gtk_tut.sgml

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</verb></tscreen>

This is a packing call, which will be explained in depth later on in
<ref id="sec_packing_widgets" name="Packing Widgets">. But it is
fairly easy to understand. It simply tells GTK that the button is to
be placed in the window where it will be displayed. Note that a GTK
container can only contain one widget. There are other widgets, that
are described later, which are designed to layout multiple widgets in
various ways.
 
<tscreen><verb>
    gtk_container_add (GTK_CONTAINER (window), button);
</verb></tscreen>

Now we have everything set up the way we want it to be. With all the
signal handlers in place, and the button placed in the window where it
should be, we ask GTK to "show" the widgets on the screen. The window
widget is shown last so the whole window will pop up at once rather
than seeing the window pop up, and then the button form inside of
it. Although with such a simple example, you'd never notice.

<tscreen><verb>
    gtk_widget_show (button);

    gtk_widget_show (window);
</verb></tscreen>

And of course, we call gtk_main() which waits for events to come from
the X server and will call on the widgets to emit signals when these
events come.

<tscreen><verb>
    gtk_main ();
</verb></tscreen>

And the final return. Control returns here after gtk_quit() is called.

<tscreen><verb>
    return (0);
</verb></tscreen>

Now, when we click the mouse button on a GTK button, the widget emits
a "clicked" signal. In order for us to use this information, our
program sets up a signal handler to catch that signal, which
dispatches the function of our choice. In our example, when the button
we created is "clicked", the hello() function is called with a NULL
argument, and then the next handler for this signal is called. This
calls the gtk_widget_destroy() function, passing it the window widget
as its argument, destroying the window widget. This causes the window
to emit the "destroy" signal, which is caught, and calls our destroy()
callback function, which simply exits GTK.

Another course of events is to use the window manager to kill the
window, which will cause the "delete_event" to be emitted. This will
call our "delete_event" handler. If we return TRUE here, the window
will be left as is and nothing will happen. Returning FALSE will cause
GTK to emit the "destroy" signal which of course calls the "destroy"
callback, exiting GTK.

<!-- ***************************************************************** -->
<sect>Moving On
<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>Data Types
<p>
There are a few things you probably noticed in the previous examples
that need explaining. The gint, gchar, etc. that you see are typedefs
to int and char, respectively, that are part of the GLlib system. This
is done to get around that nasty dependency on the size of simple data
types when doing calculations.

A good example is "gint32" which will be typedef'd to a 32 bit integer
for any given platform, whether it be the 64 bit alpha, or the 32 bit
i386. The typedefs are very straightforward and intuitive. They are
all defined in glib/glib.h (which gets included from gtk.h).

You'll also notice GTK's ability to use GtkWidget when the function
calls for an Object. GTK is an object oriented design, and a widget
is an object.

<!-- ----------------------------------------------------------------- -->
<sect1>More on Signal Handlers
<p>
Lets take another look at the gtk_signal_connect declaration.

<tscreen><verb>
gint gtk_signal_connect( GtkObject *object,
                         gchar *name,
                         GtkSignalFunc func,
                         gpointer func_data );
</verb></tscreen>

Notice the gint return value? This is a tag that identifies your
callback function. As stated above, you may have as many callbacks per
signal and per object as you need, and each will be executed in turn,
in the order they were attached.

This tag allows you to remove this callback from the list by using:

<tscreen><verb>
void gtk_signal_disconnect( GtkObject *object,
                            gint id );
</verb></tscreen>

So, by passing in the widget you wish to remove the handler from, and
the tag returned by one of the signal_connect functions, you can
disconnect a signal handler.

You can also temporarily disable signal handlers with the
gtk_signal_handler_block() and gtk_signal_handler_unblock() family of
functions.

<tscreen><verb>
void gtk_signal_handler_block( GtkObject *object,
                               guint      handler_id );

void gtk_signal_handler_block_by_func( GtkObject     *object,
                                       GtkSignalFunc  func,
                                       gpointer       data );

void gtk_signal_handler_block_by_data( GtkObject *object,
                                       gpointer   data );

void gtk_signal_handler_unblock( GtkObject *object,
                                 guint      handler_id );

void gtk_signal_handler_unblock_by_func( GtkObject     *object,
                                         GtkSignalFunc  func,
                                         gpointer       data );

void gtk_signal_handler_unblock_by_data( GtkObject *object,
                                         gpointer   data);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>An Upgraded Hello World
<p>
Let's take a look at a slightly improved <em>helloworld</em> with
better examples of callbacks. This will also introduce us to our next
topic, packing widgets.

<tscreen><verb>
/* example-start helloworld2 helloworld2.c */

#include <gtk/gtk.h>

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

/* another callback */
gint 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 (&amp;argc, &amp;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. */
    gtk_signal_connect (GTK_OBJECT (window), "delete_event",
			GTK_SIGNAL_FUNC (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 */
    gtk_signal_connect (GTK_OBJECT (button), "clicked",
			GTK_SIGNAL_FUNC (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. */
    gtk_signal_connect (GTK_OBJECT (button), "clicked",
			GTK_SIGNAL_FUNC (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);
}
/* example-end */
</verb></tscreen>

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.

Just as a side note, there is another useful define for
gtk_window_new() - <tt/GTK_WINDOW_DIALOG/. This interacts with the
window manager a little differently and should be used for transient
windows.

<!-- ***************************************************************** -->
<sect>Packing Widgets <label id="sec_packing_widgets">
<!-- ***************************************************************** -->
<p>
When creating an application, you'll want to put more than one widget
inside a window. Our first <em>helloworld</em> example only used one
widget so we could simply use a gtk_container_add call to "pack" the
widget into the window. But when you want to put more than one widget
into a window, how do you control where that widget is positioned?
This is where packing comes in.

<!-- ----------------------------------------------------------------- -->
<sect1>Theory of Packing Boxes
<p>
Most packing is done by creating boxes as in the example above. These
are invisible widget containers that we can pack our widgets into
which come in two forms, a horizontal box, and a vertical box. When
packing widgets into a horizontal box, the objects are inserted
horizontally from left to right or right to left depending on the call
used. In a vertical box, widgets are packed from top to bottom or vice
versa. You may use any combination of boxes inside or beside other
boxes to create the desired effect.

To create a new horizontal box, we use a call to gtk_hbox_new(), and
for vertical boxes, gtk_vbox_new(). The gtk_box_pack_start() and
gtk_box_pack_end() functions are used to place objects inside of these
containers. The gtk_box_pack_start() function will start at the top
and work its way down in a vbox, and pack left to right in an hbox.
gtk_box_pack_end() will do the opposite, packing from bottom to top in
a vbox, and right to left in an hbox. Using these functions allows us
to right justify or left justify our widgets and may be mixed in any
way to achieve the desired effect. We will use gtk_box_pack_start() in
most of our examples. An object may be another container or a
widget. In fact, many widgets are actually containers themselves,
including the button, but we usually only use a label inside a button.

By using these calls, GTK knows where you want to place your widgets
so it can do automatic resizing and other nifty things. There are also
a number of options as to how your widgets should be packed. As you
can imagine, this method gives us a quite a bit of flexibility when
placing and creating widgets.

<!-- ----------------------------------------------------------------- -->
<sect1>Details of Boxes
<p>
Because of this flexibility, packing boxes in GTK can be confusing at
first. There are a lot of options, and it's not immediately obvious how
they all fit together. In the end, however, there are basically five
different styles.

<? <CENTER> >
<?
<IMG SRC="gtk_tut_packbox1.gif" VSPACE="15" HSPACE="10" WIDTH="528"
HEIGHT="235" ALT="Box Packing Example Image">
>
<? </CENTER> >

Each line contains one horizontal box (hbox) with several buttons. The
call to gtk_box_pack is shorthand for the call to pack each of the
buttons into the hbox. Each of the buttons is packed into the hbox the
same way (i.e., same arguments to the gtk_box_pack_start() function).

This is the declaration of the gtk_box_pack_start function.

<tscreen><verb>
void gtk_box_pack_start( GtkBox    *box,
                         GtkWidget *child,
                         gint       expand,
                         gint       fill,
                         gint       padding );
</verb></tscreen>

The first argument is the box you are packing the object into, the
second is the object. The objects will all be buttons for now, so
we'll be packing buttons into boxes.

The expand argument to gtk_box_pack_start() and gtk_box_pack_end()
controls whether the widgets are laid out in the box to fill in all
the extra space in the box so the box is expanded to fill the area
allotted to it (TRUE); or the box is shrunk to just fit the widgets
(FALSE). Setting expand to FALSE will allow you to do right and left
justification of your widgets.  Otherwise, they will all expand to fit
into the box, and the same effect could be achieved by using only one
of gtk_box_pack_start or gtk_box_pack_end.

The fill argument to the gtk_box_pack functions control whether the
extra space is allocated to the objects themselves (TRUE), or as extra
padding in the box around these objects (FALSE). It only has an effect
if the expand argument is also TRUE.

When creating a new box, the function looks like this:

<tscreen><verb>
GtkWidget *gtk_hbox_new (gint homogeneous,
                         gint spacing);
</verb></tscreen>

The homogeneous argument to gtk_hbox_new (and the same for
gtk_vbox_new) controls whether each object in the box has the same
size (i.e., the same width in an hbox, or the same height in a
vbox). If it is set, the gtk_box_pack routines function essentially
as if the <tt/expand/ argument was always turned on.

What's the difference between spacing (set when the box is created)
and padding (set when elements are packed)? Spacing is added between
objects, and padding is added on either side of an object. The
following figure should make it clearer:

<? <CENTER> >
<?
<IMG ALIGN="center" SRC="gtk_tut_packbox2.gif" WIDTH="509"
HEIGHT="213" VSPACE="15" HSPACE="10"