## Changes that need to be done at the time of the switch

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.

### Replace size_request by get_preferred_width/height

The request-phase of the traditional GTK+ geometry management has been replaced by a more flexible height-for-width system, which is described in detail in the API documentation (see the section called “Height-for-width Geometry Management”). As a consequence, the ::size-request signal and vfunc has been removed from GtkWidgetClass. The replacement for `size_request()` can take several levels of sophistication:

- As a minimal replacement to keep current functionality, you can simply implement the `GtkWidgetClass.get_preferred_width()` and `GtkWidgetClass.get_preferred_height()` vfuncs by calling your existing `size_request()` function. So you go from

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      static void
      my_widget_class_init (MyWidgetClass *class)
      {
        GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (class);

        /* ... */

        widget_class->size_request = my_widget_size_request;

        /* ... */
      }

  to something that looks more like this:

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      static void
      my_widget_get_preferred_width (GtkWidget *widget,
                                     gint      *minimal_width,
                                     gint      *natural_width)
      {
        GtkRequisition requisition;

        my_widget_size_request (widget, &requisition);

        *minimal_width = *natural_width = requisition.width;
      }

      static void
      my_widget_get_preferred_height (GtkWidget *widget,
                                      gint      *minimal_height,
                                      gint      *natural_height)
      {
        GtkRequisition requisition;

        my_widget_size_request (widget, &requisition);

        *minimal_height = *natural_height = requisition.height;
      }

       /* ... */

      static void
      my_widget_class_init (MyWidgetClass *class)
      {
        GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (class);

        /* ... */

        widget_class->get_preferred_width = my_widget_get_preferred_width;
        widget_class->get_preferred_height = my_widget_get_preferred_height;

        /* ... */

      }

  Sometimes you can make things a little more streamlined by replacing your existing `size_request()` implementation by one that takes an orientation parameter:

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      static void
      my_widget_get_preferred_size (GtkWidget      *widget,
                                    GtkOrientation  orientation,
                                     gint          *minimal_size,
                                     gint          *natural_size)
      {

        /* do things that are common for both orientations ... */

        if (orientation == GTK_ORIENTATION_HORIZONTAL)
          {
            /* do stuff that only applies to width... */

            *minimal_size = *natural_size = ...
          }
        else
         {
            /* do stuff that only applies to height... */

            *minimal_size = *natural_size = ...
         }
      }

      static void
      my_widget_get_preferred_width (GtkWidget *widget,
                                     gint      *minimal_width,
                                     gint      *natural_width)
      {
        my_widget_get_preferred_size (widget,
                                      GTK_ORIENTATION_HORIZONTAL,
                                      minimal_width,
                                      natural_width);
      }

      static void
      my_widget_get_preferred_height (GtkWidget *widget,
                                      gint      *minimal_height,
                                      gint      *natural_height)
      {
        my_widget_get_preferred_size (widget,
                                      GTK_ORIENTATION_VERTICAL,
                                      minimal_height,
                                      natural_height);
      }

       /* ... */

- If your widget can cope with a small size, but would appreciate getting some more space (a common example would be that it contains ellipsizable labels), you can do that by making your `GtkWidgetClass.get_preferred_width()` / `GtkWidgetClass.get_preferred_height()` functions return a smaller value for `minimal` than for `natural`. For `minimal`, you probably want to return the same value that your `size_request()` function returned before (since `size_request()` was defined as returning the minimal size a widget can work with). A simple way to obtain good values for `natural`, in the case of containers, is to use `gtk_widget_get_preferred_width()` and `gtk_widget_get_preferred_height()` on the children of the container, as in the following example:

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      static void
      gtk_fixed_get_preferred_height (GtkWidget *widget,
                                      gint      *minimum,
                                      gint      *natural)
      {
        GtkFixed *fixed = GTK_FIXED (widget);
        GtkFixedPrivate *priv = fixed->priv;
        GtkFixedChild *child;
        GList *children;
        gint child_min, child_nat;

        *minimum = 0;
        *natural = 0;

        for (children = priv->children; children; children = children->next)
          {
            child = children->data;

            if (!gtk_widget_get_visible (child->widget))
              continue;

            gtk_widget_get_preferred_height (child->widget, &child_min, &child_nat);

            *minimum = MAX (*minimum, child->y + child_min);
            *natural = MAX (*natural, child->y + child_nat);
          }
      }

- Note that the `GtkWidgetClass.get_preferred_width()` / `GtkWidgetClass.get_preferred_height()` functions only allow you to deal with one dimension at a time. If your `size_request()` handler is doing things that involve both width and height at the same time (e.g. limiting the aspect ratio), you will have to implement `GtkWidgetClass.get_preferred_height_for_width()` and `GtkWidgetClass.get_preferred_width_for_height()`.

- To make full use of the new capabilities of the height-for-width geometry management, you need to additionally implement the `GtkWidgetClass.get_preferred_height_for_width()` and `GtkWidgetClass.get_preferred_width_for_height()`. For details on these functions, see the section called “Height-for-width Geometry Management”.

### Replace GdkRegion by cairo_region_t

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.

Porting your application to the cairo region API should be a straight find-and-replace task. Please refer to the following table:

Table 1.

GDK | cairo
--- | ---
GdkRegion | cairo_region_t
GdkRectangle | cairo_rectangle_int_t
`gdk_rectangle_intersect()` | this function is still there
`gdk_rectangle_union()` | this function is still there
`gdk_region_new()` | `cairo_region_create()`
`gdk_region_copy()` | `cairo_region_copy()`
`gdk_region_destroy()` | `cairo_region_destroy()`
`gdk_region_rectangle()` | `cairo_region_create_rectangle()`
`gdk_region_get_clipbox()` | `cairo_region_get_extents()`
`gdk_region_get_rectangles()` | `cairo_region_num_rectangles()` and `cairo_region_get_rectangle()`
`gdk_region_empty()` | `cairo_region_is_empty()`
`gdk_region_equal()` | `cairo_region_equal()`
`gdk_region_point_in()` | `cairo_region_contains_point()`
`gdk_region_rect_in()` | `cairo_region_contains_rectangle()`
`gdk_region_offset()` | `cairo_region_translate()`
`gdk_region_union_with_rect()` | `cairo_region_union_rectangle()`
`gdk_region_intersect()` | `cairo_region_intersect()`
`gdk_region_union()` | `cairo_region_union()`
`gdk_region_subtract()` | `cairo_region_subtract()`
`gdk_region_xor()` | `cairo_region_xor()`
`gdk_region_shrink()` | no replacement
`gdk_region_polygon()` | no replacement, use cairo paths instead

### Replace GdkPixmap by cairo surfaces

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.

Example 9. Creating custom cursors

One place where pixmaps were commonly used is to create custom cursors:

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    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, &fg, &fg, 0, 0);

    g_object_unref (pixmap);

The same can be achieved without pixmaps, by drawing onto an image surface:

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    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 (s,
                                          0, 0,
                                          3, 3);

    cairo_surface_destroy (s);

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

    g_object_unref (pixbuf);

### Replace GdkColormap by GdkVisual

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_widget_set_visual()` has been added.

Example 10. Setting up a translucent window

You might have a screen-changed handler like the following to set up a translucent window with an alpha-channel:

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    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);
    }

With visuals instead of colormaps, this will look as follows:

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    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_widget_set_visual (window, visual);
    }

### GdkDrawable is gone

GdkDrawable has been removed in GTK+ 3, together with GdkPixmap and GdkImage. The only remaining drawable class is GdkWindow. For dealing with image data, you should use a cairo_surface_t or a GdkPixbuf.

GdkDrawable functions that are useful with windows have been replaced by corresponding GdkWindow functions:

Table 2. GdkDrawable to GdkWindow

GDK 2.x | GDK 3
--- | ---
`gdk_drawable_get_visual()` | `gdk_window_get_visual()`
`gdk_drawable_get_size()` | `gdk_window_get_width()` `gdk_window_get_height()`
`gdk_pixbuf_get_from_drawable()` | `gdk_pixbuf_get_from_window()`
`gdk_drawable_get_clip_region()` | `gdk_window_get_clip_region()`
`gdk_drawable_get_visible_region()` | `gdk_window_get_visible_region()`

### Event filtering

If your application uses the low-level event filtering facilities in GDK, there are some changes you need to be aware of.

The special-purpose GdkEventClient events and the `gdk_add_client_message_filter()` and `gdk_display_add_client_message_filter()` functions have been removed. Receiving X11 ClientMessage events is still possible, using the general `gdk_window_add_filter()` API. A client message filter like

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    static GdkFilterReturn
    message_filter (GdkXEvent *xevent, GdkEvent *event, gpointer data)
    {
      XClientMessageEvent *evt = (XClientMessageEvent *)xevent;

      /* do something with evt ... */
    }

     ...

    message_type = gdk_atom_intern ("MANAGER", FALSE);
    gdk_display_add_client_message_filter (display, message_type, message_filter, NULL);

then looks like this:

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    static GdkFilterReturn
    event_filter (GdkXEvent *xevent, GdkEvent *event, gpointer data)
    {
      XClientMessageEvent *evt;
      GdkAtom message_type;

      if (((XEvent *)xevent)->type != ClientMessage)
        return GDK_FILTER_CONTINUE;

      evt = (XClientMessageEvent *)xevent;
      message_type = XInternAtom (evt->display, "MANAGER", FALSE);

      if (evt->message_type != message_type)
        return GDK_FILTER_CONTINUE;

      /* do something with evt ... */
    }

     ...

    gdk_window_add_filter (NULL, message_filter, NULL);

One advantage of using an event filter is that you can actually remove the filter when you don't need it anymore, using `gdk_window_remove_filter()`.

The other difference to be aware of when working with event filters in GTK+ 3 is that GDK now uses XI2 by default when available. That means that your application does not receive core X11 key or button events. Instead, all input events are delivered as XIDeviceEvents. As a short-term workaround for this, you can force your application to not use XI2, with `gdk_disable_multidevice()`. In the long term, you probably want to rewrite your event filter to deal with XIDeviceEvents.

### Backend-specific code

In GTK+ 2.x, GDK could only be compiled for one backend at a time, and the `GDK_WINDOWING_X11` or `GDK_WINDOWING_WIN32` macros could be used to find out which one you are dealing with:

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    #ifdef GDK_WINDOWING_X11
          if (timestamp != GDK_CURRENT_TIME)
            gdk_x11_window_set_user_time (gdk_window, timestamp);
    #endif
    #ifdef GDK_WINDOWING_WIN32
            /* ... win32 specific code ... */
    #endif

In GTK+ 3, GDK can be built with multiple backends, and currently used backend has to be determined at runtime, typically using type-check macros on a GdkDisplay or GdkWindow. You still need to use the GDK_WINDOWING macros to only compile code referring to supported backends:

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    #ifdef GDK_WINDOWING_X11
          if (GDK_IS_X11_DISPLAY (display))
            {
              if (timestamp != GDK_CURRENT_TIME)
                gdk_x11_window_set_user_time (gdk_window, timestamp);
            }
          else
    #endif
    #ifdef GDK_WINDOWING_WIN32
          if (GDK_IS_WIN32_DISPLAY (display))
            {
              /* ... win32 specific code ... */
            }
          else
    #endif
           {
             g_warning ("Unsupported GDK backend");
           }

If you used the pkg-config variable `target` to conditionally build part of your project depending on the GDK backend, for instance like this:

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    AM_CONDITIONAL(BUILD_X11, test `$PKG_CONFIG --variable=target gtk+-2.0` = "x11")

then you should now use the M4 macro provided by GTK+ itself:

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    GTK_CHECK_BACKEND([x11], [3.0.2], [have_x11=yes], [have_x11=no])
    AM_CONDITIONAL(BUILD_x11, [test "x$have_x11" = "xyes"])

### GtkPlug and GtkSocket

The GtkPlug and GtkSocket widgets are now X11-specific, and you have to include the `<gtk/gtkx.h>` header to use them. The previous section about proper handling of backend-specific code applies, if you care about other backends.

### The GtkWidget::draw signal

The GtkWidget “expose-event” signal has been replaced by a new “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.

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'.

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.

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:

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    if (event->window == widget->window1)
      {
         /* ... draw window1 ... */
      }
    else if (event->window == widget->window2)
      {
         /* ... draw window2 ... */
      }
    ...

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):

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    if (gtk_cairo_should_draw_window (cr, widget->window1)
      {
         /* ... draw window1 ... */
      }
    if (gtk_cairo_should_draw_window (cr, widget->window2)
      {
         /* ... draw window2 ... */
      }
    ...

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. You may want to use `cairo_save()` and `cairo_restore()` when modifying the cairo context in your draw function.

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.

Example 11. A simple ::draw function

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    gboolean
    gtk_arrow_draw (GtkWidget *widget,
                    cairo_t   *cr)
    {
      GtkStyleContext *context;
      gint x, y;
      gint width, height;
      gint extent;

      context = gtk_widget_get_style_context (widget);

      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_render_arrow (context, rc, G_PI / 2, x, y, extent);
    }

### GtkProgressBar orientation

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 3.

GTK+ 2.x | GTK+ 3
--- | ---
GtkProgressBarOrientation | GtkOrientation | inverted
GTK_PROGRESS_LEFT_TO_RIGHT | GTK_ORIENTATION_HORIZONTAL | FALSE
GTK_PROGRESS_RIGHT_TO_LEFT | GTK_ORIENTATION_HORIZONTAL | TRUE
GTK_PROGRESS_TOP_TO_BOTTOM | GTK_ORIENTATION_VERTICAL | FALSE
GTK_PROGRESS_BOTTOM_TO_TOP | GTK_ORIENTATION_VERTICAL | TRUE

### Check your expand and fill flags

The behaviour of expanding widgets has changed slightly in GTK+ 3, compared to GTK+ 2.x. It is now 'inherited', i.e. a container that has an expanding child is considered expanding itself. This is often the desired behaviour. In places where you don't want this to happen, setting the container explicity as not expanding will stop the expand flag of the child from being inherited. See `gtk_widget_set_hexpand()` and `gtk_widget_set_vexpand()`.

If you experience sizing problems with widgets in ported code, carefully check the GtkBox expand and GtkBox fill child properties of your boxes.

### Scrolling changes

The default values for the “hscrollbar-policy” and “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.

The ::set-scroll-adjustments signal on GtkWidget has been replaced by the GtkScrollable interface which must be implemented by a widget that wants to be placed in a GtkScrolledWindow. Instead of emitting ::set-scroll-adjustments, the scrolled window simply sets the “hadjustment” and “vadjustment” properties.

### GtkObject is gone

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 adjust your class_init function, since destroy is now a member of GtkWidgetClass:

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    GtkObjectClass *object_class = GTK_OBJECT_CLASS (class);

    object_class->destroy = my_destroy;

becomes

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    GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (class);

    widget_class->destroy = my_destroy;

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.

If your program used functions like gtk_object_get or gtk_object_set, these can be replaced directly with g_object_get or g_object_set. In fact, most every gtk_object_* function can be replaced with the corresponding g_object_ function, even in GTK+ 2 code. The one exception to this rule is gtk_object_destroy, which can be replaced with gtk_widget_destroy, again in both GTK+ 2 and GTK+ 3.

### GtkEntryCompletion signal parameters

The “match-selected” and “cursor-on-match” signals were erroneously given the internal filter model instead of the users model. This oversight has been fixed in GTK+ 3; if you have handlers for these signals, they will likely need slight adjustments.

### Resize grips

The resize grip functionality has been moved from GtkStatusbar to GtkWindow. Any window can now have resize grips, regardless whether it has a statusbar or not. The functions `gtk_statusbar_set_has_resize_grip()` and `gtk_statusbar_get_has_resize_grip()` have disappeared, and instead there are now `gtk_window_set_has_resize_grip()` and `gtk_window_get_has_resize_grip()`.

### Prevent mixed linkage

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).

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.

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.

### Install GTK+ modules in the right place

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 ``libdir`/gtk-2.0` (and its subdirectories), for GTK+ 3 the location is ``libdir`/gtk-3.0` (and its subdirectories).

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 ``sysconfdir`/gtk-2.0`. For GTK+ 3, they have been moved to ``libdir`/gtk-3.0/3.0.0/`. The commands that create these cache files have been renamed with a -3 suffix to make them parallel-installable.

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.