Why the Timer doesn't work? (about g_timeout_add())

["Lazy Fox" <lazy fox wu gmail com>]

I copy my source files "CurveWindow.h" and "testCurveWindow.cpp" below.
And attach them in this mail. You can compile them by:
$ g++ testCurveWindow.cpp -o testCurveWindow `pkg-config --cflags
--libs gtk+-2.0`

In this application, I create an 1 second timer to draw a moving line.
But in the function timer_1_test(), the timer is run only once, and
then stopped.
See the comment in this function.

The following is my two source files:

/***************************************************************************
 *              Filename : CurveWindow.h
 *                   Begin : 2008-08-14 08:46:43
 *                 Project : CurveWindow
 *                  Author : Wu Yin
 ***************************************************************************/
#include <iostream>
#include <gtk/gtk.h>
 using namespace std;

 class CurveWindow
 {
         public:
                 CurveWindow(){};
                 GtkWidget *create_drawing_win();
                 GtkWidget *get_root_widget();
                 int draw();
                 GdkPixmap *pixmap;

         private:
                 GtkWidget *drawing;
 };
//========================================================


/* Create a new backing pixmap of the appropriate size */
static gboolean configure_event( GtkWidget *widget, GdkEvent *event,
gpointer data)
{
        CurveWindow *self = (CurveWindow *)data;
        GdkColor color;
        GdkGC *gc = gdk_gc_new(widget->window);

        self->pixmap = gdk_pixmap_new(widget->window,
                        widget->allocation.width,
                        widget->allocation.height,
                        -1);
        // Draw the background
        gdk_draw_rectangle (self->pixmap,
                        widget->style->white_gc,
                        TRUE,
                        0, 0,
                        widget->allocation.width,
                        widget->allocation.height);
        return TRUE;
}

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
static gboolean redraw(GtkWidget *widget, GdkEventExpose *event, gpointer data)
{
printf("expose-event \n");
        CurveWindow *self = (CurveWindow *)data;
        gdk_draw_drawable (widget->window,
                        widget->style->fg_gc[GTK_WIDGET_STATE (widget)],
                        self->pixmap,
                        event->area.x, event->area.y,
                        event->area.x, event->area.y,
                        event->area.width, event->area.height);
        return TRUE;
}

/***************************************************************************
 * add by Wu Yin, 2008-08-19
 ***************************************************************************/
int CurveWindow::draw()
{
        GtkWidget *widget = this->get_root_widget();
        static int y = 0;

        y += 10;
        gdk_draw_line(pixmap,
                widget->style->fg_gc[GTK_WIDGET_STATE(widget)],
                0, 0, 120, y);
        gtk_widget_draw(widget, NULL);
        return 0;
}

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
GtkWidget *CurveWindow::get_root_widget()
{
        return drawing;
}

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
GtkWidget *CurveWindow::create_drawing_win()
{
        drawing = gtk_drawing_area_new();
        g_signal_connect(G_OBJECT(drawing), "expose-event",
                        G_CALLBACK(redraw), this);
        g_signal_connect (G_OBJECT (drawing),"configure_event",
                        G_CALLBACK (configure_event), this);
        return drawing;
}


/***************************************************************************
 *              Filename : testCurveWindow.cpp
 *                   Begin : 2008-08-14 08:46:43
 *                 Project : CurveWindow
 *                  Author : Wu Yin
 ***************************************************************************/
#include <gtk/gtk.h>
#include <iostream>
#include "CurveWindow.h"
using  namespace std;

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
static void timer_1_test(gpointer data)
{
        CurveWindow *cw = (CurveWindow *)data;
        cw->draw();

/* !!! Look Here !!!
 * Uncomment each statement of the following, the timer will run
correctly, WHY?
 */
//      printf("timer_1_test() \n");
//      gtk_widget_draw(cw->get_root_widget(), NULL);
}

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
static void create_gui()
{
        GtkWidget *drawing_win;
        GtkWidget *root_win;
        GtkWidget *root_table;
        CurveWindow *cw;
        GdkColor bg, fg;

        // Create root_window
        root_win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
        gtk_window_set_default_size(GTK_WINDOW(root_win), 800, 600);
        g_signal_connect(GTK_OBJECT(root_win), "destroy",
G_CALLBACK(gtk_main_quit), NULL);

        // Create root table container
        root_table = gtk_table_new(10, 10, TRUE);
        gtk_container_add(GTK_CONTAINER(root_win), root_table);

        // Create Drawing place 1, and add to the root table
        cw = new CurveWindow();
        drawing_win = cw->create_drawing_win();
        g_timeout_add(1000, (GSourceFunc)timer_1_test, cw);
        gtk_table_attach_defaults(GTK_TABLE(root_table), drawing_win, 0, 2, 0, 2);

        gtk_widget_show_all(root_win);
}

/***************************************************************************
 * add by Wu Yin, 2008-08-14 08:55:09
 ***************************************************************************/
int main(int argc, char *argv[])
{
        gtk_init(&argc, &argv);
        create_gui();
        gtk_main();
        return 0;
}