Official GTK4 example 3 (https://docs.gtk.org/gtk4/getting_started.html)

example-3.c

/** example-3.c	
 *
 * COMPILE
 *
 * gcc `pkg-config --cflags gtk4` -o example-3 example-3.c `pkg-config --libs gtk4`
 *
 * EXAMPLE
 *
 * ./example-3
 *
 * SOURCE
 *
 * https://docs.gtk.org/gtk4/getting_started.html
 */

#include <gtk/gtk.h>

#if GLIB_CHECK_VERSION(2, 74, 0)
static int app_flags = G_APPLICATION_DEFAULT_FLAGS;
#else
static int app_flags = G_APPLICATION_FLAGS_NONE;
#endif

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);
}

static void
resize_cb (GtkWidget *widget,
           int width,	
           int height,
           gpointer data)
{
	if (surface)
	  {
	    cairo_surface_destroy (surface);
	    surface = NULL;
	  }
	
	if (gtk_native_get_surface (gtk_widget_get_native (widget)))
	  {
	  	surface = gdk_surface_create_similar_surface (gtk_native_get_surface (gtk_widget_get_native (widget)),
							     CAIRO_CONTENT_COLOR,
							     gtk_widget_get_width (widget),
							     gtk_widget_get_height (widget));
	  	clear_surface ();
	  }
}

static void
draw_cb (GtkDrawingArea *drawing_area,
         cairo_t *cr,
	 int width,
	 int height,
	 gpointer data)
{
  cairo_set_source_surface (cr, surface, 0, 0);
  cairo_paint (cr);
}

static void
draw_brush (GtkWidget *widget,
            double x,
	    double y)
{
  cairo_t *cr;

  cr = cairo_create (surface);

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

  cairo_fill (cr);

  cairo_destroy (cr);

  gtk_widget_queue_draw (widget);
}

static double start_x;
static double start_y;

static void
drag_begin (GtkGestureDrag *gesture,
	    double x,
            double y,
	    GtkWidget *area)
{
  start_x = x;
  start_y = y;

  draw_brush (area, x, y);
}

static void
drag_update (GtkGestureDrag *gesture,
             double x,
	     double y,
	     GtkWidget *area)
{
  draw_brush (area, start_x + x, start_y + y);
}

static void
drag_end (GtkGestureDrag *gesture,
	  double x,
          double y,
	  GtkWidget *area)
{
  draw_brush (area, start_x + x, start_y + y);
}

static void
pressed(GtkGestureClick *gesture,
	double x,
        double y,
	GtkWidget *area)
{
  clear_surface ();
  gtk_widget_queue_draw (area);
}

static void 
close_window(GtkApplication *app,
	     gpointer user_data)
{
  if (surface)
    cairo_surface_destroy (surface);
}

static void
activate (GtkApplication *app,
          gpointer user_data)
{
  GtkWidget *window, *frame, *drawing_area, *drag, *press;
  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Drawing Area");
  g_signal_connect (window, "destroy", G_CALLBACK (close_window), NULL);
  frame = gtk_frame_new (NULL);

  gtk_window_set_child (GTK_WINDOW (window), frame);

  drawing_area = gtk_drawing_area_new ();

  gtk_widget_set_size_request (drawing_area, 100, 100);

  gtk_frame_set_child (GTK_FRAME (frame), drawing_area);
  gtk_drawing_area_set_draw_func (GTK_DRAWING_AREA (drawing_area), draw_cb, NULL, NULL);
  g_signal_connect_after (drawing_area, "resize", G_CALLBACK (resize_cb), NULL);

  drag = GTK_WIDGET (gtk_gesture_drag_new ());
  gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (drag), GDK_BUTTON_PRIMARY);
  gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (drag));
  g_signal_connect(drag, "drag-begin", G_CALLBACK (drag_begin), drawing_area);
  g_signal_connect(drag, "drag-update", G_CALLBACK (drag_update), drawing_area);
  g_signal_connect(drag, "drag-end", G_CALLBACK (drag_end), drawing_area);

  press = GTK_WIDGET (gtk_gesture_click_new ());
  gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (press), GDK_BUTTON_SECONDARY);
  gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (press));
  g_signal_connect (press, "pressed", G_CALLBACK (pressed), drawing_area);

  gtk_widget_show (window);
}

int
main (int argc, char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", app_flags);
  g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}