Re: newbie

[Owen Taylor <otaylor redhat com>]

<khosro@iglou.com> writes:

> Hi all,
> 
> Newbie with a question:  I modified/simplified the example code, provided
> with the gtk-tutorial.  My goal is to draw a simple window, in which 
> geometric figures are drawn sequentially, not stamped form a backing pixmap. 
> Now, the window appears, but the line doesn't!  Can someone help me? 
>
> TIA,
> 
> Khosro
> =================================================================
> here is the code:
> =================================================================
> 
> 
> /* GTK - The GIMP Toolkit
>  * Copyright (C) 1995-1997 Peter Mattis, Spencer Kimball and Josh MacDonald
>  *...
>  */
> 
> #include <gtk/gtk.h>
> 
> static GdkPixmap *pixmap = NULL;
> 
> void
> quit ()
> {
>   gtk_exit (0);
> }
> 
> int
> main (int argc, char *argv[])
> {
>   GtkWidget *window;
>   GtkWidget *drawing_area;
>   GtkWidget *vbox;
>   GtkWidget *widget;
>   GtkWidget *button;
> 
>   
>   gtk_init (&argc, &argv);
> 
>   window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
> 
>   vbox = gtk_vbox_new (FALSE, 0);
>   gtk_container_add (GTK_CONTAINER (window), vbox);
> 
>   gtk_signal_connect (GTK_OBJECT (window), "destroy",
> 		      GTK_SIGNAL_FUNC (quit), NULL);
> 
>   /* Create the drawing area */
> 
>   drawing_area = gtk_drawing_area_new ();
>   gtk_drawing_area_size (GTK_DRAWING_AREA (drawing_area), 200, 200);
>   gtk_box_pack_start (GTK_BOX (vbox), drawing_area, TRUE, TRUE, 0);
> 
>   
>   widget = drawing_area;
>   
>   pixmap = gdk_pixmap_new(widget->window,
> 			  widget->allocation.width,
> 			  widget->allocation.height,
> 			  -1);

widget->window will be NULL here, since the widget
hasn't been "realized" yet. Your better off doing:
   
   pixmap = gdk_pixmap_new(NULL,
 			   widget->allocation.width,
 			   widget->allocation.height,
 			   gtk_widget_get_visual (widget)->depth);

>   gdk_draw_rectangle (pixmap,
> 		      widget->style->white_gc,
> 		      TRUE,
> 		      0, 0,
> 		      widget->allocation.width,
> 		      widget->allocation.height);
> 
>   gdk_draw_line (pixmap,
>                  widget->style->black_gc,
>                  100, 50, 50, 100);               /** ??? ***/
> 
>   gdk_draw_pixmap(widget->window,
>                       widget->style->fg_gc[GTK_WIDGET_STATE (widget)],
>                       pixmap,
>                       0,0,
>                       0,0,
>                       widget->allocation.width,
> 		      widget->allocation.height);

To draw widget->window and have it appear, it must be non-NULL (at
this point the widget still hasn't been realized), the
widget and all its parents must have been shown, and the
X server must have got around to mapping the window on
the screen. Because of the last condition, just showing
a widget and immediately drawing on it doesn't work.
You can use gtk_widget_show_now() which waits for the window
to be mapped, or probably better, just do your drawing
in a "expose_event" handler.

Take a look at the "scribble" section of the tutorial for
a working example of code that draws on a backing pixmap
and then copies that to the screen.

>   gtk_widget_show (drawing_area);
> 
>   gtk_widget_show (vbox);
> 
>   gtk_widget_show (button);
> 
>   gtk_widget_show (window);

'gtk_widget_show_all (window)' would be more compact. ;-)

                                        Owen