gtk2/docs/reference/gtk/migrating-GtkStyleContext.xml

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<chapter id="gtk-migrating-GtkStyleContext">
  <title>Migrating from GtkStyle to GtkStyleContext</title>

  <para>
    In GTK+ 3.0, #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.
  </para>

  <refsect2 id="gtk-migrating-GtkStyleContext-themes">
    <title>Migrating themes</title>

    <para>
      From GTK+ 3.0 on, theme engines must implement #GtkThemingEngine and be
      installed in <filename>$libdir/gtk+-3.0/$GTK_VERSION/theming-engines</filename>,
      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
      <filename>$datadir/themes/Clearlooks/gtk-3.0/gtk.css</filename>,
      with possible variants such as the dark theme being named
      <filename>gtk-dark.css</filename> in the same directory.
    </para>
  </refsect2>

  <refsect2 id="gtk-migrating-theme-GtkStyleContext-engines">
    <title>Migrating theme engines</title>

    <para>
      Migrating a #GtkStyle based engine to a #GtkThemingEngine based one
      should be straightforward for most of the vfuncs. 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:
    </para>

    <orderedlist>
      <listitem>
        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.
      </listitem>
      <listitem>
        gtk_paint_resize_grip() has been subsumed by gtk_render_handle()
        with a #GTK_STYLE_CLASS_GRIP class set in the style context.
      </listitem>
      <listitem>
        gtk_paint_spinner() disappears in favor of gtk_render_activity()
        with a #GTK_STYLE_CLASS_SPINNER class set in the style context.
      </listitem>
    </orderedlist>

    <para>
      The list of available render methods is:
    </para>

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

    <para>
      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.
    </para>

    <para>
      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 <filename>gtkstylecontext.h</filename>,
      although custom widgets may define their own, which themes may
      attempt to handle.
    </para>

  </refsect2>

  <refsect2 id="gtk-migrating-GtkStyleContext-parser-extensions">
    <title>Extending the CSS parser</title>

    <para>
      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.
    </para>

    <para>
      The installed properties depend on the #GtkThemeEngine::name property,
      so they should be added in the <literal>constructed()</literal> vfunc.
      For example, if an engine with the name "Clearlooks" installs a
      "focus-color" property with the type #GdkRGBA, the property
      <literal>-Clearlooks-focus-color</literal> will be registered and
      accepted in CSS like this:
      <informalexample><programlisting>
      GtkEntry {
        -Clearlooks-focus-color: rgba(255, 0, 0, 1.0);
      }
      </programlisting></informalexample>
    </para>

    <para>
      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
      <literal>-GtkWidget-focus-line-width</literal>.
    </para>
  </refsect2>

  <refsect2 id="gtk-migrating-GtkStyleContext-css">
    <title>Using the CSS file format</title>

    <para>
      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.
    </para>
    <para>
      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:
    </para>

    <example>
      <title>Sample RC code</title>
      <programlisting>
        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] = &num;fff;
        }

        class "GtkWidget" style "default"
        class "GtkEntry" style "entry"
        widget_class "*&lt;GtkButton&gt;" style "button"
        widget "gtk-tooltip*" style "tooltips"
        widget_class "window-name.*.GtkButton" style "other"
      </programlisting>
    </example>

    <para>
      would roughly translate to this CSS:
    </para>

    <example>
      <title>CSS translation</title>
      <programlisting>
        * {
          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 */
        &num;window-name .button,
        GtkWindow&num;window-name GtkButton.button {
          background-color: &num;fff;
        }
      </programlisting>
    </example>

    <para>
      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.
    </para>

    <para>
      Access to colors has also changed a bit. With #GtkStyle, the common
      way to access colors is:
      <informalexample><programlisting>
      GdkColor *color1;
      GdkColor color2;

      color1 = &amp;style->bg[GTK_STATE_PRELIGHT];
      gtk_style_lookup_color (style, "focus_color", &amp;color2);
      </programlisting></informalexample>
      With #GtkStyleContext, you generally use #GdkRGBA instead of #GdkColor
      and the code looks like this:
      <informalexample><programlisting>
      GdkRGBA *color1;
      GdkRGBA  color2;

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

      ...

      gdk_rgba_free (color1);
      </programlisting></informalexample>
      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.
    </para>

    <para>
      It is worth mentioning that the new file format does not support
      custom keybindings nor stock icon mappings as the RC format did.
    </para>
  </refsect2>

  <refsect2 id="gtk-migrating-GtkStyleContext-checklist">
    <title>A checklist for widgets</title>

    <para>
      When porting your widgets to use #GtkStyleContext, this checklist
      might be useful.
    </para>

    <orderedlist>
      <listitem>
        Replace <literal>style_set()</literal> calls with
        <literal>style_updated()</literal>.
      </listitem>

      <listitem>
        <para>
          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:
        </para>

        <example>
          <title>Setting a permanent CSS class</title>
          <programlisting>
            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);
            }
          </programlisting>
        </example>

        <para>
          Or
        </para>

        <example>
          <title>Using dynamic CSS classes for different elements</title>
          <programlisting>
            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);

              ...
            }
          </programlisting>
        </example>

        <para>
          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.
        </para>
      </listitem>

      <listitem>
        Replace all <literal>gtk_paint_*()</literal> calls with corresponding
        <literal>gtk_render_*()</literal> calls. The most distinctive changes
        are the use of #GtkStateFlags to represent the widget state and the
        lack of #GtkShadowType. 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.
      </listitem>

      <listitem>
        The various <literal>gtk_widget_modify_*()</literal> functions to
        override colors or fonts for individual widgets have been replaced
        by similar <literal>gtk_widget_override_*()</literal> functions.
      </listitem>

      <listitem>
        It is no longer necessary to call gtk_widget_style_attach(),
        gtk_style_attach(), gtk_style_detach() or gtk_widget_ensure_style().
      </listitem>

      <listitem>
        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.
      </listitem>
    </orderedlist>
  </refsect2>

  <refsect2 id="gtk-migrating-GtkStyleContext-parsing">
    <title>Parsing of custom resources</title>
    <para>
      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 parse CSS
      information from a file or from a string.
    </para>

    <para>
      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.
    </para>
  </refsect2>

  <refsect2 id="gtk-migrating-GtkStyleContext-bonus-points">
    <title>Bonus points</title>

    <para>
      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.
    </para>

    <orderedlist>
      <listitem>
        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.

        <example>
          <title>Theming widget regions</title>
          <programlisting>
            GtkNotebook tab {
              background-color: &num;f3329d;
            }

            GtkTreeView row::nth-child (even) {
              background-color: &num;dddddd;
            }
          </programlisting>
        </example>
      </listitem>

      <listitem>
        <para>
          If your container renders child widgets within different regions,
          make it implement GtkContainer::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.
        </para>

        <para>
          For example, a #GtkNotebook modifies the tab labels' #GtkWidgetPath
          so the "tab" region is added. This makes it possible to theme tab
          labels through:
        </para>

        <example>
          <title>Theming a widget within a parent container region</title>
          <programlisting>
            GtkNotebook tab GtkLabel {
              font: Sans 8;
            }
          </programlisting>
        </example>

      </listitem>

      <listitem>
        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.
      </listitem>

      <listitem>
        <para>
          #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.
        </para>
        <para>
          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().
        </para>

        <para>
          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.
        </para>
      </listitem>
    </orderedlist>
  </refsect2>
</chapter>