## Migrating from GTK+ 2.x to GTK+ 3

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.

## Preparation in GTK+ 2.x

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.

### Do not include individual headers

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

`gtk/gtk.h` | for GTK
`gtk/gtkx.h` | for the X-specfic widgets GtkSocket and GtkPlug
`gtk/gtkunixprint.h` | for low-level, UNIX-specific printing functions
`gdk/gdk.h` | for GDK
`gdk/gdkx.h` | for GDK functions that are X11-specific
`gdk/gdkwin32.h` | for GDK functions that are Windows-specific

(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 `pkg-config --cflags gtk+-2.0.pc`.)

To check that your application only includes the allowed headers, you can use defines to disable inclusion of individual headers, as follows:

        make CFLAGS+="-DGTK_DISABLE_SINGLE_INCLUDES"

### Do not use deprecated symbols

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 for GTK+ 2 also includes an index of all deprecated symbols.

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:

        make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"

Note that some parts of our API, such as enumeration values, are not well covered by the deprecation warnings. In most cases, using them will require you to also use deprecated functions, which will trigger warnings. But some things, like the `GTK_DIALOG_NO_SEPARATOR` flag that has disappeared in GTK+ 3, may not.

### Use accessor functions instead of direct access

GTK+ 3 removes many implementation details and struct members from its public headers.

To ensure that your application does not have problems with this, you define the preprocessor symbol `GSEAL_ENABLE` while building your application against GTK+ 2.x. 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.

        make CFLAGS+="-DGSEAL_ENABLE"

While it may be painful to convert, this helps us keep API and ABI compatibility when we change internal interfaces. As a quick example, when adding GSEAL_ENABLE, if you see an error like:

        error: 'GtkToggleButton' has no member named 'active'

this means that you are accessing the public structure of GtkToggleButton directly, perhaps with some code like:

          static void
          on_toggled (GtkToggleButton *button)
          {
            if (button->active)
              frob_active ();
            else
              frob_inactive ();
          }

In most cases, this can easily be replaced with the correct accessor method. The main rule is that if you have code like the above which accesses the "active" field of a "GtkToggleButton", then the accessor method becomes "gtk_toggle_button_get_active":

          static void
          on_toggled (GtkToggleButton *button)
          {
            if (gtk_toggle_button_get_active (button))
              frob_active ();
            else
              frob_inactive ();
          }

In the case of setting field members directly, there's usually a corresponding setter method.

### Replace GDK_<keyname> with GDK_KEY_<keyname>

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

### Use GIO for launching applications

The `gdk_spawn` family of functions has been deprecated in GDK 2.24 and removed from GDK 3. Various replacements exist; the best replacement depends on the circumstances:

- If you are opening a document or URI by launching a command like `firefox http://my-favourite-website.com` or `gnome-open ghelp:epiphany`, it is best to just use `gtk_show_uri()`; as an added benefit, your application will henceforth respect the users preference for what application to use.
- If you are launching a regular, installed application that has a desktop file, it is best to use GIOs GAppInfo with a suitable launch context.

              GAppInfo *info;
              GAppLaunchContext *context;
              GError *error = NULL;

              info = (GAppInfo*) g_desktop_app_info_new ("epiphany.desktop");
              context = (GAppLaunchContext*) gdk_display_get_app_launch_context (display);
              g_app_info_launch (info, NULL, context, &error);

              if (error)
                {
                  g_warning ("Failed to launch epiphany: %s", error->message);
                  g_error_free (error);
                }

              g_object_unref (info);
              g_object_unref (context);

  Remember that you have to include `gio/gdesktopappinfo.h` and use the `gio-unix-2.0` pkg-config file when using `g_desktop_app_info_new()`.
- If you are launching a custom commandline, you can still use `g_app_info_launch()` with a GAppInfo that is constructed with `g_app_info_create_from_commandline()`, or you can use the more lowlevel `g_spawn` family of functions (e.g. `g_spawn_command_line_async()`), and pass `DISPLAY` in the environment. `gdk_screen_make_display_name()` can be used to find the right value for the `DISPLAY` environment variable.

### Use cairo for drawing

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.

The GdkGC and GdkImage objects, as well as all the functions using them, are gone. This includes the `gdk_draw` 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 GdkWindows, which cairo supports automatically, a transition is usually straightforward.

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.

Example 6. Drawing a GdkPixbuf onto a GdkWindow

Drawing a pixbuf onto a drawable used to be done like this:

    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);

Doing the same thing with cairo:

    cairo_t *cr = gdk_cairo_create (window);
    gdk_cairo_set_source_pixbuf (cr, pixbuf, x, y);
    cairo_paint (cr);
    cairo_destroy (cr);

Note that very similar code can be used when porting code using GdkPixmap to cairo_surface_t by calling `cairo_set_source_surface()` instead of `gdk_cairo_set_source_pixbuf()`.

Example 7. Drawing a tiled GdkPixmap to a GdkWindow

Tiled pixmaps are often used for drawing backgrounds. Old code looked something like this:

    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 (window, 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);

The equivalent cairo code to draw a tiled surface looks like this:

    cairo_t *cr;
    cairo_surface_t *surface;

    surface = ...
    cr = gdk_cairo_create (window);
    cairo_set_source_surface (cr, surface, 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);

The surface here can be either an image surface or a X surface, and can either be created on the spot or kept around for caching purposes. Another alternative is to use pixbufs instead of surfaces with `gdk_cairo_set_source_pixbuf()` instead of `cairo_set_source_surface()`.

Example 8. Drawing a PangoLayout to a clipped area

Drawing layouts clipped is often used to avoid overdraw or to allow drawing selections. Code would have looked like this:

    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);

With cairo, the same effect can be achieved using:

    GtkStyleContext *context;
    GtkStateFlags flags;
    GdkRGBA rgba;
    cairo_t *cr;

    cr = gdk_cairo_create (drawable);
    /* clip */
    gdk_cairo_rectangle (cr, &area);
    cairo_clip (cr);
    /* set the correct source color */
    context = gtk_widget_get_style_context (widget));
    state = gtk_widget_get_state_flags (widget);
    gtk_style_context_get_color (context, state, &rgba);
    gdk_cairo_set_source_rgba (cr, &rgba);
    /* draw the text */
    cairo_move_to (cr, x, y);
    pango_cairo_show_layout (cr, layout);
    cairo_destroy (cr);

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.

#### What should you be aware of ?

No more stippling. 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:

    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;
    }

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.

Using the target also as source or mask. 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:

    gdk_draw_drawable (pixmap,
                       gc,
                       pixmap,
                       area.x + dx, area.y + dy,
                       area.x, area.y,
                       area.width, area.height);

By using this code:

    cairo_t *cr = cairo_create (surface);
    /* 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);
    cairo_set_source_surface (cr, surface, dx, dy);
    cairo_paint (cr);
    /* Now copy the intermediate target back */
    cairo_pop_group_to_source (cr);
    cairo_paint (cr);
    cairo_destroy (cr);

The surface here can be either an image surface or a X surface, and can either be created on the spot or kept around for caching purposes. Another alternative is to use pixbufs instead of surfaces with `gdk_cairo_set_source_pixbuf()` instead of `cairo_set_source_surface()`. 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.

Using pango_cairo_show_layout() instead of gdk_draw_layout_with_colors(). 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.