gtk: Add GtkWindow property for session identifiers

This identifier will be used for session management purposes,
and is meant to be stable across executions.

gtk/gtkwindow.c

@@ -284,6 +284,8 @@ typedef struct
 
   GdkCursor *resize_cursor;
 
+  char *session_id;
+
   GtkEventController *menubar_controller;
 } GtkWindowPrivate;
 
@@ -333,6 +335,8 @@ enum {
   PROP_MAXIMIZED,
   PROP_FULLSCREENED,
 
+  PROP_SESSION_ID,
+
   LAST_ARG
 };
 
@@ -1076,6 +1080,24 @@ gtk_window_class_init (GtkWindowClass *klass)
                             TRUE,
                             GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY);
 
+  /**
+   * GtkWindow:session-id: (attributes org.gtk.Property.get=gtk_window_get_session_id org.gtk.Property.set=gtk_window_set_session_id)
+   *
+   * The identifier of this toplevel in the session.
+   *
+   * In windowing environments that allow it, this identifier will be
+   * used to identify windows in a persistent manner across runs, and
+   * restore window state (e.g. position, size) for them.
+   *
+   * Currently, this is only implemented for the Wayland backend.
+   *
+   * Since: 4.16
+   */
+  window_props[PROP_SESSION_ID] =
+      g_param_spec_string ("session-id", NULL, NULL,
+                           NULL,
+                           GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY);
+
   g_object_class_install_properties (gobject_class, LAST_ARG, window_props);
 
   /**
@@ -1895,6 +1917,9 @@ gtk_window_set_property (GObject      *object,
     case PROP_HANDLE_MENUBAR_ACCEL:
       gtk_window_set_handle_menubar_accel (window, g_value_get_boolean (value));
       break;
+    case PROP_SESSION_ID:
+      gtk_window_set_session_id (window, g_value_get_string (value));
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -1984,6 +2009,9 @@ gtk_window_get_property (GObject      *object,
     case PROP_HANDLE_MENUBAR_ACCEL:
       g_value_set_boolean (value, gtk_window_get_handle_menubar_accel (window));
       break;
+    case PROP_SESSION_ID:
+      g_value_set_string (value, gtk_window_get_session_id (window));
+      break;
     default:
       G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
       break;
@@ -6987,3 +7015,52 @@ gtk_window_get_handle_menubar_accel (GtkWindow *window)
 
   return phase == GTK_PHASE_CAPTURE;
 }
+
+/**
+ * gtk_window_set_session_id:
+ * @window: a `GtkWindow`
+ * @session_id: (nullable): A persistent identifier for this window
+ *
+ * Sets the identifier to be used for session management purposes.
+ *
+ * State of this window may be restored from prior executions, by using this
+ * identifier to match the related state. This identifier should be constant,
+ * or at least stable between executions, and unique among the windows of the
+ * application.
+ *
+ * Different backends may have different requirements for this string, therefore
+ * it is best to be conservative and keep the string relatively short and stick
+ * to ASCII without leading or trailing whitespace.
+ *
+ * Since: 4.16
+ **/
+void
+gtk_window_set_session_id (GtkWindow  *window,
+                           const char *session_id)
+{
+  GtkWindowPrivate *priv = gtk_window_get_instance_private (window);
+
+  if (g_set_str (&priv->session_id, session_id))
+    g_object_notify_by_pspec (G_OBJECT (window), window_props[PROP_SESSION_ID]);
+}
+
+/**
+ * gtk_window_get_session_id:
+ * @window: a `GtkWindow`
+ *
+ * Gets the window identifier to be used for session management purposes.
+ *
+ * See [method@Gtk.Window.set_session_id] for more details about window
+ * identifiers for session management.
+ *
+ * Returns: (nullable): the session identifier
+ *
+ * Since: 4.16
+ */
+const char *
+gtk_window_get_session_id (GtkWindow *window)
+{
+  GtkWindowPrivate *priv = gtk_window_get_instance_private (window);
+
+  return priv->session_id;
+}

gtk/gtkwindow.h

@@ -259,6 +259,13 @@ void     gtk_window_set_handle_menubar_accel (GtkWindow *window,
 GDK_AVAILABLE_IN_4_2
 gboolean gtk_window_get_handle_menubar_accel (GtkWindow *window);
 
+GDK_AVAILABLE_IN_4_16
+void         gtk_window_set_session_id       (GtkWindow  *window,
+                                              const char *session_id);
+
+GDK_AVAILABLE_IN_4_16
+const char * gtk_window_get_session_id       (GtkWindow  *window);
+
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GtkWindow, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GtkWindowGroup, g_object_unref)