GtkToolTip: new API to add css classes to GtkTooltipWindow

we add a simple API to allow styling GtkTooltip's window
by setting a css class on it:

void gtk_tooltip_set_css_class (GtkTooltip *tooltip,
                                const char *css_class);

By passing NULL it will remove any previously set css class
by this function.

Issue #5925

gtk/gtktooltip.c

@@ -1065,3 +1065,26 @@ gtk_tooltip_unset_surface (GtkNative *native)
   gtk_tooltip_set_surface (tooltip, NULL);
 }
 
+/**
+ * gtk_tooltip_set_css_class:
+ * @tooltip: a #GtkTooltip
+ * @css_class: (allow-none): a css class name, or %NULL
+ *
+ * This function allows to add a single css class
+ * to @tooltip window, that means it will remove any
+ * css class previously added by this function before
+ * adding @css_class as the currently active one.
+ *
+ * if %NULL is passed then any active css class (which
+ * was added by this function) will be cleared.
+ *
+ * Since: 4.12
+ */
+void
+gtk_tooltip_set_css_class (GtkTooltip *tooltip,
+                           const char *css_class)
+{
+  g_return_if_fail (GTK_IS_TOOLTIP (tooltip));
+
+  gtk_tooltip_window_set_css_class (GTK_TOOLTIP_WINDOW (tooltip->window), css_class);
+}

gtk/gtktooltip.h

@@ -56,6 +56,9 @@ void gtk_tooltip_set_custom	         (GtkTooltip         *tooltip,
 GDK_AVAILABLE_IN_ALL
 void gtk_tooltip_set_tip_area            (GtkTooltip         *tooltip,
                                           const GdkRectangle *rect);
+GDK_AVAILABLE_IN_4_12
+void gtk_tooltip_set_css_class           (GtkTooltip *tooltip,
+                                          const char *css_class);
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GtkTooltip, g_object_unref)
 

gtk/gtktooltipwindow.c

@@ -59,6 +59,7 @@ struct _GtkTooltipWindow
   GtkWidget *image;
   GtkWidget *label;
   GtkWidget *custom_widget;
+  char      *css_class;
 };
 
 struct _GtkTooltipWindowClass
@@ -392,6 +393,7 @@ static void
 gtk_tooltip_window_init (GtkTooltipWindow *self)
 {
   gtk_widget_init_template (GTK_WIDGET (self));
+  self->css_class = NULL;
 }
 
 GtkWidget *
@@ -520,3 +522,30 @@ gtk_tooltip_window_position (GtkTooltipWindow *window,
 
   gtk_tooltip_window_relayout (window);
 }
+
+/* See gtk_tooltip_set_css_class() description */
+void
+gtk_tooltip_window_set_css_class (GtkTooltipWindow *window,
+                                  const char       *css_class)
+{
+  GtkWidget *wid = GTK_WIDGET (window);
+
+  if (g_strcmp0 (window->css_class, css_class) == 0)
+    return;
+
+  if (css_class)
+    {
+      if (window->css_class)
+        {
+          gtk_widget_remove_css_class (wid, window->css_class);
+          g_free (window->css_class);
+        }
+      window->css_class = g_strdup (css_class);
+      gtk_widget_add_css_class (wid, css_class);
+    }
+  else
+    {
+      gtk_widget_remove_css_class (wid, window->css_class);
+      g_clear_pointer (&window->css_class, g_free);
+    }
+}

gtk/gtktooltipwindowprivate.h

@@ -59,6 +59,8 @@ void            gtk_tooltip_window_position                     (GtkTooltipWindo
                                                                  GdkAnchorHints    anchor_hints,
                                                                  int               dx,
                                                                  int               dy);
+void            gtk_tooltip_window_set_css_class                (GtkTooltipWindow *window,
+                                                                 const char       *css_class);
 
 G_END_DECLS