gtk2/docs/reference/gtk/question_index.sgml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
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]>
<refentry id="gtk-question-index">
<refmeta>
<refentrytitle>Common Questions</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>Common Questions</refmiscinfo>
</refmeta>

<refnamediv>
<refname>Common Questions</refname>
<refpurpose>
Find answers to common questions in the GTK+ manual
</refpurpose>
</refnamediv>

<refsect1>
<title>Questions and Answers</title>

<para>
This is an "index" of the reference manual organized by common "How do
I..." questions. If you aren't sure which documentation to read for
the question you have, this list is a good place to start.
</para>

<qandaset>

<qandadiv><title>General</title>

<qandaentry>
<question><para>
How do I get started with GTK+?
</para></question>

<answer><para>
The GTK+ <ulink url="http://www.gtk.org">website</ulink> offers a 
<ulink url="http://www.gtk.org/tutorial">tutorial</ulink> and a 
<ulink url="http://www.gtk.org/faq">FAQ</ulink>. More documentation ranging
from whitepapers to online books can be found at the
<ulink url="http://library.gnome.org/devel/">GNOME developer's site</ulink>.
After studying these materials you should be well prepared to come back to
this reference manual for details.
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
Where can I get help with GTK+, submit a bug report, or make a feature 
request?
</para></question>

<answer>

<para>
See the <link linkend="gtk-resources">documentation on this topic</link>.
</para>

</answer>

</qandaentry>


<qandaentry>
<question><para>How do I port from one GTK+
version to another?</para></question>

<answer>

<para>
See <xref linkend="gtk-migrating-2-to-3"/>.
You may also find useful information in the documentation for
specific widgets and functions.
</para>

<para>
If you have a question not covered in the manual, feel free to
ask on the mailing lists and please <ulink
url="http://bugzilla.gnome.org">file a bug report</ulink> against the
documentation.
</para>

</answer>

</qandaentry>


<qandaentry>
<question><para>
How does memory management work in GTK+? Should I free data returned
from functions?
</para></question>

<answer>

<para>
See the documentation for #GObject and #GInitiallyUnowned. For #GObject note
specifically g_object_ref() and g_object_unref(). #GInitiallyUnowned is a
subclass of #GObject so the same points apply, except that it has a "floating"
state (explained in its documentation).
</para>

<para>
For strings returned from functions, they will be declared "const" (using 
#G_CONST_RETURN) if they should not be freed. Non-const strings should be 
freed with g_free(). Arrays follow the same rule.  (If you find an exception 
to the rules, please report a bug to <ulink 
url="http://bugzilla.gnome.org">http://bugzilla.gnome.org</ulink>.)
</para>

</answer>
</qandaentry>

<qandaentry>
<question>
<para>
Why does my program leak memory, if I destroy a widget immediately
after creating it ?
</para>
</question>

<answer>
<para>
If <structname>GtkFoo</structname> isn't a toplevel window, then
<informalexample><programlisting>
 foo = gtk_foo_new (<!-- -->);
 gtk_widget_destroy (foo);
</programlisting></informalexample>
is a memory leak, because no one assumed the initial floating
reference. If you are using a widget and you aren't immediately
packing it into a container, then you probably want standard
reference counting, not floating reference counting.
</para>

<para>
To to get this, you must acquire a reference to the widget and drop the
floating reference (<quote>ref and sink</quote> in GTK+ parlance) after
creating it:
<informalexample><programlisting>
 foo = gtk_foo_new (<!-- -->);
 g_object_ref_sink (foo);
</programlisting></informalexample>
When you want to get rid of the widget, you must call gtk_widget_destroy()
to break any external connections to the widget before dropping your
reference:
<informalexample><programlisting>
 gtk_widget_destroy (foo);
 g_object_unref (foo);
</programlisting></informalexample>
When you immediately add a widget to a container, it takes care of
assuming the initial floating reference and you don't have to worry
about reference counting at all ... just call gtk_widget_destroy()
to get rid of the widget.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
How do I use GTK+ with threads?
</para></question>

<answer>

<para>
This is covered in the <link linkend="gdk-Threads">GDK threads 
documentation</link>. See also the <link linkend="glib-Threads">GThread</link> 
documentation for portable threading primitives.
</para>

</answer>

</qandaentry>

<qandaentry>
<question><para>
How do I internationalize a GTK+ program?
</para></question>

<answer>
<para>
Most people use <ulink url="http://www.gnu.org/software/gettext/">GNU
gettext</ulink>, already required in order to install GLib. On a UNIX
or Linux system with gettext installed, type <literal>info gettext</literal> 
to read the documentation.
</para>
<para>
The short checklist on how to use gettext is: call bindtextdomain() so gettext 
can find the files containing your translations, call textdomain() to set the 
default translation domain, call bind_textdomain_codeset() to request that
all translated strings are returned in UTF-8, then call gettext() to look up 
each string to be translated in the default domain. 
Conventionally, people define macros as follows for convenience:
<informalexample>
<programlisting>
  #define  _(x)  gettext (x)
  #define N_(x)  x
</programlisting>
</informalexample>
You use N_() (N stands for no-op) to mark a string for translation in a 
context where a function call to gettext() is not allowed, such as in an 
array initializer. 
You eventually have to call gettext() on the string to actually fetch the
translation.  _() both marks the string for translation and actually 
translates it.
</para>
<para>
Nowadays, GLib provides the common shorthand macros in the header file
<filename>gi18n.h</filename>, so you don't have to define them yourself, 
just include that header.
</para>
<para>
Code using these macros ends up looking like this:
<informalexample>
<programlisting>
 #include &lt;gi18n.h&gt;

 static const char *global_variable = N_("Translate this string");

 static void
 make_widgets (void)
 {
    GtkWidget *label1;
    GtkWidget *label2;

    label1 = gtk_label_new (_("Another string to translate"));
    label2 = gtk_label_new (_(global_variable));
...
</programlisting>
</informalexample>
</para>
<para>
Libraries using gettext should use dgettext() instead of gettext(), which 
allows them to specify the translation domain each time they ask for a 
translation. Libraries should also avoid calling textdomain(), since they
will be specifying the domain instead of using the default. For dgettext() 
the _() macro can be defined as:
<informalexample>
<programlisting>
  #define _(x) dgettext ("MyDomain", x)
</programlisting>
</informalexample>
</para>
<para>
Again, GLib comes with the <filename>gi18n-lib.h</filename>, saving you the 
trouble of defining the macros by hand. The macros in that header expect the 
translation domain to be specified by the %GETTEXT_PACKAGE macro. 
</para>
</answer>
</qandaentry>

<qandaentry>
<question>
<para>
How do I use non-ASCII characters in GTK+ programs ?
</para>
</question>

<answer>
<para>
GTK+ uses <ulink url="http://www.unicode.org">Unicode</ulink> (more exactly 
UTF-8) for all text. UTF-8 encodes each Unicode codepoint as a sequence of 
one to six bytes and has a number of nice properties which make it a good 
choice for working with Unicode text in C programs:
<itemizedlist>
<listitem><para>
ASCII characters are encoded by their familiar ASCII codepoints.
</para></listitem>
<listitem><para>
ASCII characters never appear as part of any other character.
</para></listitem>
<listitem><para>
The zero byte doesn't occur as part of a character, so that UTF-8 strings 
can be manipulated with the usual C library functions for handling 
zero-terminated strings.
</para></listitem>
</itemizedlist>
More information about Unicode and UTF-8 can be found in the 
<ulink url="http://www.cl.cam.ac.uk/~mgk25/unicode.html">UTF-8 and Unicode i
FAQ for Unix/Linux</ulink>.
GLib provides functions for converting strings between UTF-8 and other
encodings, see g_locale_to_utf8() and g_convert().
</para>
<para>
Text coming from external sources (e.g. files or user input), has to be
converted to UTF-8 before being handed over to GTK+. The following example 
writes the content of a IS0-8859-1 encoded text file to 
<literal>stdout</literal>:
<informalexample><programlisting>
gchar *text, *utf8_text;
gsize length;
GError *error = NULL;

if (g_file_get_contents (filename, &amp;text, &amp;length, NULL)) 
  {
     utf8_text = g_convert (text, length, "UTF-8", "ISO-8859-1", 
                            NULL, NULL, &amp;error);
     if (error != NULL)
       {
         fprintf ("Couldn't convert file &percnt;s to UTF-8\n", filename);
         g_error_free (error);
       }
     else
       g_print (utf8_text);
  }
else 
  fprintf (stderr, "Unable to read file &percnt;s\n", filename);
</programlisting></informalexample>
</para>
<para>
For string literals in the source code, there are several alternatives for
handling non-ASCII content:
<variablelist>
<varlistentry><term>direct UTF-8</term>
<listitem><para>
If your editor and compiler are capable of handling UTF-8 encoded sources,
it is very convenient to simply use UTF-8 for string literals, since it allows
you to edit the strings in "wysiwyg". Note that choosing this option may 
reduce the portability of your code.  
</para></listitem>
</varlistentry>

<varlistentry><term>escaped UTF-8</term>
<listitem><para>
Even if your toolchain can't handle UTF-8 directly, you can still encode string
literals in UTF-8 by using octal or hexadecimal escapes like 
<literal>\212</literal> or <literal>\xa8</literal> to
encode each byte. This is portable, but modifying the escaped strings is not
very convenient. Be careful when mixing hexadecimal escapes with ordinary text;
<literal>"\xa8abcd"</literal> is a string of length 1 !
</para></listitem>
</varlistentry>

<varlistentry><term>runtime conversion</term>
<listitem><para>
If the string literals can be represented in an encoding which your toolchain
can handle (e.g. IS0-8859-1), you can write your source files in that encoding
and use g_convert() to convert the strings to UTF-8 at runtime. Note that this 
has some runtime overhead, so you may want to move the conversion out of inner 
loops.
</para></listitem>
</varlistentry>
</variablelist>
Here is an example showing the three approaches using the copyright sign 
&copy; which has Unicode and ISO-8859-1 codepoint 169 and is represented in
UTF-8 by the two bytes 194, 169:
<informalexample><programlisting>
g_print ("direct UTF-8: &copy;");
g_print ("escaped UTF-8: \302\251");
text = g_convert ("runtime conversion: &copy;", -1, "ISO-8859-1", "UTF-8", NULL, NULL, NULL);
g_print(text);
g_free (text);
</programlisting></informalexample>
</para>
<para>
If you are using gettext() to localize your application, you need to
call bind_textdomain_codeset() to ensure that translated strings are
returned in UTF-8 encoding.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
How do I use GTK+ with C++?
</para></question>

<answer>
<para>
There are two ways to approach this. The GTK+ header files use the subset 
of C that's also valid C++, so you can simply use the normal GTK+ API 
in a C++ program. Alternatively, you can use a "C++ binding" 
such as <ulink url="http://gtkmm.sourceforge.net/">gtkmm</ulink>
which provides a native C++ API.
</para>
<para>
When using GTK+ directly, keep in mind that only functions can be
connected to signals, not methods. So you will need to use global
functions or "static" class functions for signal connections.
</para>
<para>
Another common issue when using GTK+ directly is that 
C++ will not implicitly convert an integer to an enumeration. 
This comes up when using bitfields; in C you can write the following
code:
<informalexample>
<programlisting>
  gdk_window_set_events (gdk_window, 
                         GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
</programlisting>
</informalexample>
while in C++ you must write:
<informalexample>
<programlisting>
  gdk_window_set_events (gdk_window, 
                         (GdkEventMask) GDK_BUTTON_PRESS_MASK | GDK_BUTTON_RELEASE_MASK);
</programlisting>
</informalexample>
There are very few functions that require this cast, however.
</para>
</answer>

</qandaentry>

<qandaentry>
<question><para>
How do I use GTK+ with other non-C languages?
</para></question>

<answer>
<para>
See the <ulink url="http://www.gtk.org/bindings.html">list of language
bindings</ulink> on <ulink
url="http://www.gtk.org">http://www.gtk.org</ulink>.
</para>

</answer>

</qandaentry>

<qandaentry>
<question><para>
How do I load an image or animation from a file?
</para></question>

<answer>

<para>
To load an image file straight into a display widget, use 
gtk_image_new_from_file() <footnote><para> If the file load fails, 
gtk_image_new_from_file() will display no image graphic &mdash; to detect 
a failed load yourself, use gdk_pixbuf_new_from_file() directly, then 
gtk_image_new_from_pixbuf().</para></footnote>. 
To load an image for another purpose, use gdk_pixbuf_new_from_file(). To i
load an animation, use gdk_pixbuf_animation_new_from_file().
gdk_pixbuf_animation_new_from_file() can also load non-animated images, so 
use it in combination with gdk_pixbuf_animation_is_static_image() to load a 
file of unknown type. 
</para>
<para>
To load an image or animation file asynchronously (without blocking), use 
#GdkPixbufLoader.
</para>
</answer>

</qandaentry>

<qandaentry>
<question><para>
How do I draw text ?
</para></question>

<answer>
<para>
To draw a piece of text, use a Pango layout and gdk_draw_layout(), 
using code like the following:
<informalexample>
<programlisting>
 layout = gtk_widget_create_pango_layout (widget, text);
 fontdesc = pango_font_description_from_string ("Luxi Mono 12");
 pango_layout_set_font_description (layout, fontdesc); 
 gdk_draw_layout (..., layout);
 pango_font_description_free (fontdesc);
 g_object_unref (layout);
</programlisting>
</informalexample>
Do not use the deprecated #GdkFont and gdk_draw_text().
</para>

<para>
See also the
<ulink url="http://library.gnome.org/devel/pango/stable/pango-Cairo-Rendering.html">Cairo Rendering</ulink>
section of <ulink url="http://library.gnome.org/devel/pango/stable/">Pango manual</ulink>.
</para>
</answer>

</qandaentry>

<qandaentry>
<question>
<para>
How do I measure the size of a piece of text ?
</para>
</question>

<answer>
<para>
To obtain the size of a piece of text, use a Pango layout and 
pango_layout_get_pixel_size(), using code like the following:
<informalexample>
<programlisting>
 layout = gtk_widget_create_pango_layout (widget, text);
 fontdesc = pango_font_description_from_string ("Luxi Mono 12");
 pango_layout_set_font_description (layout, fontdesc); 
 pango_layout_get_pixel_size (layout, &amp;width, &amp;height);
 pango_font_description_free (fontdesc);
 g_object_unref (layout);
</programlisting>
</informalexample>
Do not use the deprecated function gdk_text_width().
</para>

<para>
See also the
<ulink url="http://library.gnome.org/devel/pango/stable/pango-Layout-Objects.html">Layout Objects</ulink>
section of <ulink url="http://library.gnome.org/devel/pango/stable/">Pango manual</ulink>.
</para>
</answer>
</qandaentry>

<qandaentry>
<question>
<para>
Why are types not registered if I use their <literal>GTK_TYPE_BLAH</literal> 
macro ?
</para>
</question>

<answer>
<para>
The <literal>GTK_TYPE_BLAH</literal> macros are defined as calls to 
<literal>gtk_blah_get_type()</literal>, and the <literal>_get_type()</literal> i
functions are declared as %G_GNUC_CONST which allows the compiler to optimize
the call away if it appears that the value is not being used.
</para>

<para>
A common workaround for this problem is to store the result in a volatile 
variable, which keeps the compiler from optimizing the call away.
<informalexample><programlisting>
volatile GType dummy = GTK_TYPE_BLAH;
</programlisting></informalexample>
</para>
</answer>
</qandaentry>

<qandaentry>
<question>
<para>
How do I create a transparent toplevel window ?
</para>
</question>

<answer>
<para>
To make a window transparent, it needs to use a visual which supports that.
This is done by getting the RGBA colormap of the screen with 
gdk_screen_get_rgba_colormap() and setting it on the window. Note that
gdk_screen_get_rgba_colormap() will return %NULL if transparent windows
are not supported on the screen; also note that this may change from
screen to screen, so it needs to be repeated whenever the window is moved
to a different screen.
<informalexample><programlisting>
GdkColormap *colormap;

colormap = gdk_screen_get_rgba_colormap (screen);
if (!colormap)
  colormap = gdk_screen_get_rgb_colormap (screen);

gtk_widget_set_colormap (widget, colormap);
</programlisting></informalexample>
One possibility to fill the alpha channel on the window is to use
gdk_draw_rgb_32_image().
</para>
<para>
Note that the presence of an RGBA visual is no guarantee that the
window will actually appear transparent on screen. On X11, this 
requires a compositing manager to be running. See 
gtk_widget_is_composited() for a way to find out if the alpha
channel will be respected.
</para>
</answer>
</qandaentry>

</qandadiv>

<qandadiv><title>Which widget should I use...</title>

<qandaentry>
<question><para>
...for lists and trees?
</para></question>

<answer>
<para>
See <link linkend="TreeWidget">tree widget overview</link> &mdash; you
should use the #GtkTreeView widget. (A list is just a tree with no branches, 
so the tree widget is used for lists as well.) Do not use the deprecated 
widgets #GtkTree or #GtkCList/#GtkCTree in newly-written code, they are
less flexible and result in an inferior user interface.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
...for multi-line text display or editing?
</para></question>

<answer>
<para>
See <link linkend="TextWidget">text widget overview</link> &mdash; you
should use the #GtkTextView widget. Do not use the deprecated widget #GtkText 
in newly-written code, it has a number of problems that are best avoided.
</para>
<para>
If you only have a small amount of text, #GtkLabel may also be appropriate 
of course. It can be made selectable with gtk_label_set_selectable(). For a 
single-line text entry, see #GtkEntry.
</para>
</answer>
</qandaentry>


<qandaentry>
<question><para>
...to display an image or animation?
</para></question>

<answer>
<para>
#GtkImage can display images in just about any format GTK+ understands. 
You can also use #GtkDrawingArea if you need to do something more complex, 
such as draw text or graphics over the top of the image.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
...for presenting a set of mutually-exclusive choices, where Windows
would use a combo box?
</para></question>

<answer>
<para>
With GTK+, a #GtkComboBox is the recommended widget to use for this use case.
This widget looks like either a combo box or the current option menu, depending
on the current theme. If you need an editable text entry, use #GtkComboBoxEntry.
</para>
</answer>
</qandaentry>

</qandadiv>

<qandadiv><title>GtkWidget</title>

<qandaentry>
<question><para>
How do I change the color of a widget?
</para></question>

<answer><para>
See gtk_widget_modify_fg(), gtk_widget_modify_bg(), gtk_widget_modify_base(),
and gtk_widget_modify_text().  See <link linkend="gtk-Resource-Files">GTK+ 
resource files</link> for more discussion. You can also change widget color 
by installing a resource file and parsing it with gtk_rc_add_default_file().
The advantage of a resource file is that users can then override the
color you've chosen.
</para>

<para>To change the background color for widgets such as #GtkLabel that have 
no background, place them in a #GtkEventBox and set the background of the 
event box.  
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
How do I change the font of a widget?
</para></question>

<answer><para>
This has several possible answers, depending on what exactly you want to 
achieve. One option is gtk_widget_modify_font(). Note that this function 
can be used to change only the font size, as in the following example:
<informalexample><programlisting>
 PangoFontDesc *font_desc = pango_font_description_new (<!-- -->);
 pango_font_description_set_size (font_desc, 40);
 gtk_widget_modify_font (widget, font);
 pango_font_description_free (font_desc);
</programlisting></informalexample>
</para>
<para>
If you want to make the text of a label larger, you can use 
gtk_label_set_markup():
<informalexample><programlisting>
gtk_label_set_markup (label, "&lt;big&gt;big text&lt;/big&gt;");
</programlisting></informalexample>
This is preferred for many apps because it's a relative size to the 
user's chosen font size. See g_markup_escape_text() if you are 
constructing such strings on the fly.
</para>
<para>
You can also change the font of a widget by putting
<programlisting>
 gtk-font-name = "Sans 30"
</programlisting>
in a resource file and parsing it with gtk_rc_add_default_file(). 
The advantage of a resource file is that users can then override the font you
have chosen. See <link linkend="gtk-Resource-Files">GTK+ resource files</link> 
for more discussion. 
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
How do I disable/ghost/desensitize a widget?
</para></question>

<answer><para> In GTK+ a disabled widget is termed "insensitive." See
gtk_widget_set_sensitive().
</para></answer>
</qandaentry>

</qandadiv>


<qandadiv><title>GtkTextView</title>

<qandaentry>
<question><para>
How do I get the contents of the entire text widget as a string?
</para></question>

<answer><para>
See gtk_text_buffer_get_bounds() and gtk_text_buffer_get_text()
or gtk_text_iter_get_text().
</para>
<para>
<informalexample><programlisting>
  GtkTextIter start, end;
  GtkTextBuffer *buffer;
  char *text;

  buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (text_view));
  gtk_text_buffer_get_bounds (buffer, &amp;start, &amp;end);
  text = gtk_text_iter_get_text (&amp;start, &amp;end);
  /* use text */
  g_free (text);
</programlisting></informalexample>
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
How do I make a text widget display its complete contents in a specific font?
</para></question>

<answer><para>
If you use gtk_text_buffer_insert_with_tags() with appropriate tags to select 
the font, the inserted text will have the desired appearance, but text typed 
in by the user before or after the tagged block will appear in the default 
style. 
</para>
<para>
To ensure that all text has the desired appearance, use gtk_widget_modify_font() 
to change the default font for the widget.
</para></answer>
</qandaentry>

<qandaentry>
<question>
<para>
How do I make a text view scroll to the end of the buffer automatically ?
</para>
</question>

<answer>
<para>
A good way to keep a text buffer scrolled to the end is to place a
<link linkend="GtkTextMark">mark</link> at the end of the buffer, and
give it right gravity. The gravity has the effect that text inserted
at the mark gets inserted <emphasis>before</emphasis>, keeping the mark
at the end. 
</para>

<para> 
To ensure that the end of the buffer remains visible, use
gtk_text_view_scroll_to_mark() to scroll to the mark after
inserting new text.
</para>

<para>
The gtk-demo application contains an example of this technique. 
</para>
</answer>
</qandaentry>
</qandadiv>



<qandadiv><title>#GtkTreeView</title>

<qandaentry>
<question><para>
How do I associate some data with a row in the tree?
</para></question>

<answer>
<para>
Remember that the #GtkTreeModel columns don't necessarily have to be displayed. 
So you can put non-user-visible data in your model just like any other data, 
and retrieve it with gtk_tree_model_get(). See the 
<link linkend="TreeWidget">tree widget overview</link>.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
What's the #GtkTreeView equivalent of gtk_clist_find_row_from_data()?
</para></question>

<answer>
<para>
As there is no separate data column in the #GtkTreeModel, there's no
built in function to find the iter from data.  You can write a custom
searching function to walk the tree and find the data, or use
gtk_tree_model_foreach().
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
How do I put an image and some text in the same column?
</para></question>

<answer>
<para>
You can pack more than one #GtkCellRenderer into a single #GtkTreeViewColumn 
using gtk_tree_view_column_pack_start() or gtk_tree_view_column_pack_end(). 
So pack both a #GtkCellRendererPixbuf and a #GtkCellRendererText into the 
column.
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
I can set data easily on my #GtkTreeStore/#GtkListStore models using 
gtk_list_store_set() and gtk_tree_store_set(), but can't read it back?
</para></question>

<answer>
<para>
Both the #GtkTreeStore and the #GtkListStore implement the #GtkTreeModel
interface.  Consequentially, the can use any function this interface 
implements. The easiest way to read a set of data back is to use 
gtk_tree_model_get().
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
How do I change the way that numbers are formatted by #GtkTreeView?
</para></question>
<answer><para>
Use gtk_tree_view_insert_column_with_data_func()
or gtk_tree_view_column_set_cell_data_func() and do the conversion from i
number to string yourself (with, say, g_strdup_printf()).
</para>

<para>
The following example demonstrates this:
<informalexample><programlisting>
enum 
{
  DOUBLE_COLUMN,
  N_COLUMNS
};

GtkListStore *mycolumns;
GtkTreeView *treeview;

void 
my_cell_double_to_text (GtkTreeViewColumn *tree_column,
	                GtkCellRenderer   *cell, 
                        GtkTreeModel      *tree_model,
	                GtkTreeIter       *iter, 
                        gpointer           data)
{
  GtkCellRendererText *cell_text = (GtkCellRendererText *)cell;
  gdouble d;
  gchar *text;

  /* Get the double value from the model. */
  gtk_tree_model_get (tree_model, iter, (gint)data, &amp;d, -1);
  /* Now we can format the value ourselves. */
  text = g_strdup_printf ("&percnt;.2f", d);
  g_object_set (cell, "text", text, NULL);
  g_free (text);
}

void 
set_up_new_columns (GtkTreeView *myview)
{
  GtkCellRendererText *renderer;
  GtkTreeViewColumn *column;
  GtkListStore *mycolumns;

  /* Create the data model and associate it with the given TreeView */
  mycolumns = gtk_list_store_new (N_COLUMNS, G_TYPE_DOUBLE);
  gtk_tree_view_set_model (myview, GTK_TREE_MODEL (mycolumns));

  /* Create a GtkCellRendererText */
  renderer = gtk_cell_renderer_text_new (<!-- -->);

  /* Create a new column that has a title ("Example column"),
   * uses the above created renderer that will render the double
   * value into text from the associated model's rows. 
   */
  column = gtk_tree_view_column_new (<!-- -->);
  gtk_tree_view_column_set_title  (column, "Example column");
  renderer = gtk_cell_renderer_text_new (<!-- -->);
  gtk_tree_view_column_pack_start (column, renderer, TRUE);

  /* Append the new column after the GtkTreeView's previous columns. */
  gtk_tree_view_append_column (GTK_TREE_VIEW (myview), column);
  /* Since we created the column by hand, we can set it up for our
   * needs, e.g. set its minimum and maximum width, etc.
   */
  /* Set up a custom function that will be called when the column content
   * is rendered. We use the func_data pointer as an index into our
   * model. This is convenient when using multi column lists. 
   */
  gtk_tree_view_column_set_cell_data_func (column, renderer,
	                                   my_cell_double_to_text, 
                                           (gpointer)DOUBLE_COLUMN, NULL);
}
</programlisting></informalexample>
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
How do I hide the expander arrows in my tree view ?
</para></question>

<answer><para>
Set the expander-column property of the tree view to a hidden column.
See gtk_tree_view_set_expander_column() and gtk_tree_view_column_set_visible().
</para></answer>
</qandaentry>

</qandadiv>

<qandadiv><title>Using cairo with GTK+</title>

<qandaentry>
<question><para>
How do I use cairo to draw in GTK+ applications ?
</para></question>

<answer><para>
Use gdk_cairo_create() to obtain a cairo context for drawing
on a GDK window or pixmap. See <link linkend="gdk-Cairo-Interaction">Cairo 
Interaction</link> for some more useful functions.
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
I have created a cairo context with gdk_cairo_create(), but when I
later use it, my drawing does not show up. Why is that ?
</para></question>

<answer>
<para>
All drawing in GTK+ is normally done in an expose handler, and GTK+
creates a temporary pixmap for double-buffering the drawing. If you 
create a cairo context outside the expose handler, it is backed
by the GDK window itself, not the double-buffering pixmap. Consequently,
any drawing you do with that cairo context gets overwritten at the 
end of the expose handler, when the double-buffering pixmap is copied
back.
</para>
<para>
Possible solutions to this problem are:
<itemizedlist>
<listitem><para>
Turn off double-buffering, with gtk_widget_set_double_buffered().
This is not ideal, since it can cause some flickering.
</para></listitem>
<listitem><para>
Create the cairo context inside the expose handler. If you do this,
gdk_cairo_create() arranges for it to be backed by the double-buffering
pixmap. This is the preferred solution, and is used throughout GTK+
itself.
</para></listitem>
</itemizedlist>
</para>
</answer>
</qandaentry>

<qandaentry>
<question><para>
Can I improve the performance of my application by using the
Glitz backend of cairo ?
</para></question>

<answer><para>
No. The GDK X11 backend uses the cairo X backend (and the other
GDK backends use their respective native cairo backends). The
GTK+ developers believe that the best way to improving the GDK
drawing performance is to optimize the cairo X backend and the
relevant code paths in the X server that is uses (mostly the
Render extension).
</para></answer>
</qandaentry>

<qandaentry>
<question><para>
Can I use cairo to draw on a #GdkPixbuf ?
</para></question>

<answer><para>
No, at least not yet. The cairo image surface does not support the
pixel format used by GdkPixbuf. 
</para></answer>
</qandaentry>

</qandadiv>

</qandaset>

</refsect1>

</refentry>