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This chapter explains how to write a GnomeCanvasItem. Custom canvas items
allows you to extend the canvas; consider writing a canvas
item if the stock items (or some combination of them placed
in a GnomeCanvasGroup) do
not meet your needs. As an example, the chapter describes
the implementation of
GnomeCanvasRect.
To write a
GnomeCanvasItem, you create a concrete
implemenation of the
GnomeCanvasItem abstract base class. This chapter
assumes you've read the
chapter called The GTK+ Object and Type System
and understand how a
GtkObject works; you will need to understand
objects to write your own.
Canvas items can support GDK mode, antialiased mode, or
both. The canvas has a flag indicating which kind of
canvas it is; items can check this flag at runtime:
if (item->canvas->aa)
{
/* antialiased mode */
}
else
{
/* Gdk mode */
}
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However, most code will be the same for both canvas
types. The only real difference is in the drawing
process: GDK mode draws to a pixmap, antialiased mode
draws to an RGB buffer. You do not have to support both
kinds of canvas, just be careful not to use your item
with the unsupported canvas type.
Here is the
GnomeCanvasItem type you will be subclassing:
typedef struct _GnomeCanvasItem GnomeCanvasItem;
typedef struct _GnomeCanvasItemClass GnomeCanvasItemClass;
struct _GnomeCanvasItem {
GtkObject object;
/* Canvas we are on */
GnomeCanvas *canvas;
/* Parent group */
GnomeCanvasItem *parent;
/* Bounding box for this item */
double x1, y1, x2, y2;
/* If NULL, the identity transform */
double *xform;
};
struct _GnomeCanvasItemClass {
GtkObjectClass parent_class;
void (* update) (GnomeCanvasItem *item, double *affine,
ArtSVP *clip_path, int flags);
void (* realize) (GnomeCanvasItem *item);
void (* unrealize) (GnomeCanvasItem *item);
void (* map) (GnomeCanvasItem *item);
void (* unmap) (GnomeCanvasItem *item);
/* Unused in Gnome 1.0 */
ArtUta *(* coverage) (GnomeCanvasItem *item);
/* Used only in Gdk mode */
void (* draw) (GnomeCanvasItem *item, GdkDrawable *drawable,
int x, int y, int width, int height);
/* Used only in RGB mode */
void (* render) (GnomeCanvasItem *item, GnomeCanvasBuf *buf);
double (* point) (GnomeCanvasItem *item, double x, double y,
int cx, int cy,
GnomeCanvasItem **actual_item);
/* Obsolete; not used in Gnome 1.0 */
void (* translate) (GnomeCanvasItem *item, double dx, double dy);
/* Deprecated, but occasionally used in Gnome 1.0 */
void (* bounds) (GnomeCanvasItem *item,
double *x1, double *y1,
double *x2, double *y2);
/* The only canvas item class function that is also a signal */
gint (* event) (GnomeCanvasItem *item, GdkEvent *event);
};
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This chapter explains everything in more detail; keep
reading.
GnomeCanvasRect and
GnomeCanvasEllipse have
almost identical implementations; in fact, all but
three of
GnomeCanvasItem's methods are implemented by the
GnomeCanvasRE base
class. GnomeCanvasRE
handles their user-visible interface, as discussed in
the previous chapter.
To understand the implementation of GnomeCanvasRect presented in this
chapter, you should first read the previous chapter's
discussion of the object arguments it supports. You'll
also want to see the object itself:
typedef struct _GnomeCanvasRE GnomeCanvasRE;
struct _GnomeCanvasRE {
GnomeCanvasItem item;
double x1, y1, x2, y2; /* Corners of item, item coordinates */
double width; /* Outline width, item coordinates */
guint fill_color; /* Fill color, RGBA */
guint outline_color; /* Outline color, RGBA */
gulong fill_pixel; /* Fill color */
gulong outline_pixel; /* Outline color */
GdkBitmap *fill_stipple; /* Stipple for fill */
GdkBitmap *outline_stipple; /* Stipple for outline */
GdkGC *fill_gc; /* GC for filling */
GdkGC *outline_gc; /* GC for outline */
/* Antialiased specific stuff follows */
ArtSVP *fill_svp; /* The SVP for the filled shape */
ArtSVP *outline_svp; /* The SVP for the outline shape */
/* Configuration flags */
unsigned int fill_set : 1; /* Is fill color set? */
unsigned int outline_set : 1; /* Is outline color set? */
unsigned int width_pixels : 1;/* Is outline width specified in pixels or units? */
};
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GnomeCanvasRect adds no
new members not found in
GnomeCanvasRE. The method implementations
discussed in this chapter should make clear the purpose
of the various struct members.
This chapter discusses all the interesting parts of
GnomeCanvasRect;
complete source code comes with the Gnome libraries.
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