
Animation is integral to many parts of GUIs, including both the fancy effects when things change used in window managers, and of course games.   The core of any animation system is a scheme for periodically changing the display while still remaining responsive to the user.   This task demonstrates this.



Task

Create a window containing the string "Hello World! " (the trailing space is significant).

Make the text appear to be rotating right by periodically removing one letter from the end of the string and attaching it to the front.

When the user clicks on the (windowed) text, it should reverse its direction.




#include <stdlib.h>
#include <string.h>
#include <gtk/gtk.h>
 
const gchar *hello = "Hello World! ";
gint direction = -1;
gint cx=0;
gint slen=0;
 
GtkLabel *label;
 
void change_dir(GtkLayout *o, gpointer d)
{
  direction = -direction;
}
 
gchar *rotateby(const gchar *t, gint q, gint l)
{
  gint i, cl = l, j;
  gchar *r = malloc(l+1);
  for(i=q, j=0; cl > 0; cl--, i = (i + 1)%l, j++)
    r[j] = t[i];
  r[l] = 0;
  return r;
}
 
gboolean scroll_it(gpointer data)
{
  if ( direction > 0 )
    cx = (cx + 1) % slen;
  else
    cx = (cx + slen - 1 ) % slen;
  gchar *scrolled = rotateby(hello, cx, slen);
  gtk_label_set_text(label, scrolled);
  free(scrolled);
  return TRUE;
}
 
 
int main(int argc, char **argv)
{
  GtkWidget *win;
  GtkButton *button;
  PangoFontDescription *pd;
 
  gtk_init(&argc, &argv);
  win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
  gtk_window_set_title(GTK_WINDOW(win), "Basic Animation");
  g_signal_connect(G_OBJECT(win), "delete-event", gtk_main_quit, NULL);
 
  label = (GtkLabel *)gtk_label_new(hello);
 
  // since we shift a whole character per time, it's better to use
  // a monospace font, so that the shifting seems done at the same pace
  pd = pango_font_description_new();
  pango_font_description_set_family(pd, "monospace");
  gtk_widget_modify_font(GTK_WIDGET(label), pd);
 
  button = (GtkButton *)gtk_button_new();
  gtk_container_add(GTK_CONTAINER(button), GTK_WIDGET(label));
 
  gtk_container_add(GTK_CONTAINER(win), GTK_WIDGET(button));
  g_signal_connect(G_OBJECT(button), "clicked", G_CALLBACK(change_dir), NULL);
 
  slen = strlen(hello);
 
  g_timeout_add(125, scroll_it, NULL);
 
  gtk_widget_show_all(GTK_WIDGET(win));
  gtk_main();
  return 0;
}