how to save Drawing area's content after resizing window at cairo

Question

how can I change the below code so the contents and drawings at the Drawing-area won't clean after changing window's size.
I want to copy smaller surface content to top left corner of the bigger content
and for copying bigger surface to smaller surface there should be some data loss
code is from gtk+-3.0 reference manual

#include <gtk/gtk.h>

/* Surface to store current scribbles */

static cairo_surface_t *surface = NULL;

static void
clear_surface (void)
{
  cairo_t *cr;

  cr = cairo_create (surface);

  cairo_set_source_rgb (cr, 1, 1, 1);
  cairo_paint (cr);

  cairo_destroy (cr);
}

/* Create a new surface of the appropriate size to store our scribbles */
static gboolean
configure_event_cb (GtkWidget         *widget,
                    GdkEventConfigure *event,
                    gpointer           data)
{
  if (surface)
    cairo_surface_destroy (surface);

  surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget),
                                               CAIRO_CONTENT_COLOR,
                                               gtk_widget_get_allocated_width (widget),
                                               gtk_widget_get_allocated_height (widget));

  /* Initialize the surface to white */
  clear_surface ();

  /* We've handled the configure event, no need for further processing. */
  return TRUE;
}

/* Redraw the screen from the surface. Note that the ::draw
 * signal receives a ready-to-be-used cairo_t that is already
 * clipped to only draw the exposed areas of the widget
 */
static gboolean
draw_cb (GtkWidget *widget,
         cairo_t   *cr,
         gpointer   data)
{
  cairo_set_source_surface (cr, surface, 0, 0);
  cairo_paint (cr);

  return FALSE;
}

/* Draw a rectangle on the surface at the given position */
static void
draw_brush (GtkWidget *widget,
            gdouble    x,
            gdouble    y)
{
  cairo_t *cr;

  /* Paint to the surface, where we store our state */
  cr = cairo_create (surface);

  cairo_rectangle (cr, x - 3, y - 3, 6, 6);
  cairo_fill (cr);

  cairo_destroy (cr);

  /* Now invalidate the affected region of the drawing area. */
  gtk_widget_queue_draw_area (widget, x - 3, y - 3, 6, 6);
}

/* Handle button press events by either drawing a rectangle
 * or clearing the surface, depending on which button was pressed.
 * The ::button-press signal handler receives a GdkEventButton
 * struct which contains this information.
 */
static gboolean
button_press_event_cb (GtkWidget      *widget,
                       GdkEventButton *event,
                       gpointer        data)
{
  /* paranoia check, in case we haven't gotten a configure event */
  if (surface == NULL)
    return FALSE;

  if (event->button == GDK_BUTTON_PRIMARY)
    {
      draw_brush (widget, event->x, event->y);
    }
  else if (event->button == GDK_BUTTON_SECONDARY)
    {
      clear_surface ();
      gtk_widget_queue_draw (widget);
    }

  /* We've handled the event, stop processing */
  return TRUE;
}

/* Handle motion events by continuing to draw if button 1 is
 * still held down. The ::motion-notify signal handler receives
 * a GdkEventMotion struct which contains this information.
 */
static gboolean
motion_notify_event_cb (GtkWidget      *widget,
                        GdkEventMotion *event,
                        gpointer        data)
{
  /* paranoia check, in case we haven't gotten a configure event */
  if (surface == NULL)
    return FALSE;

  if (event->state & GDK_BUTTON1_MASK)
    draw_brush (widget, event->x, event->y);

  /* We've handled it, stop processing */
  return TRUE;
}

static void
close_window (void)
{
  if (surface)
    cairo_surface_destroy (surface);

  gtk_main_quit ();
}

int
main (int   argc,
      char *argv[])
{
  GtkWidget *window;
  GtkWidget *frame;
  GtkWidget *da;

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title (GTK_WINDOW (window), "Drawing Area");

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

  gtk_container_set_border_width (GTK_CONTAINER (window), 8);

  frame = gtk_frame_new (NULL);
  gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN);
  gtk_container_add (GTK_CONTAINER (window), frame);

  da = gtk_drawing_area_new ();
  /* set a minimum size */
  gtk_widget_set_size_request (da, 100, 100);

  gtk_container_add (GTK_CONTAINER (frame), da);

  /* Signals used to handle the backing surface */
  g_signal_connect (da, "draw",
                    G_CALLBACK (draw_cb), NULL);
  g_signal_connect (da,"configure-event",
                    G_CALLBACK (configure_event_cb), NULL);

  /* Event signals */
  g_signal_connect (da, "motion-notify-event",
                    G_CALLBACK (motion_notify_event_cb), NULL);
  g_signal_connect (da, "button-press-event",
                    G_CALLBACK (button_press_event_cb), NULL);

  /* Ask to receive events the drawing area doesn't normally
   * subscribe to. In particular, we need to ask for the
   * button press and motion notify events that want to handle.
   */
  gtk_widget_set_events (da, gtk_widget_get_events (da)
                             | GDK_BUTTON_PRESS_MASK
                             | GDK_POINTER_MOTION_MASK);

  gtk_widget_show_all (window);

  gtk_main ();

  return 0;
}

Answer 1

Change your configure_event_cb() function to this:

/* Create a new surface of the appropriate size to store our scribbles */
static gboolean
configure_event_cb (GtkWidget         *widget,
                    GdkEventConfigure *event,
                    gpointer           data)
{
  cairo_surface_t *old_surface = surface;
  cairo_t *cr;

  surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget),
                                               CAIRO_CONTENT_COLOR,
                                               gtk_widget_get_allocated_width (widget),
                                               gtk_widget_get_allocated_height (widget));

  /* Initialize the surface to white */
  clear_surface ();

  if (old_surface) {
    /* Copy over the old surface */
    cr = cairo_create (surface);
    cairo_set_source_surface (old_surface, 0, n0);
    cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
    cairo_paint (cr);
    cairo_destroy (cr);
    cairo_surface_destroy (old_surface);
  }

  /* We've handled the configure event, no need for further processing. */
  return TRUE;
}