How to create a cairo_t on a Gtk 2 window

Question

I have an old app, now compiling on Gtk 2, but I need to introduce the use of Cairo. I can't figure out how to create the necessary cairo context (cairo_t) from my Widgets.

Here's the code I'm trying to learn with so far, modified (*ahem* cribbed) from a Gtk 3 tutorial. The crux of the matter is creating a cairo surface from the window widget. As it stands, that call is a sketch and it won't compile, let alone run. Remember, my target is Gtk2, not 3, at least at this point.

/* Snitched from http://zetcode.com/gfx/cairo/cairobackends/ on 13 Jan 2014 */
#include <cairo.h>
#include <cairo-xlib.h>
#include <gtk/gtk.h>

int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *darea;
  cairo_surface_t *surface;
  cairo_t *cr;

  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  darea = gtk_drawing_area_new();
  gtk_container_add(GTK_CONTAINER(window), darea);

  g_signal_connect(window,          "destroy", G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), 400, 90);
  gtk_window_set_title(GTK_WINDOW(window), "GTK window");

  surface = cairo_xlib_surface_create(
      gtk_widget_get_display(window),   /* must be Display *, not struct GdkDisplay * */
      window,                           /* must be Drawable (no star) */
      gtk_widget_get_visual(window),    /* must be Visual * */
      gtk_widget_get_width(window),     /* int */
      gtk_widget_get_height(window)     /* int */
      );
  cr = cairo_create(surface);
  cairo_set_source_rgb(cr, 0, 0, 0);
  cairo_select_font_face(cr, "Sans", CAIRO_FONT_SLANT_NORMAL, 
     CAIRO_FONT_WEIGHT_NORMAL);
  cairo_set_font_size(cr, 40.0);

  cairo_move_to(cr, 10.0, 50.0);
  cairo_show_text(cr, "Discipline is power.");

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}

Answer 1

I hope the tutorial did not do cairo drawing in main()... The meaningful place to draw onto a GTK2 widget is in the expose-event (and if you want to force a redraw from somewhere else, just call gtk_widget_queue_draw()). It is easier to use gdk_cairo_create() to get a cairo context.

Something like this:

static gboolean
on_expose_event (GtkWidget *widget,
                 GdkEventExpose *event,
                 gpointer data)
{
    cairo_t *cr;

    cr = gdk_cairo_create (gtk_widget_get_window (widget));
    cairo_move_to (cr, 30, 30);
    cairo_show_text (cr, "Text");

    cairo_destroy (cr);

    return FALSE;
}

g_signal_connect(darea, "expose-event",
                 G_CALLBACK(on_expose_event), NULL);

Jan Bodnar has a more complete example (in the end).

This is all a lot nicer in GTK3 in my opinion. Still, even if your goal is to port to GTK3 it may make sense to change the drawing to use cairo first as you're doing -- changing to GTK3 afterwards should just simplify the code.