#include <gtk/gtk.h>

#define DRAG_THRESHOLD 20

typedef struct _appcontext AppContext;
struct _appcontext {
  GtkWidget *window;
  GtkWidget *swin;
  GtkWidget *view;
  GtkWidget *vbox;
  
  GtkAdjustment *vadj;
  gboolean       in_drag;
  gdouble        start_drag_y;
  gdouble        start_drag_vadj;
};


/* record current scrolled window position / mouse coordinate on click */
gboolean
button_drag_begin (GtkWidget       *widget,
                   GdkEventButton  *event,
                   gpointer         user_data) 
{
  AppContext *context = (AppContext *) user_data;
  
  context->in_drag = FALSE;
  context->start_drag_y = event->y_root;
  context->start_drag_vadj = gtk_adjustment_get_value (context->vadj);
  
  return FALSE;
}


/*
 * on motion with button pressed (GDK_BUTTON_MOTION_MASK) check if we started
 * scrolling (y-distance > DRAG_THRESHOLD) and update vadj accordingly
 */
gboolean
button_drag_motion (GtkWidget       *widget,
                    GdkEventMotion  *event,
                    gpointer         user_data) 
{
  AppContext *context = (AppContext *) user_data;
  gdouble cur_val;
  gdouble cur_y;
  
  cur_y = event->y_root;
  if (ABS (context->start_drag_y - cur_y) > DRAG_THRESHOLD)
    context->in_drag = TRUE;
  
  if (!context->in_drag)
    return FALSE;
  
  cur_val = gtk_adjustment_get_value (context->vadj);
  gtk_adjustment_set_value (context->vadj,
                            CLAMP (context->start_drag_vadj +
                                   context->start_drag_y - cur_y,
                                   context->vadj->lower,
                                   context->vadj->upper - context->vadj->page_size));
                            
  return FALSE;
}


/* for buttons we might need to stop the "clicked" signal emission to
 * avoid further handlers to run. After scrolling we don't want to activate
 * the button */
void
button_drag_end (GtkWidget       *widget,
                 gpointer         user_data) 
{
  AppContext *context = (AppContext *) user_data;
  
  if (context->in_drag)
    g_signal_stop_emission_by_name (widget, "clicked");
  
  context->in_drag = FALSE;
}


/* just some sample callback for the buttons. Note that these need to be
 * connected *after* the previous callback */
void
button_activity (GtkWidget       *widget,
                 gpointer         user_data) 
{
  AppContext *context = (AppContext *) user_data;
  GtkWidget *dlg;
  
  dlg = gtk_dialog_new_with_buttons ("pressed!\n",
                                     GTK_WINDOW (context->window),
                                     GTK_DIALOG_MODAL,
                                     GTK_STOCK_OK,
                                     GTK_RESPONSE_ACCEPT,
                                     NULL);
  gtk_widget_new (gtk_label_get_type (),
                  "label", gtk_button_get_label (GTK_BUTTON (widget)),
                  "visible", TRUE,
                  "parent", gtk_dialog_get_content_area (GTK_DIALOG (dlg)),
                  NULL);
  
  gtk_dialog_run (GTK_DIALOG (dlg));
  gtk_widget_destroy (dlg);
}



int
main (int argc, char *argv[])
{
  AppContext *context;
  GtkWidget *w;
  int i;
  
  gtk_init (&argc, &argv);

  context = g_new0 (AppContext, 1);
  
  /* top level window */
  context->window = GTK_WIDGET (gtk_window_new (GTK_WINDOW_TOPLEVEL));
  gtk_window_set_default_size (GTK_WINDOW (context->window), 400, 400);
  g_signal_connect (G_OBJECT (context->window), "delete-event",
                    G_CALLBACK (gtk_main_quit), NULL);

  /* a scrolled window with a viewport for the button-vbox */
  context->swin = gtk_widget_new (gtk_scrolled_window_get_type(),
                                  "vscrollbar-policy", GTK_POLICY_ALWAYS,
                                  "hscrollbar-policy", GTK_POLICY_NEVER,
                                  "parent", context->window,
                                  NULL);
  
  context->view = gtk_widget_new (gtk_viewport_get_type(),
                                  "parent", context->swin,
                                  NULL);
  /* motion/press handlers for the viewport */
  g_signal_connect (G_OBJECT (context->view), "button-press-event",
                    G_CALLBACK (button_drag_begin), context);
  g_signal_connect (G_OBJECT (context->view), "motion-notify-event",
                    G_CALLBACK (button_drag_motion), context);
  gtk_widget_add_events (GTK_WIDGET (context->view),
                         GDK_BUTTON_MOTION_MASK);
  
  /* the vbox containing a lot of buttons */
  context->vbox = gtk_widget_new (gtk_vbox_get_type(),
                                  "spacing", 10,
                                  "parent", context->view,
                                  NULL);
  
  context->vadj = gtk_scrolled_window_get_vadjustment (GTK_SCROLLED_WINDOW (context->swin));
  
  for (i = 0; i < 50; i++)
    {
      gchar *label;
      
      label = g_strdup_printf ("Button No. %d", i+1);
      w = gtk_widget_new (gtk_button_get_type(),
                          "label", label,
                          "parent", context->vbox,
                          NULL);
      /* first connect the signal handlers for the scroll handling */
      
      g_signal_connect (G_OBJECT (w), "button-press-event",
                        G_CALLBACK (button_drag_begin), context);
      g_signal_connect (G_OBJECT (w), "motion-notify-event",
                        G_CALLBACK (button_drag_motion), context);
      g_signal_connect (G_OBJECT (w), "clicked",
                        G_CALLBACK (button_drag_end), context);
      gtk_widget_add_events (GTK_WIDGET (w),
                             GDK_BUTTON_MOTION_MASK);
      
      /* *then* connect the GUI functionality */
      g_signal_connect (G_OBJECT (w), "clicked",
                        G_CALLBACK (button_activity), context);
      g_free (label);
    }
  
  gtk_widget_show_all (GTK_WIDGET (context->window));

  gtk_main ();

  return 0;

}