gtk2/docs/reference/gtk/migrating-2to3.xml

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
               "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
]>
<chapter id="gtk-migrating-2-to-3">
  <title>Migrating from GTK+ 2.x to GTK+ 3</title>

  <para>
    GTK+ 3 is a major new version of GTK+ that breaks both API and ABI
    compared to GTK+ 2.x, which has remained API- and ABI-stable for a
    long time. Thankfully, most of the changes are not hard to adapt to
    and there are a number of steps that you can take to prepare your
    GTK+ 2.x application for the switch to GTK+ 3. After that, there's
    a small number of adjustments that you may have to do when you actually
    switch your application to build against GTK+ 3.
  </para>

  <section>
    <title>Preparation in GTK+ 2.x</title>

    <para>
      The steps outlined in the following sections assume that your
      application is working with GTK+ 2.24, which is the final stable
      release of GTK+ 2.x. It includes all the necessary APIs and tools
      to help you port your application to GTK+ 3. If you are still using
      an older version of GTK+ 2.x, you should first get your application
      to build and work with 2.24.
    </para>

  <section>
  <title>Do not include individual headers</title>
  <para>
    With GTK+ 2.x it was common to include just the header files for
    a few widgets that your application was using, which could lead
    to problems with missing definitions, etc. GTK+ 3 tightens the
    rules about which header files you are allowed to include directly.
    The allowed header files are are
    <variablelist>
      <varlistentry>
        <term><filename>gtk/gtk.h</filename></term>
        <listitem>for GTK</listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>gtk/gtkunixprint.h</filename></term>
        <listitem>for low-level, UNIX-specific printing functions</listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>gdk/gdk.h</filename></term>
        <listitem>for GDK</listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>gdk/gdkx.h</filename></term>
        <listitem>for GDK functions that are X11-specific</listitem>
      </varlistentry>
      <varlistentry>
        <term><filename>gdk/gdkwin32.h</filename></term>
        <listitem>for GDK functions that are Windows-specific</listitem>
      </varlistentry>
    </variablelist>
    (these relative paths are assuming that you are using the include
     paths that are specified in the gtk+-2.0.pc file, as returned by
     <literal>pkg-config --cflags gtk+-2.0.pc</literal>.)
  </para>
  <para>
    To check that your application only includes the allowed headers,
    you can use defines to disable inclusion of individual headers,
    as follows:
    <programlisting>
    make CFLAGS+="-DGTK_DISABLE_SINGLE_INCLUDES"
    </programlisting>
  </para>
  </section>

  <section>
  <title>Do not use deprecated symbols</title>
  <para>
    Over the years, a number of functions, and in some cases, entire
    widgets have been deprecated. These deprecations are clearly spelled
    out in the API reference, with hints about the recommended replacements.
    The API reference also includes an
    <link linkend="api-index-deprecated">index</link> of all deprecated
    symbols.
  </para>
  <para>
    To verify that your program does not use any deprecated symbols,
    you can use defines to remove deprecated symbols from the header files,
    as follows:
    <programlisting>
    make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
    </programlisting>
  </para>
  </section>

  <section>
  <title>Use accessor functions instead of direct access</title>
  <para>
    GTK+ 3 removes many implementation details and struct members from
    its public headers.
  </para>
  <para>
    To ensure that your application does not have problems with this, you
    define the preprocessor symbol <literal>GSEAL_ENABLE</literal>. This
    will make the compiler catch all uses of direct access to struct fields
    so that you can go through them one by one and replace them with a call
    to an accessor function instead.
    <programlisting>
    make CFLAGS+="-DGSEAL_ENABLE"
    </programlisting>
  </para>
  </section>

  <section>
    <title>Replace GDK_&lt;keyname&gt; with GDK_KEY_&lt;keyname&gt;</title>

    <para>
      Key constants have gained a <literal>_KEY_</literal> infix.
      For example, <literal>GDK_a</literal> is now
      <literal>GDK_KEY_a</literal>.  In GTK+ 2, the old names continue
      to be available.  In GTK+ 3 however, the old names will require
      an explicit include of the <literal>gdkkeysyms-compat.h</literal> header.
    </para>

  </section>

  <section>
    <title>Use cairo for drawing</title>
    <para>
      In GTK+ 3, the GDK drawing API (which closely mimics the X
      drawing API, which is itself modeled after PostScript) has been
      removed. All drawing in GTK+ 3 is done via cairo.
    </para>
    <para>
      The #GdkGC and #GdkImage objects, as well as all the functions using
      them, are gone. This includes the <literal>gdk_draw</literal> family
      of functions like gdk_draw_rectangle() and gdk_draw_drawable(). As
      #GdkGC is roughly equivalent to #cairo_t and #GdkImage was used for
      drawing images to GdkDrawables, which cairo supports automatically,
      a transition is usually straightforward.
    </para>
    <para>
      The following examples show a few common drawing idioms used by
      applications that have been ported to use cairo and how the code
      was replaced.
    </para>
    <example>
       <title>Drawing a GdkPixbuf onto a GdkDrawable</title>
         <para>
           Drawing a pixbuf onto a drawable used to be done like this:
<programlisting><![CDATA[
gdk_draw_pixbuf (window,
                 gtk_widget_get_style (widget)->black_gc,
                 pixbuf,
                 0, 0
                 x, y,
                 gdk_pixbuf_get_width (pixbuf),
                 gdk_pixbuf_get_height (pixbuf),
                 GDK_RGB_DITHER_NORMAL,
                 0, 0);
]]></programlisting>
           Doing the same thing with cairo:
<programlisting><![CDATA[
cairo_t *cr = gdk_cairo_create (window);
gdk_cairo_set_source_pixbuf (cr, pixbuf, x, y);
cairo_paint (cr);
cairo_destroy (cr);
]]></programlisting>
           Note that very similar code can be used for drawing pixmaps
           by using gdk_cairo_set_source_pixmap() instead of
           gdk_cairo_set_source_pixbuf().
         </para>
      </example>
      <example>
        <title>Drawing a tiled GdkPixmap to a GdkDrawable</title>
        <para>
          Tiled pixmaps are often used for drawing backgrounds.
          Old code looked something like this:
          <programlisting><![CDATA[
GdkGCValues gc_values;
GdkGC *gc;

/* setup */
gc = gtk_widget_get_style (widget)->black_gc;
gdk_gc_set_tile (gc, pixmap);
gdk_gc_set_fill (gc, GDK_TILED);
gdk_gc_set_ts_origin (gc, x_origin, y_origin);
/* use */
gdk_draw_rectangle (drawable, gc, TRUE, 0, 0, width, height);
/* restore */
gdk_gc_set_tile (gc, NULL);
gdk_gc_set_fill (gc, GDK_SOLID);
gdk_gc_set_ts_origin (gc, 0, 0);
]]></programlisting>
          The equivalent cairo code looks like this:
<programlisting><![CDATA[
cairo_t *cr;

cr = gdk_cairo_create (drawable);
gdk_cairo_set_source_pixmap (cr, pixmap, x_origin, y_origin);
cairo_pattern_set_extend (cairo_get_source (cr), CAIRO_EXTEND_REPEAT);
cairo_rectangle (cr, 0, 0, width, height);
cairo_fill (cr);
cairo_destroy (cr);
]]></programlisting>
          Again, you can exchange pixbufs and pixmaps by using
          gdk_cairo_set_source_pixbuf() instead of
          gdk_cairo_set_source_pixmap().
        </para>
      </example>
      <example>
        <title>Drawing a PangoLayout to a clipped area</title>
        <para>
          Drawing layouts clipped is often used to avoid overdraw or to
          allow drawing selections. Code would have looked like this:
<programlisting><![CDATA[
GdkGC *gc;

/* setup */
gc = gtk_widget_get_style (widget)->text_gc[state];
gdk_gc_set_clip_rectangle (gc, &area);
/* use */
gdk_draw_layout (drawable, gc, x, y, layout);
/* restore */
gdk_gc_set_clip_rectangle (gc, NULL);
]]></programlisting>
          With cairo, the same effect can be achieved using:
<programlisting><![CDATA[
cairo_t *cr;

cr = gdk_cairo_create (drawable);
/* clip */
gdk_cairo_rectangle (cr, &area);
cairo_clip (cr);
/* set the correct source color */
gdk_cairo_set_source_color (cr, &gtk_widget_get_style (widget)->text[state]);
/* draw the text */
cairo_move_to (cr, x, y);
pango_cairo_show_layout (cr, layout);
cairo_destroy (cr);
]]></programlisting>
          Clipping using cairo_clip() is of course not restricted to text
          rendering and can be used everywhere where GC clips were used.
          And using gdk_cairo_set_source_color() with style colors should
          be used in all the places where a style’s GC was used to achieve
          a particular color.
        </para>
      </example>
    <section>
      <title>What should you be aware of ?</title>
      <formalpara><title>No more stippling</title>
        <para>
          Stippling is the usage of a bi-level mask, called a #GdkBitmap.
          It was often used to achieve a checkerboard effect. You can use
          cairo_mask() to achieve this effect. To get a checkerbox mask,
          you can use code like this:
<programlisting><![CDATA[
static cairo_pattern_t *
gtk_color_button_get_checkered (void)
{
    /* need to respect pixman's stride being a multiple of 4 */
    static unsigned char data[8] = { 0xFF, 0x00, 0x00, 0x00,
                                     0x00, 0xFF, 0x00, 0x00 };
    cairo_surface_t *surface;
    cairo_pattern_t *pattern;

    surface = cairo_image_surface_create_for_data (data,
                                                   CAIRO_FORMAT_A8,
                                                   2, 2,
                                                   4);
    pattern = cairo_pattern_create_for_surface (surface);
    cairo_surface_destroy (surface);
    cairo_pattern_set_extend (pattern, CAIRO_EXTEND_REPEAT);
    cairo_pattern_set_filter (pattern, CAIRO_FILTER_NEAREST);

    return pattern;
}
]]></programlisting>
          Note that stippling looks very outdated in UIs, and is rarely
          used in modern applications. All properties that made use of
          stippling have been removed from GTK+ 3. Most prominently,
          stippling is absent from text rendering, in particular #GtkTextTag.
        </para>
      </formalpara>
      <formalpara><title>Using the the target drawable also as source or mask</title>
        <para>
          The gdk_draw_drawable() function allowed using the same drawable
          as source and target. This was often used to achieve a scrolling
          effect. Cairo does not allow this yet. You can however use
          cairo_push_group() to get a different intermediate target that
          you can copy to. So you can replace this code:
<programlisting><![CDATA[
gdk_draw_drawable (pixmap,
                   gc,
                   pixmap,
                   area.x + dx, area.y + dy,
                   area.x, area.y,
                   area.width, area.height);
]]></programlisting>
          By using this code:
<programlisting><![CDATA[
cairo_t *cr = gdk_cairo_create (pixmap);
/* clipping restricts the intermediate surface's size, so it's a good idea
 * to use it. */
gdk_cairo_rectangle (cr, &area);
cairo_clip (cr);
/* Now push a group to change the target */
cairo_push_group (cr);
gdk_cairo_set_source_pixmap (cr, pixmap, dx, dy);
cairo_paint (cr);
/* Now copy the intermediate target back */
cairo_pop_group_to_source (cr);
cairo_paint (cr);
cairo_destroy (cr);
]]></programlisting>
          The cairo developers plan to add self-copies in the future to allow
          exactly this effect, so you might want to keep up on cairo
          development to be able to change your code.
        </para>
      </formalpara>
      <formalpara><title>Using pango_cairo_show_layout(<!-- -->) instead of gdk_draw_layout_with_colors(<!-- -->)</title>
        <para>
          GDK provided a way to ignore the color attributes of text and use
          a hardcoded text color with the gdk_draw_layout_with_colors()
          function. This is often used to draw text shadows or selections.
          Pango’s cairo support does not yet provide this functionality. If
          you use Pango layouts that change colors, the easiest way to achieve
          a similar effect is using pango_cairo_layout_path() and cairo_fill()
          instead of gdk_draw_layout_with_colors(). Note that this results in
          a slightly uglier-looking text, as subpixel anti-aliasing is not
          supported.
        </para>
      </formalpara>
    </section>
    </section>
  </section>

  <section>
    <title>Changes that need to be done at the time of the switch</title>

  <para>
    This section outlines porting tasks that you need to tackle when
    you get to the point that you actually build your application against
    GTK+ 3. Making it possible to prepare for these in GTK+ 2.24 would
    have been either impossible or impractical.
  </para>

  <section>
    <title>Replace GdkRegion by cairo_region_t</title>

    <para>
      Starting with version 1.10, cairo provides a region API that is
      equivalent to the GDK region API (which was itself copied from
      the X server). Therefore, the region API has been removed in GTK+ 3.
    </para>
    <para>
      Porting your application to the cairo region API should be a straight
      find-and-replace task. Please refer to the following table:
      <table>
        <tgroup cols="2">
          <thead>
            <row><entry>GDK</entry><entry>cairo</entry></row>
          </thead>
          <tbody>
            <row><entry>#GdkRegion</entry><entry>#cairo_region_t</entry></row>
            <row><entry>#GdkRectangle</entry><entry>#cairo_rectangle_int_t</entry></row>
            <row><entry>gdk_rectangle_intersect()</entry><entry>this function is still there</entry></row>
            <row><entry>gdk_rectangle_union()</entry><entry>this function is still there</entry></row>
            <row><entry>gdk_region_new()</entry><entry>cairo_region_create()</entry></row>
            <row><entry>gdk_region_copy()</entry><entry>cairo_region_copy()</entry></row>
            <row><entry>gdk_region_destroy()</entry><entry>cairo_region_destroy()</entry></row>
            <row><entry>gdk_region_rectangle()</entry><entry>cairo_region_create_rectangle()</entry></row>
            <row><entry>gdk_region_get_clipbox()</entry><entry>cairo_region_get_extents()</entry></row>
            <row><entry>gdk_region_get_rectangles()</entry><entry>cairo_region_num_rectangles() and
                                cairo_region_get_rectangle()</entry></row>
            <row><entry>gdk_region_empty()</entry><entry>cairo_region_is_empty()</entry></row>
            <row><entry>gdk_region_equal()</entry><entry>cairo_region_equal()</entry></row>
            <row><entry>gdk_region_point_in()</entry><entry>cairo_region_contains_point()</entry></row>
            <row><entry>gdk_region_rect_in()</entry><entry>cairo_region_contains_rectangle()</entry></row>
            <row><entry>gdk_region_offset()</entry><entry>cairo_region_translate()</entry></row>
            <row><entry>gdk_region_union_with_rect()</entry><entry>cairo_region_union_rectangle()</entry></row>
            <row><entry>gdk_region_intersect()</entry><entry>cairo_region_intersect()</entry></row>
            <row><entry>gdk_region_union()</entry><entry>cairo_region_union()</entry></row>
            <row><entry>gdk_region_subtract()</entry><entry>cairo_region_subtract()</entry></row>
            <row><entry>gdk_region_xor()</entry><entry>cairo_region_xor()</entry></row>
            <row><entry>gdk_region_shrink()</entry><entry>no replacement</entry></row>
            <row><entry>gdk_region_polygon()</entry><entry>no replacement, use cairo paths instead</entry></row>
          </tbody>
        </tgroup>
      </table>
    </para>
  </section>

  <section>
    <title>Replace GdkPixmap by cairo surfaces</title>
    <para>
      The #GdkPixmap object and related functions have been removed.
      In the cairo-centric world of GTK+ 3, cairo surfaces take over
      the role of pixmaps.
    </para>
    <example>
      <title>Creating custom cursors</title>
      <para>
        One place where pixmaps were commonly used is to create custom
        cursors:
      <programlisting>
GdkCursor *cursor;
GdkPixmap *pixmap;
cairo_t *cr;
GdkColor fg = { 0, 0, 0, 0 };

pixmap = gdk_pixmap_new (NULL, 1, 1, 1);

cr = gdk_cairo_create (pixmap);
cairo_rectangle (cr, 0, 0, 1, 1);
cairo_fill (cr);
cairo_destroy (cr);

cursor = gdk_cursor_new_from_pixmap (pixmap, pixmap, &amp;fg, &amp;fg, 0, 0);

g_object_unref (pixmap);
      </programlisting>
      The same can be achieved without pixmaps, by drawing onto
      an image surface:
      <programlisting>
GdkCursor *cursor;
cairo_surface_t *s;
cairo_t *cr;
GdkPixbuf *pixbuf;

s = cairo_image_surface_create (CAIRO_FORMAT_A1, 3, 3);
cr = cairo_create (s);
cairo_arc (cr, 1.5, 1.5, 1.5, 0, 2 * M_PI);
cairo_fill (cr);
cairo_destroy (cr);

pixbuf = gdk_pixbuf_get_from_surface (NULL, s,
                                      0, 0, 0, 0,
                                      3, 3);

cairo_surface_destroy (s);

cursor = gdk_cursor_new_from_pixbuf (display, pixbuf, 0, 0);

g_object_unref (pixbuf);
      </programlisting>
      </para>
    </example>
  </section>

  <section>
    <title>Replace colormaps by visuals</title>
    <para>
      For drawing with cairo, it is not necessary to allocate colors, and
      a #GdkVisual provides enough information for cairo to handle colors
      in 'native' surfaces. Therefore, #GdkColormap and related functions
      have been removed in GTK+ 3, and visuals are used instead. The
      colormap-handling functions of #GtkWidget (gtk_widget_set_colormap(),
      etc) have been removed and gtk_window_set_visual() has been added.
    </para>
    <example><title>Setting up a translucent window</title>
    <para>You might have a screen-changed handler like the following
     to set up a translucent window with an alpha-channel:
    </para>
    <programlisting>
static void
on_alpha_screen_changed (GtkWidget *widget,
                         GdkScreen *old_screen,
                         GtkWidget *label)
{
  GdkScreen *screen = gtk_widget_get_screen (widget);
  GdkColormap *colormap = gdk_screen_get_rgba_colormap (screen);

  if (colormap == NULL)
    colormap = gdk_screen_get_default_colormap (screen);

  gtk_widget_set_colormap (widget, colormap);
}
    </programlisting>
    <para>
    With visuals instead of colormaps, this will look as follows:
    </para>
    <programlisting>
static void
on_alpha_screen_changed (GtkWindow *window,
                         GdkScreen *old_screen,
                         GtkWidget *label)
{
  GdkScreen *screen = gtk_widget_get_screen (GTK_WIDGET (window));
  GdkVisual *visual = gdk_screen_get_rgba_visual (screen);

  if (visual == NULL)
    visual = gdk_screen_get_system_visual (screen);

  gtk_window_set_visual (window, visual);
}
    </programlisting>
    </example>
  </section>

  <section>
    <title>The GtkWidget::draw signal</title>
    <para>
      The GtkWidget #GtkWidget::expose-event signal has been replaced by
      a new #GtkWidget::draw signal, which takes a #cairo_t instead of
      an expose event. The cairo context is being set up so that the origin
      at (0, 0) coincides with the upper left corner of the widget, and
      is properly clipped.
    </para>
    <note><para>In other words, the cairo context of the draw signal is set
      up in 'widget coordinates', which is different from traditional expose
      event handlers, which always assume 'window coordinates'.
    </para></note>
    <para>
      The widget is expected to draw itself with its allocated size, which
      is available via the new gtk_widget_get_allocated_width() and
      gtk_widget_get_allocated_height() functions. It is not necessary to
      check for GTK_WIDGET_IS_DRAWABLE(), since GTK+ already does this check
      before emitting the ::draw signal.
    </para>
    <para>
      There are some special considerations for widgets with multiple windows.
      Expose events are window-specific, and widgets with multiple windows
      could expect to get an expose event for each window that needs to be
      redrawn. Therefore, multi-window expose event handlers typically look
      like this:
      <informalexample><programlisting>
      if (event->window == widget->window1)
        {
           /* ... draw window1 ... */
        }
      else if (event->window == widget->window2)
        {
           /* ... draw window2 ... */
        }
      ...
      </programlisting></informalexample>
      In contrast, the ::draw signal handler may have to draw multiple
      windows in one call. GTK+ has a convenience function
      gtk_cairo_should_draw_window() that can be used to find out if
      a window needs to be drawn. With that, the example above would look
      like this (note that the 'else' is gone):
      <informalexample><programlisting>
      if (gtk_cairo_should_draw_window (cr, widget->window1)
        {
           /* ... draw window1 ... */
        }
      if (gtk_cairo_should_draw_window (cr, widget->window2)
        {
           /* ... draw window2 ... */
        }
      ...
      </programlisting></informalexample>
      Another convenience function that can help when implementing
      ::draw for multi-window widgets is gtk_cairo_transform_to_window(),
      which transforms a cairo context from widget-relative coordinates
      to window-relative coordinates.
    </para>
    <para>
      All GtkStyle drawing functions (gtk_paint_box(), etc) have been changed
      to take a #cairo_t instead of a window and a clip area. ::draw
      implementations will usually just use the cairo context that has been
      passed in for this.
    </para>
    <example><title>A simple ::draw function</title>
    <programlisting>
gboolean
gtk_arrow_draw (GtkWidget *widget,
                cairo_t   *cr)
{
  gint x, y;
  gint width, height;
  gint extent;

  width = gtk_widget_get_allocated_width (widget);
  height = gtk_widget_get_allocated_height (widget);

  extent = MIN (width - 2 * PAD, height - 2 * PAD);
  x = PAD;
  y = PAD;

  gtk_paint_arrow (gtk_widget_get_style (widget),
                   cr,
                   gtk_widget_get_state (widget),
                   GTK_SHADOW_OUT,
                   widget,
                   "arrow",
                   widget->priv->arrow_type,
                   TRUE,
                   x, y, extent, extent);
}
    </programlisting>
    </example>
  </section>

  <section>
    <title>GtkProgressBar orientation</title>

    <para>
      In GTK+ 2.x, #GtkProgressBar and #GtkCellRendererProgress were using the
      GtkProgressBarOrientation enumeration to specify their orientation and
      direction. In GTK+ 3, both the widget and the cell renderer implement
      #GtkOrientable, and have an additional 'inverted' property to determine
      their direction. Therefore, a call to gtk_progress_bar_set_orientation()
      needs to be replaced by a pair of calls to
      gtk_orientable_set_orientation() and gtk_progress_bar_set_inverted().
      The following values correspond:
      <table>
        <tgroup cols="3">
          <colspec colname="1"/>
          <colspec colname="2"/>
          <colspec colname="3"/>
          <thead>
            <row><entry>GTK+ 2.x</entry><entry namest="2" nameend="3">GTK+ 3</entry></row>
            <row><entry>GtkProgressBarOrientation</entry><entry>GtkOrientation</entry><entry>inverted</entry></row>
          </thead>
          <tbody>
            <row><entry>GTK_PROGRESS_LEFT_TO_RIGHT</entry><entry>GTK_ORIENTATION_HORIZONTAL</entry><entry>FALSE</entry></row>
            <row><entry>GTK_PROGRESS_RIGHT_TO_LEFT</entry><entry>GTK_ORIENTATION_HORIZONTAL</entry><entry>TRUE</entry></row>
            <row><entry>GTK_PROGRESS_TOP_TO_BOTTOM</entry><entry>GTK_ORIENTATION_VERTICAL</entry><entry>FALSE</entry></row>
            <row><entry>GTK_PROGRESS_BOTTOM_TO_TOP</entry><entry>GTK_ORIENTATION_VERTICAL</entry><entry>TRUE</entry></row>
          </tbody>
        </tgroup>
      </table>
    </para>
  </section>

  <section>
    <title>GtkScrolledWindow policy</title>

    <para>
      The default values for the #GtkScrolledWindow:hscrollbar-policy and
      #GtkScrolledWindow:vscrollbar-policy properties have been changed from
      'never' to 'automatic'. If your application was relying on the default
      value, you will have explicitly set it explicitly.
    </para>
  </section>

  <section>
    <title>GtkObject is gone</title>

    <para>
      GtkObject has been removed in GTK+ 3. Its remaining functionality,
      the ::destroy signal, has been moved to GtkWidget. If you have non-widget
      classes that are directly derived from GtkObject, you have to make
      them derive from #GInitiallyUnowned (or, if you don't need the floating
      functionality, #GObject). If you have widgets that override the
      destroy class handler, you have to adust your class_init function,
      since destroy is now a member of GtkWidgetClass:
      <informalexample><programlisting>
      GtkObjectClass *object_class = GTK_OBJECT_CLASS (class);

      object_class->destroy = my_destroy;
      </programlisting></informalexample>
      becomes
      <informalexample><programlisting>
      GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (class);

      widget_class->destroy = my_destroy;
      </programlisting></informalexample>
      In the unlikely case that you have a non-widget class that is derived
      from GtkObject and makes use of the destroy functionality, you have
      to implement ::destroy yourself.
    </para>
  </section>

  <section>
    <title>Prevent mixed linkage</title>
    <para>
      Linking against GTK+ 2.x and GTK+ 3 in the same process is problematic
      and can lead to hard-to-diagnose crashes. The gtk_init() function in
      both GTK+ 2.22 and in GTK+ 3 tries to detect this situation and abort
      with a diagnostic message, but this check is not 100% reliable (e.g. if
      the problematic linking happens only in loadable modules).
    </para>
    <para>
      Direct linking of your application against both versions of GTK+ is
      easy to avoid; the problem gets harder when your application is using
      libraries that are themselves linked against some version of GTK+.
      In that case, you have to verify that you are using a version of the
      library that is linked against GTK+ 3.
    </para>
    <para>
      If you are using packages provided by a distributor, it is likely that
      parallel installable versions of the library exist for GTK+ 2.x and
      GTK+ 3, e.g for vte, check for vte3; for webkitgtk look for webkitgtk3,
      and so on.
    </para>
  </section>

  <section>
  <title>Install GTK+ modules in the right place</title>
  <para>
    Some software packages install loadable GTK+ modules such as theme engines,
    gdk-pixbuf loaders or input methods. Since GTK+ 3 is parallel-installable
    with GTK+ 2.x, the two GTK+ versions have separate locations for their
    loadable modules. The location for GTK+ 2.x is
    <filename><replaceable>libdir</replaceable>/gtk-2.0</filename>
    (and its subdirectories), for GTK+ 3 the location is
    <filename><replaceable>libdir</replaceable>/gtk-3.0</filename>
    (and its subdirectories).
  </para>
  <para>
    For some kinds of modules, namely input methods and pixbuf loaders,
    GTK+ keeps a cache file with extra information about the modules.
    For GTK+ 2.x, these cache files are located in
    <filename><replaceable>sysconfdir</replaceable>/gtk-2.0</filename>.
    For GTK+ 3, they have been moved to
    <filename><replaceable>libdir</replaceable>/gtk-3.0/3.0.0/</filename>.
    The commands that create these cache files have been renamed with a -3
    suffix to make them parallel-installable.
  </para>
  <para>
    Note that GTK+ modules often link against libgtk, libgdk-pixbuf, etc.
    If that is the case for your module, you have to be careful to link the
    GTK+ 2.x version of your module against the 2.x version of the libraries,
    and the GTK+ 3 version against hte 3.x versions. Loading a module linked
    against libgtk 2.x into an application using GTK+ 3 will lead to
    unhappiness and must be avoided.
  </para>
  </section>

  </section>

</chapter>