#include <stdlib.h>
#include <gtk/gtk.h>
#include <cairo.h>

/* compile with
 * gcc -o inputshapedemo `pkg-config --libs --cflags gtk+-3.0` inputshapedemo.c
 */

static void screen_changed (GtkWidget *widget, GdkScreen *old_screen, gpointer user_data);
static gboolean draw (GtkWidget *widget, cairo_t *cr, gpointer user_data);


#define LATE_INPUT_SHAPE

static void
screen_changed (GtkWidget *widget,
                GdkScreen *old_screen,
                gpointer   userdata)
{
  GdkScreen *screen = gtk_widget_get_screen(widget);
  GdkVisual *visual = gdk_screen_get_rgba_visual(screen);

  if (visual)
    {
      gtk_widget_set_visual (widget, visual);
    }
  else
    {
      g_printerr ("Your screen does not support alpha channels!\n");
      exit (-1);
    }
}


static gboolean
draw (GtkWidget *widget,
      cairo_t   *cr,
      gpointer   userdata)
{
  gint width, height;

#ifdef LATE_INPUT_SHAPE
  static gboolean have_input_shape = FALSE;

  if (!have_input_shape)
    {
      cairo_rectangle_int_t rect = { 0, 0, 0, 0 };
      cairo_region_t *reg = cairo_region_create_rectangle (&rect);

      gtk_widget_input_shape_combine_region (widget, reg);
      cairo_region_destroy (reg);

      have_input_shape = TRUE;
    }
#endif

  cairo_set_source_rgba (cr, 1.0, 1.0, 1.0, 0.0); /* transparent */

  cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
  cairo_paint (cr);

  gtk_window_get_size (GTK_WINDOW (widget), &width, &height);

  cairo_set_source_rgba (cr, 1, 0.2, 0.2, 0.5);
  cairo_arc (cr,
             width / 2, height / 2,
             MIN (width, height) / 2 - 8 , 0, 2 * 3.14);
  cairo_fill(cr);

  return FALSE;
}


int
main (int   argc,
      char *argv[])
{
  GtkWindow *window;

  gtk_init (&argc, &argv);

  window = GTK_WINDOW (gtk_window_new (GTK_WINDOW_TOPLEVEL));
  gtk_window_set_title (window, "Alpha Demo");
  gtk_window_resize (window, 300, 300);
  gtk_window_set_position (window, GTK_WIN_POS_CENTER);


  gtk_widget_set_app_paintable (GTK_WIDGET (window), TRUE);

  g_signal_connect (window, "delete-event", gtk_main_quit, NULL);
  g_signal_connect (window, "draw", G_CALLBACK (draw), NULL);
  g_signal_connect (window, "screen-changed", G_CALLBACK (screen_changed), NULL);

  gtk_window_set_decorated (window, FALSE);

  if (!gdk_display_supports_input_shapes (gdk_display_get_default ()))
    {
      g_printerr ("shapes are unsupported - aborting.\n");
      exit (-1);
    }

  screen_changed (GTK_WIDGET (window), NULL, NULL);

  gtk_widget_realize (GTK_WIDGET (window));

#ifndef LATE_INPUT_SHAPE
  cairo_rectangle_int_t rect = { 0, 0, 0, 0 };
  cairo_region_t *reg = cairo_region_create_rectangle (&rect);

  gtk_widget_input_shape_combine_region (GTK_WIDGET (window), reg);
  cairo_region_destroy (reg);
#endif

  /* Run the program */
  gtk_widget_show_all (GTK_WIDGET (window));

  gtk_main ();

  return 0;
}