window: add an is-maximized property to GtkWindow

With proper notifications, plus an accessor method for that state. This
allows client to just listen to notify::is-maximized instead of tracking
window-state-event.

https://bugzilla.gnome.org/show_bug.cgi?id=698786

docs/reference/gtk/gtk3-sections.txt

@@ -5580,6 +5580,7 @@ gtk_window_set_hide_titlebar_when_maximized
 gtk_window_set_screen
 gtk_window_get_screen
 gtk_window_is_active
+gtk_window_is_maximized
 gtk_window_has_toplevel_focus
 gtk_window_list_toplevels
 gtk_window_add_mnemonic

gtk/gtkheaderbar.c

@@ -433,7 +433,7 @@ _gtk_header_bar_update_window_buttons (GtkHeaderBar *bar)
                        type_hint == GDK_WINDOW_TYPE_HINT_NORMAL)
                 {
                   const gchar *icon_name;
-                  gboolean maximized = _gtk_window_get_maximized (window);
+                  gboolean maximized = gtk_window_is_maximized (window);
 
                   icon_name = maximized ? "window-restore-symbolic" : "window-maximize-symbolic";
                   button = gtk_button_new ();

gtk/gtkwindow.c

@@ -280,6 +280,8 @@ enum {
   PROP_MNEMONICS_VISIBLE,
   PROP_FOCUS_VISIBLE,
 
+  PROP_IS_MAXIMIZED,
+
   LAST_ARG
 };
 
@@ -1075,6 +1077,14 @@ gtk_window_class_init (GtkWindowClass *klass)
                                                         GTK_TYPE_WIDGET,
                                                         GTK_PARAM_READWRITE | G_PARAM_CONSTRUCT));
 
+  g_object_class_install_property (gobject_class,
+                                   PROP_IS_MAXIMIZED,
+                                   g_param_spec_boolean ("is-maximized",
+                                                         P_("Is maximized"),
+                                                         P_("Whether the window is maximized"),
+                                                         FALSE,
+                                                         GTK_PARAM_READABLE));
+
   /* Style properties.
    */
   gtk_widget_class_install_style_property (widget_class,
@@ -1233,11 +1243,29 @@ gtk_window_class_init (GtkWindowClass *klass)
   gtk_widget_class_set_accessible_type (widget_class, GTK_TYPE_WINDOW_ACCESSIBLE);
 }
 
+/**
+ * gtk_window_is_maximized:
+ * @window: a #GtkWindow
+ *
+ * Retrieves the current maximized state of @window.
+ *
+ * Note that since maximization is ultimately handled by the window
+ * manager and happens asynchronously to an application request, you
+ * shouldn't assume the return value of this function changing
+ * immediately (or at all), as an effect of calling
+ * gtk_window_maximize() or gtk_window_unmaximize().
+ *
+ * Returns: whether the window has a maximized state.
+ *
+ * Since: 3.12
+ */
 gboolean
-_gtk_window_get_maximized (GtkWindow *window)
+gtk_window_is_maximized (GtkWindow *window)
 {
   GtkWindowPrivate *priv = window->priv;
 
+  g_return_val_if_fail (GTK_IS_WINDOW (window), FALSE);
+
   return priv->maximized;
 }
 
@@ -1594,6 +1622,9 @@ gtk_window_get_property (GObject      *object,
     case PROP_FOCUS_VISIBLE:
       g_value_set_boolean (value, priv->focus_visible);
       break;
+    case PROP_IS_MAXIMIZED:
+      g_value_set_boolean (value, gtk_window_is_maximized (window));
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -6776,6 +6807,7 @@ gtk_window_state_event (GtkWidget           *widget,
     {
       priv->maximized =
         (event->new_window_state & GDK_WINDOW_STATE_MAXIMIZED) ? 1 : 0;
+      g_object_notify (G_OBJECT (widget), "is-maximized");
     }
 
   if (event->changed_mask & (GDK_WINDOW_STATE_FULLSCREEN | GDK_WINDOW_STATE_MAXIMIZED | GDK_WINDOW_STATE_TILED))
@@ -9696,8 +9728,9 @@ gtk_window_unstick (GtkWindow *window)
  * initially.
  *
  * You can track maximization via the "window-state-event" signal
- * on #GtkWidget.
+ * on #GtkWidget, or by listening to notifications on the
- * 
+ * #GtkWindow:is-maximized property.
+ *
  **/
 void
 gtk_window_maximize (GtkWindow *window)

gtk/gtkwindow.h

@@ -467,6 +467,9 @@ GDK_AVAILABLE_IN_3_10
 void     gtk_window_set_titlebar           (GtkWindow    *window,
                                             GtkWidget    *titlebar);
 
+GDK_AVAILABLE_IN_3_12
+gboolean gtk_window_is_maximized           (GtkWindow    *window);
+
 G_END_DECLS
 
 #endif /* __GTK_WINDOW_H__ */

gtk/gtkwindowprivate.h

@@ -89,7 +89,6 @@ gboolean        _gtk_window_titlebar_shows_app_menu (GtkWindow *window);
 void            _gtk_window_get_shadow_width (GtkWindow *window,
                                               GtkBorder *border);
 
-gboolean        _gtk_window_get_maximized (GtkWindow *window);
 void            _gtk_window_toggle_maximized (GtkWindow *window);
 
 G_END_DECLS