Gtk: Migrating Themes from GTK 2.x to GTK 3

Migrating Themes from GTK 2.x to GTK 3

Theming changes

In GTK 3, GtkStyleContext was added to replace GtkStyle and the theming infrastructure available in 2.x. GtkStyleContext is an object similar in spirit to GtkStyle, as it contains theming information, although in a more complete and tokenized fashion. There are two aspects to switching to GtkStyleContext: porting themes and theme engines, and porting applications, libraries and widgets.

Migrating themes

From GTK 3.0 on, theme engines must implement GtkThemingEngine and be installed in $libdir/gtk+-3.0/$GTK_VERSION/theming-engines, and the files containing style information must be written in the CSS-like format that is understood by GtkCssProvider. For a theme named “Clearlooks”, the CSS file parsed by default is $datadir/themes/Clearlooks/gtk-3.0/gtk.css, with possible variants such as the dark theme being named gtk-dark.css in the same directory. Starting from GTK 3.14, the theming engine modules are deprecated in favor of pure CSS themes, and GTK will not load them.

If your theme RC file was providing values for GtkSettings, you can install a settings.ini keyfile along with the gtk.css to provide theme-specific defaults for settings.

Key themes have been converted to CSS syntax too. See the GtkCssProvider documentation information about the syntax. GTK looks for key themes in the file $datadir/themes/theme/gtk-3.0/gtk-keys.css, where theme is the current key theme name.

Migrating theme engines

Migrating a GtkStyle based engine to a GtkThemingEngine based one should be straightforward for most of the virtual functions. Besides a cleanup in the available paint methods and a simplification in the passed arguments (in favor of GtkStyleContext containing all the information), the available render methods resemble those of GtkStyle quite evidently. Notable differences include:

All variations of gtk_paint_box(), gtk_paint_flat_box(), gtk_paint_shadow(), gtk_paint_box_gap() and gtk_paint_shadow_gap() are replaced by gtk_render_background(), gtk_render_frame() and gtk_render_frame_gap(). The first function renders frameless backgrounds and the last two render frames in various forms.
gtk_paint_resize_grip() has been subsumed by gtk_render_handle() with a GTK_STYLE_CLASS_GRIP class set in the style context.
gtk_paint_spinner() disappears in favor of gtk_render_activity() with a GTK_STYLE_CLASS_SPINNER class set in the style context.

The list of available render methods is:

gtk_render_background(): Renders a widget/area background.
gtk_render_frame(): Renders a frame border around the given rectangle. Usually the detail of the border depends on the theme information, plus the current widget state.
gtk_render_frame_gap(): Renders a frame border with a gap on one side.
gtk_render_layout(): Renders a PangoLayout.
gtk_render_handle(): Renders all kind of handles and resize grips, depending on the style class.
gtk_render_check(): Render checkboxes.
gtk_render_option(): Render radiobuttons.
gtk_render_arrow(): Renders an arrow pointing to a direction.
gtk_render_expander(): Renders an expander indicator, such as in GtkExpander.
gtk_render_focus(): Renders the indication that a widget has the keyboard focus.
gtk_render_line(): Renders a line from one coordinate to another.
gtk_render_slider(): Renders a slider, such as in GtkScale.
gtk_render_extension(): Renders an extension that protrudes from a UI element, such as a notebook tab.
gtk_render_activity(): Renders an area displaying activity, be it a progressbar or a spinner.
gtk_render_icon_pixbuf(): Renders an icon into a GdkPixbuf.

One of the main differences to GtkStyle-based engines is that the rendered widget is totally isolated from the theme engine, all style information is meant to be retrieved from the GtkThemingEngine API, or from the GtkWidgetPath obtained from gtk_theming_engine_get_path(), which fully represents the rendered widget’s hierarchy from a styling point of view.

The detail string available in GtkStyle-based engines has been replaced by widget regions and style classes. Regions are a way for complex widgets to associate different styles with different areas, such as even and odd rows in a treeview. Style classes allow sharing of style information between widgets, regardless of their type. Regions and style classes can be used in style sheets to associate styles, and them engines can also access them. There are several predefined classes and regions such as GTK_STYLE_CLASS_BUTTON or GTK_STYLE_REGION_TAB in gtkstylecontext.h, although custom widgets may define their own, which themes may attempt to handle.

Extending the CSS parser

In GtkStyle-based engines, GtkRCStyle provided ways to extend the gtkrc parser with engine-specific extensions. This has been replaced by gtk_theming_engine_register_property(), which lets a theme engine register new properties with an arbitrary type. While there is built-in support for most basic types, it is possible to use a custom parser for the property.

The installed properties depend on the “name” property, so they should be added in the constructed() vfunc. For example, if an engine with the name “Clearlooks” installs a “focus-color” property with the type GdkRGBA, the property -Clearlooks-focus-color will be registered and accepted in CSS like this:

GtkEntry {
  -Clearlooks-focus-color: rgba(255, 0, 0, 1.0);
}

Widget style properties also follow a similar syntax, with the widget type name used as a prefix. For example, the GtkWidget focus-line-width style property can be modified in CSS as -GtkWidget-focus-line-width.

Using the CSS file format

The syntax of RC and CSS files formats is obviously different. The CSS-like syntax will hopefully be much more familiar to many people, lowering the barrier for custom theming.

Instead of going through the syntax differences one-by-one, we will present a more or less comprehensive example and discuss how it can be translated into CSS:

style "default" {
        xthickness = 1
        ythickness = 1

        GtkButton::child-displacement-x = 1
        GtkButton::child-displacement-y = 1
        GtkCheckButton::indicator-size = 14

        bg[NORMAL]        = @bg_color
        bg[PRELIGHT]      = shade (1.02, @bg_color)
        bg[SELECTED]      = @selected_bg_color
        bg[INSENSITIVE]   = @bg_color
        bg[ACTIVE]        = shade (0.9, @bg_color)

        fg[NORMAL]        = @fg_color
        fg[PRELIGHT]      = @fg_color
        fg[SELECTED]      = @selected_fg_color
        fg[INSENSITIVE]   = darker (@bg_color)
        fg[ACTIVE]        = @fg_color

        text[NORMAL]      = @text_color
        text[PRELIGHT]    = @text_color
        text[SELECTED]    = @selected_fg_color
        text[INSENSITIVE] = darker (@bg_color)
        text[ACTIVE]      = @selected_fg_color

        base[NORMAL]      = @base_color
        base[PRELIGHT]    = shade (0.95, @bg_color)
        base[SELECTED]    = @selected_bg_color
        base[INSENSITIVE] = @bg_color
        base[ACTIVE]      = shade (0.9, @selected_bg_color)

        engine "clearlooks" {
                colorize_scrollbar = TRUE
                style = CLASSIC
        }
}

style "tooltips" {
        xthickness = 4
        ythickness = 4

        bg[NORMAL]        = @tooltip_bg_color
        fg[NORMAL]        = @tooltip_fg_color
}

style "button" {
        xthickness = 3
        ythickness = 3

        bg[NORMAL]        = shade (1.04, @bg_color)
        bg[PRELIGHT]      = shade (1.06, @bg_color)
        bg[ACTIVE]        = shade (0.85, @bg_color)
}

style "entry" {
        xthickness = 3
        ythickness = 3

        bg[SELECTED] = mix (0.4, @selected_bg_color, @base_color)
        fg[SELECTED] = @text_color

        engine "clearlooks" {
                focus_color = shade (0.65, @selected_bg_color)
        }
}

style "other" {
        bg[NORMAL] = #fff;
}

class "GtkWidget" style "default"
class "GtkEntry" style "entry"
widget_class "*<GtkButton>" style "button"
widget "gtk-tooltip*" style "tooltips"
widget_class "window-name.*.GtkButton" style "other"

would roughly translate to this CSS:

* {
  padding: 1;
  -GtkButton-child-displacement-x: 1;
  -GtkButton-child-displacement-y: 1;
  -GtkCheckButton-indicator-size: 14;

  background-color: @bg_color;
  color: @fg_color;

  -Clearlooks-colorize-scrollbar: true;
  -Clearlooks-style: classic;
}

*:hover {
  background-color: shade (@bg_color, 1.02);
}

*:selected {
  background-color: @selected_bg_color;
  color: @selected_fg_color;
}

*:insensitive {
  color: shade (@bg_color, 0.7);
}

*:active {
  background-color: shade (@bg_color, 0.9);
}

.tooltip {
  padding: 4;

  background-color: @tooltip_bg_color;
  color: @tooltip_fg_color;
}

.button {
  padding: 3;
  background-color: shade (@bg_color, 1.04);
}

.button:hover {
  background-color: shade (@bg_color, 1.06);
}

.button:active {
  background-color: shade (@bg_color, 0.85);
}

.entry {
  padding: 3;

  background-color: @base_color;
  color: @text_color;
}

.entry:selected {
  background-color: mix (@selected_bg_color, @base_color, 0.4);
  -Clearlooks-focus-color: shade (0.65, @selected_bg_color)
}

/* The latter selector is an specification of the first,
   since any widget may use the same classes or names */
#window-name .button,
GtkWindow#window-name GtkButton.button {
  background-color: #fff;
}

One notable difference is the reduction from fg/bg/text/base colors to only foreground/background, in exchange the widget is able to render its various elements with different CSS classes, which can be themed independently.

In the same vein, the light, dark and mid color variants that were available in GtkStyle should be replaced by a combination of symbolic colors and custom CSS, where necessary. text_aa should really not be used anywhere, anyway, and the white and black colors that were available in GtkStyle can just be replaced by literal GdkRGBA values.

Access to colors has also changed a bit. With GtkStyle, the common way to access colors is:

GdkColor *color1;
GdkColor color2;

color1 = &style->bg[GTK_STATE_PRELIGHT];
gtk_style_lookup_color (style, "focus_color", &color2);

With GtkStyleContext, you generally use GdkRGBA instead of GdkColor and the code looks like this:

GdkRGBA *color1;
GdkRGBA  color2;

gtk_style_context_get (context, GTK_STATE_FLAG_PRELIGHT,
                       "background-color", &color1,
                       NULL);
gtk_style_context_lookup_color (context, "focus_color", &color2);

/* ... */

gdk_rgba_free (color1);

Note that the memory handling here is different: gtk_style_context_get() expects the address of a GdkRGBA* and returns a newly allocated struct; gtk_style_context_lookup_color() expects the address of an existing struct, and fills it.

It is worth mentioning that the new file format does not support stock icon mappings as the RC format did.

A checklist for widgets

When porting your widgets to use GtkStyleContext, this checklist might be useful.

Replace “style-set” handlers with GtkWidget::style-updated handlers.
Try to identify the role of what you’re rendering with any number of classes. This will replace the detail string. There is a predefined set of CSS classes which you can reuse where appropriate. Doing so will give you theming ‘for free’, whereas custom classes will require extra work in the theme. Note that complex widgets are likely to need different styles when rendering different parts, and style classes are one way to achieve this. This could result in code like the following (simplified) examples:

static void
gtk_button_init (GtkButton *button)
{
  GtkStyleContext *context;

  /* ... */

  context = gtk_widget_get_style_context (GTK_WIDGET (button));

  /* Set the "button" class */
  gtk_style_context_add_class (context, GTK_STYLE_CLASS_BUTTON);

  /* ... */
}

static gboolean
gtk_spin_button_draw (GtkSpinButton *spin,
                      cairo_t       *cr)
{
  GtkStyleContext *context;

  /* ... */

  context = gtk_widget_get_style_context (GTK_WIDGET (spin));

  gtk_style_context_save (context);
  gtk_style_context_add_class (context, GTK_STYLE_CLASS_ENTRY);

  /* Call to entry draw impl with "entry" class */
  parent_class->draw (spin, cr);

  gtk_style_context_restore (context);
  gtk_style_context_save (context);

  /* Render up/down buttons with the "button" class */
  gtk_style_context_add_class (context, GTK_STYLE_CLASS_BUTTON);
  draw_up_button (spin, cr);
  draw_down_button (spin, cr);

  gtk_style_context_restore (context);

  /* ... */
}

Note that GtkStyleContext only provides fg/bg colors, so text/base is done through distinctive theming of the different classes. For example, an entry would usually be black on white while a button would usually be black on light grey.

Replace all gtk_paint_*() calls with corresponding gtk_render_*() calls.
The most distinctive changes are the use of GtkStateFlags to represent the widget state and the lack of GtkShadowType. Note that widget state is now passed implicitly via the context, so to render in a certain state, you have to temporarily set the state on the context, as in the following example:

gtk_style_context_save (context);
gtk_style_context_set_state (context, GTK_STATE_FLAG_ACTIVE);
gtk_render_check (context, cr, x, y, width, height);
gtk_style_context_restore (context);

For gtk_render_check() and gtk_render_option(), the shadow_type parameter is replaced by the GTK_STATE_FLAG_ACTIVE and GTK_STATE_FLAG_INCONSISTENT state flags. For things such as pressed/unpressed button states, GTK_STATE_FLAG_ACTIVE is used, and the CSS may style normal/active states differently to render outset/inset borders, respectively.
The various gtk_widget_modify_*() functions to override colors or fonts for individual widgets have been replaced by similar gtk_widget_override_*() functions.
It is no longer necessary to call gtk_widget_style_attach(), gtk_style_attach(), gtk_style_detach() or gtk_widget_ensure_style().
Replace all uses of xthickness/ythickness. GtkStyleContext uses the CSS box model, and there are border-width/padding/margin properties to replace the different applications of X and Y thickness.

Note that all of this is merely a guideline. Widgets may choose to follow it or not.

Parsing of custom resources

As a consequence of the RC format going away, calling gtk_rc_parse() or gtk_rc_parse_string() won’t have any effect on a widgets appearance. The way to replace these calls is using a custom GtkStyleProvider, either for an individual widget through gtk_style_context_add_provider() or for all widgets on a screen through gtk_style_context_add_provider_for_screen(). Typically, the provider will be a GtkCssProvider, which parses CSS information from a file or from a string.

GtkStyleContext *context;
GtkCssProvider *provider;

context = gtk_widget_get_style_context (widget);
provider = gtk_css_provider_new ();
gtk_css_provider_load_from_data (GTK_CSS_PROVIDER (provider),
                                 ".frame1 {\n"
                                 "   border-image: url('gradient1.png') 10 10 10 10 stretch;\n"
                                 "}\n", -1, NULL);
gtk_style_context_add_provider (context,
                                GTK_STYLE_PROVIDER (provider),
                                GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
g_object_unref (provider);

Notice that you can also get style information from custom resources by implementing the GtkStyleProvider interface yourself. This is an advanced feature that should be rarely used.

Bonus points

There are some features in GtkStyleContext that were not available in GtkStyle, or were made available over time for certain widgets through extending the detail string in obscure ways. There is a lot more information available when rendering UI elements, and it is accessible in more uniform, less hacky ways. By going through this list you’ll ensure your widget is a good citizen in a fully themable user interface.

If your widget renders a series of similar elements, such as tabs in a GtkNotebook or rows/column in a GtkTreeView, consider adding regions through gtk_style_context_add_region(). These regions can be referenced in CSS and the :nth-child pseudo-class may be used to match the elements depending on the flags passed.

GtkNotebook tab {
  background-color: #f3329d;
}

GtkTreeView row::nth-child (even) {
  background-color: #dddddd;
}

If your container renders child widgets within different regions, make it implement Gtk.ContainerClass.get_path_for_child. This function lets containers assign a special GtkWidgetPath to child widgets depending on their role/region. This is necessary to extend the concept above throughout the widget hierarchy. For example, a GtkNotebook modifies the tab labels’ GtkWidgetPath so the “tab” region is added. This makes it possible to theme tab labels through:

GtkNotebook tab GtkLabel {
  font: Sans 8;
}

If you intend several visual elements to look interconnected, make sure you specify rendered elements’ connection areas with gtk_style_context_set_junction_sides(). It is of course up to the theme to make use of this information or not.
GtkStyleContext supports implicit animations on state changes for the most simple case out-of-the-box: widgets with a single animatable area, whose state is changed with gtk_widget_set_state_flags() or gtk_widget_unset_state_flags(). These functions trigger animated transitions for the affected state flags. Examples of widgets for which this kind of animation may be sufficient are GtkButton or GtkEntry.
If your widget consists of more than a simple area, and these areas may be rendered with different states, make sure to mark the rendered areas with gtk_style_context_push_animatable_region() and gtk_style_context_pop_animatable_region().
gtk_style_context_notify_state_change() may be used to trigger a transition for a given state. The region ID will determine the animatable region that is affected by this transition.