standard GObject header file. Our item derives from GooCanvasItemSimple and just has its own x, y, width and height variables: 

  /*
   * GooCanvas Demo. Copyright (C) 2006 Damon Chaplin.
   * Released under the GNU LGPL license. See COPYING for details.
   *
   * demo-item.c - a simple demo item.
   */
  #ifndef __GOO_DEMO_ITEM_H__
  #define __GOO_DEMO_ITEM_H__
  
  #include <gtk/gtk.h>
  #include "goocanvasitemsimple.h"
  
  G_BEGIN_DECLS
  
  
  #define GOO_TYPE_DEMO_ITEM            (goo_demo_item_get_type ())
  #define GOO_DEMO_ITEM(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), GOO_TYPE_DEMO_ITEM, GooDemoItem))
  #define GOO_DEMO_ITEM_CLASS(klass)    (G_TYPE_CHECK_CLASS_CAST ((klass), GOO_TYPE_DEMO_ITEM, GooDemoItemClass))
  #define GOO_IS_DEMO_ITEM(obj)         (G_TYPE_CHECK_INSTANCE_TYPE ((obj), GOO_TYPE_DEMO_ITEM))
  #define GOO_IS_DEMO_ITEM_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GOO_TYPE_DEMO_ITEM))
  #define GOO_DEMO_ITEM_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), GOO_TYPE_DEMO_ITEM, GooDemoItemClass))
  
  
  typedef struct _GooDemoItem       GooDemoItem;
  typedef struct _GooDemoItemClass  GooDemoItemClass;
  
  struct _GooDemoItem
  {
    GooCanvasItemSimple parent_object;
  
    gdouble x, y, width, height;
  };
  
  struct _GooDemoItemClass
  {
    GooCanvasItemSimpleClass parent_class;
  };
  
  
  GType               goo_demo_item_get_type  (void) G_GNUC_CONST;
  
  GooCanvasItem*      goo_demo_item_new       (GooCanvasItem      *parent,
                                               gdouble             x,
                                               gdouble             y,
                                               gdouble             width,
                                               gdouble             height,
                                               ...);
  
  
  G_END_DECLS
  
  #endif /* __GOO_DEMO_ITEM_H__ */

And here's the source file: 

  /*
   * GooCanvas Demo. Copyright (C) 2006 Damon Chaplin.
   * Released under the GNU LGPL license. See COPYING for details.
   *
   * demo-item.c - a simple demo item.
   */
  #include <config.h>
  #include "goocanvas.h"
  #include "demo-item.h"
  
  
  /* Use the GLib convenience macro to define the type. GooDemoItem is the
     class struct, goo_demo_item is the function prefix, and our class is a
     subclass of GOO_TYPE_CANVAS_ITEM_SIMPLE. */
  G_DEFINE_TYPE (GooDemoItem, goo_demo_item, GOO_TYPE_CANVAS_ITEM_SIMPLE)
  
  
  /* The standard object initialization function. */
  static void
  goo_demo_item_init (GooDemoItem *demo_item)
  {
    demo_item->x = 0.0;
    demo_item->y = 0.0;
    demo_item->width = 0.0;
    demo_item->height = 0.0;
  }
  
  
  /* The convenience function to create new items. This should start with a 
     parent argument and end with a variable list of object properties to fit
     in with the standard canvas items. */
  GooCanvasItem*
  goo_demo_item_new (GooCanvasItem      *parent,
                     gdouble             x,
                     gdouble             y,
                     gdouble             width,
                     gdouble             height,
                     ...)
  {
    GooCanvasItem *item;
    GooDemoItem *demo_item;
    const char *first_property;
    va_list var_args;
  
    item = g_object_new (GOO_TYPE_DEMO_ITEM, NULL);
  
    demo_item = (GooDemoItem*) item;
    demo_item->x = x;
    demo_item->y = y;
    demo_item->width = width;
    demo_item->height = height;
  
    va_start (var_args, height);
    first_property = va_arg (var_args, char*);
    if (first_property)
      g_object_set_valist ((GObject*) item, first_property, var_args);
    va_end (var_args);
  
    if (parent)
      {
        goo_canvas_item_add_child (parent, item, -1);
        g_object_unref (item);
      }
  
    return item;
  }
  
  
  /* The update method. This is called when the canvas is initially shown and
     also whenever the object is updated and needs to change its size and/or
     shape. It should calculate its new bounds in its own coordinate space,
     storing them in simple->bounds. */
  static void
  goo_demo_item_update  (GooCanvasItemSimple *simple,
                         cairo_t             *cr)
  {
    GooDemoItem *demo_item = (GooDemoItem*) simple;
  
    /* Compute the new bounds. */
    simple->bounds.x1 = demo_item->x;
    simple->bounds.y1 = demo_item->y;
    simple->bounds.x2 = demo_item->x + demo_item->width;
    simple->bounds.y2 = demo_item->y + demo_item->height;
  }
  
  
  /* The paint method. This should draw the item on the given cairo_t, using
     the item's own coordinate space. */
  static void
  goo_demo_item_paint (GooCanvasItemSimple   *simple,
                       cairo_t               *cr,
                       const GooCanvasBounds *bounds)
  {
    GooDemoItem *demo_item = (GooDemoItem*) simple;
  
    cairo_move_to (cr, demo_item->x, demo_item->y);
    cairo_line_to (cr, demo_item->x, demo_item->y + demo_item->height);
    cairo_line_to (cr, demo_item->x + demo_item->width,
                   demo_item->y + demo_item->height);
    cairo_line_to (cr, demo_item->x + demo_item->width, demo_item->y);
    cairo_close_path (cr);
    goo_canvas_style_set_fill_options (simple->simple_data->style, cr);
    cairo_fill (cr);
  }
  
  
  /* Hit detection. This should check if the given coordinate (in the item's
     coordinate space) is within the item. If it is it should return TRUE,
     otherwise it should return FALSE. */
  static gboolean
  goo_demo_item_is_item_at (GooCanvasItemSimple *simple,
                            gdouble              x,
                            gdouble              y,
                            cairo_t             *cr,
                            gboolean             is_pointer_event)
  {
    GooDemoItem *demo_item = (GooDemoItem*) simple;
  
    if (x < demo_item->x || (x > demo_item->x + demo_item->width)
        || y < demo_item->y || (y > demo_item->y + demo_item->height))
      return FALSE;
  
    return TRUE;
  }
  
  
  /* The class initialization function. Here we set the class' update(), paint()
     and is_item_at() methods. */
  static void
  goo_demo_item_class_init (GooDemoItemClass *klass)
  {
    GooCanvasItemSimpleClass *simple_class = (GooCanvasItemSimpleClass*) klass;
  
    simple_class->update        = goo_demo_item_update;
    simple_class->paint         = goo_demo_item_paint;
    simple_class->is_item_at    = goo_demo_item_is_item_at;
  }