I'm struggling to understand the code from lines 125 to around 157. The function call started on line 154 particularly confuses me.

I don't know what a static prefix means for a data type (which I am guessing is a
boolean value here). I thought this code looks pretty important.

static gboolean
on_timeout_cb (gpointer user_data)
{
  GDBusConnection *connection = G_DBUS_CONNECTION (user_data);
  GVariantBuilder *builder;
  GVariantBuilder *invalidated_builder;
  GError *error;

  swap_a_and_b = !swap_a_and_b;

  error = NULL;
  builder = g_variant_builder_new (G_VARIANT_TYPE_ARRAY);
  invalidated_builder = g_variant_builder_new (G_VARIANT_TYPE ("as"));
  g_variant_builder_add (builder,
                         "{sv}",
                         "Foo",
                         g_variant_new_string (swap_a_and_b ? "Tock" : "Tick"));
  g_variant_builder_add (builder,
                         "{sv}",
                         "Bar",
                         g_variant_new_string (swap_a_and_b ? "Tick" : "Tock"));
  g_dbus_connection_emit_signal (connection,
                                 NULL,
                                 "/org/gtk/GDBus/TestObject",
                                 "org.freedesktop.DBus.Properties",
                                 "PropertiesChanged",
                                 g_variant_new ("(sa{sv}as)",
                                                "org.gtk.GDBus.TestInterface",
                                                builder,
                                                invalidated_builder),
                                 &error);
  g_assert_no_error (error);


  return TRUE;
}