4.3. An Upgraded Hello World

Let's take a look at a slightly improved helloworld with better examples of callbacks. This will also introduce us to our next topic, packing widgets.

#include <gtk/gtk.h>

/* Our new improved callback.  The data passed to this function
 * is printed to stdout. */
static void callback( GtkWidget *widget,
                      gpointer   data )
{
    g_print ("Hello again - %s was pressed\n", (gchar *) data);
}

/* another callback */
static gboolean delete_event( GtkWidget *widget,
                              GdkEvent  *event,
                              gpointer   data )
{
    gtk_main_quit ();
    return FALSE;
}

int main( int   argc,
          char *argv[] )
{
    /* GtkWidget is the storage type for widgets */
    GtkWidget *window;
    GtkWidget *button;
    GtkWidget *box1;

    /* This is called in all GTK applications. Arguments are parsed
     * from the command line and are returned to the application. */
    gtk_init (&argc, &argv);

    /* Create a new window */
    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);

    /* This is a new call, which just sets the title of our
     * new window to "Hello Buttons!" */
    gtk_window_set_title (GTK_WINDOW (window), "Hello Buttons!");

    /* Here we just set a handler for delete_event that immediately
     * exits GTK. */
    g_signal_connect (G_OBJECT (window), "delete_event",
		      G_CALLBACK (delete_event), NULL);

    /* Sets the border width of the window. */
    gtk_container_set_border_width (GTK_CONTAINER (window), 10);

    /* We create a box to pack widgets into.  This is described in detail
     * in the "packing" section. The box is not really visible, it
     * is just used as a tool to arrange widgets. */
    box1 = gtk_hbox_new (FALSE, 0);

    /* Put the box into the main window. */
    gtk_container_add (GTK_CONTAINER (window), box1);

    /* Creates a new button with the label "Button 1". */
    button = gtk_button_new_with_label ("Button 1");
    
    /* Now when the button is clicked, we call the "callback" function
     * with a pointer to "button 1" as its argument */
    g_signal_connect (G_OBJECT (button), "clicked",
		      G_CALLBACK (callback), (gpointer) "button 1");

    /* Instead of gtk_container_add, we pack this button into the invisible
     * box, which has been packed into the window. */
    gtk_box_pack_start (GTK_BOX(box1), button, TRUE, TRUE, 0);

    /* Always remember this step, this tells GTK that our preparation for
     * this button is complete, and it can now be displayed. */
    gtk_widget_show (button);

    /* Do these same steps again to create a second button */
    button = gtk_button_new_with_label ("Button 2");

    /* Call the same callback function with a different argument,
     * passing a pointer to "button 2" instead. */
    g_signal_connect (G_OBJECT (button), "clicked",
		      G_CALLBACK (callback), (gpointer) "button 2");

    gtk_box_pack_start(GTK_BOX (box1), button, TRUE, TRUE, 0);

    /* The order in which we show the buttons is not really important, but I
     * recommend showing the window last, so it all pops up at once. */
    gtk_widget_show (button);

    gtk_widget_show (box1);

    gtk_widget_show (window);
    
    /* Rest in gtk_main and wait for the fun to begin! */
    gtk_main ();

    return 0;
}

Compile this program using the same linking arguments as our first example. You'll notice this time there is no easy way to exit the program, you have to use your window manager or command line to kill it. A good exercise for the reader would be to insert a third "Quit" button that will exit the program. You may also wish to play with the options to gtk_box_pack_start() while reading the next section. Try resizing the window, and observe the behavior.