demo: Make gtk4-demo use session management

Use it as an example of what is to be expected to make your
application windows restore-able between executions, the session
ID is preserved in a setting between runs, so that it's fed to
the GtkApplication on startup. The actual session ID used is likewise
always stored in this setting, for later runs.

The main window gets an identifier within the session, so that its
state may be restored when first mapped.

demos/gtk-demo/main.c

@@ -1140,6 +1140,19 @@ local_options (GApplication *app,
   return -1;
 }
 
+static void
+startup (GApplication *app,
+         gpointer      data)
+{
+  GSettings *settings = data;
+  const char *session_id;
+
+  session_id = gtk_application_get_current_session_id (GTK_APPLICATION (app));
+
+  if (session_id)
+    g_settings_set_string (settings, "session-id", session_id);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -1156,6 +1169,8 @@ main (int argc, char **argv)
     { "app.about", { "F1", NULL } },
     { "app.quit", { "<Control>q", NULL } },
   };
+  GSettings *settings;
+  char *session_id;
   int i;
 
   app = gtk_application_new ("org.gtk.Demo4", G_APPLICATION_NON_UNIQUE|G_APPLICATION_HANDLES_COMMAND_LINE);
@@ -1164,6 +1179,11 @@ main (int argc, char **argv)
                                    app_entries, G_N_ELEMENTS (app_entries),
                                    app);
 
+  settings = g_settings_new ("org.gtk.Demo4");
+  session_id = g_settings_get_string (settings, "session-id");
+  gtk_application_set_session_id (app, session_id);
+  g_free (session_id);
+
   for (i = 0; i < G_N_ELEMENTS (accels); i++)
     gtk_application_set_accels_for_action (app, accels[i].action_and_target, accels[i].accelerators);
 
@@ -1175,8 +1195,11 @@ main (int argc, char **argv)
   g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
   g_signal_connect (app, "command-line", G_CALLBACK (command_line), NULL);
   g_signal_connect (app, "handle-local-options", G_CALLBACK (local_options), NULL);
+  g_signal_connect (app, "startup", G_CALLBACK (startup), settings);
 
   g_application_run (G_APPLICATION (app), argc, argv);
 
+  g_object_unref (settings);
+
   return 0;
 }

demos/gtk-demo/main.ui

@@ -19,6 +19,7 @@
   <object class="GtkApplicationWindow" id="window">
     <property name="default-width">800</property>
     <property name="default-height">600</property>
+    <property name="session-id">main-window</property>
     <child type="titlebar">
       <object class="GtkHeaderBar" id="headerbar">
         <child>

demos/gtk-demo/org.gtk.Demo4.gschema.xml

@@ -21,6 +21,10 @@
     <key name='fullscreen' type='b'>
       <default>false</default>
     </key>
+
+    <key name='session-id' type='s'>
+      <default>''</default>
+    </key>
   </schema>
 
 </schemalist>