Stateful action

Some actions have states. The typical values of states is boolean or string. However, other types of states are possible if you want.

Actions which have states are called stateful.

Stateful action without a parameter

Some menus are called toggle menu. For example, fullscreen menu has a state which has two values – fullscreen and non-fullscreen. The value of the state is changed every time the menu is clicked. An action corresponds to the fullscreen menu also have a state. Its value is TRUE or FALSE and it is called boolean value. TRUE corresponds to fullscreen and FALSE to non-fullscreen.

The following is an example code to implement a fullscreen menu except the signal handler. The signal handler will be shown later.

GSimpleAction *act_fullscreen = g_simple_action_new_stateful ("fullscreen",
                                NULL, g_variant_new_boolean (FALSE));
g_signal_connect (act_fullscreen, "change-state", G_CALLBACK (fullscreen_changed), win);
g_action_map_add_action (G_ACTION_MAP (win), G_ACTION (act_fullscreen));
... ... ...
GMenuItem *menu_item_fullscreen = g_menu_item_new ("Full Screen", "win.fullscreen");
static void
fullscreen_changed(GSimpleAction *action, GVariant *value, GtkWindow *win) {
  if (g_variant_get_boolean (value))
    gtk_window_maximize (win);
  else
    gtk_window_unmaximize (win);
  g_simple_action_set_state (action, value);
}

You can use “activate” signal instead of “change-state” signal, or both signals. But the way above is the simplest and the best.

GVariant

GVarient can contain boolean, string or other type values. For example, the following program assigns TRUE to value whose type is GVariant.

GVariant *value = g_variant_new_boolean (TRUE);

Another example is:

GVariant *value2 = g_variant_new_string ("Hello");

value2 is a GVariant and it has a string type value “Hello”. GVariant can contain other types like int16, int32, int64, double and so on.

If you want to get the original value, use g_variant_get series functions. For example, you can get the boolean value with g_variant_get_boolean.

gboolean bool = g_variant_get_boolean (value);

Because value has been created as a boolean type GVariant and TRUE value, bool equals TRUE. In the same way, you can get a string from value2

const char *str = g_variant_get_string (value2, NULL);

The second parameter is a pointer to gsize type variable (gsize is defined as unsigned long). If it isn’t NULL, then the length of the string will be set by the function. If it is NULL, nothing happens. The returned string str can’t be changed.

Stateful action with a parameter

Another example of stateful actions is an action corresponds to color select menus. For example, there are three menus and each menu has red, green or blue color respectively. They determine the background color of a GtkLabel widget. One action is connected to the three menus. The action has a state whose value is “red”, “green” or “blue”. The values are string. Those colors are given to the signal handler as a parameter.

... ... ...
GSimpleAction *act_color = g_simple_action_new_stateful ("color",
                   g_variant_type_new("s"), g_variant_new_string ("red"));
GMenuItem *menu_item_red = g_menu_item_new ("Red", "app.color::red");
GMenuItem *menu_item_green = g_menu_item_new ("Green", "app.color::green");
GMenuItem *menu_item_blue = g_menu_item_new ("Blue", "app.color::blue");
g_signal_connect (act_color, "activate", G_CALLBACK (color_activated), NULL);
... ... ...

The following is the “activate” signal handler.

static void
color_activated(GSimpleAction *action, GVariant *parameter) {
  char *color = g_strdup_printf ("label.lb {background-color: %s;}",
                                   g_variant_get_string (parameter, NULL));
  gtk_css_provider_load_from_data (provider, color, -1);
  g_free (color);
  g_action_change_state (G_ACTION (action), parameter);
}

Note: If you haven’t set an “activate” signal handler, the signal is forwarded to “change-state” signal. So, you can use “change-state” signal instead of “activate” signal. See src/menu/menu2_change_state.c.

GVariantType

GVariantType gives a type of GVariant. GVariant can contain many kinds of types. And the type often needs to be recognized at runtime.

GVariantType is created with a string which expresses a type.

The following program is a simple example. It finally outputs the string “s”.

#include <glib.h>

int
main (int argc, char **argv) {
  GVariantType *vtype = g_variant_type_new ("s");
  const char *type_string = g_variant_type_peek_string (vtype);
  g_print ("%s\n",type_string);
}

Example

The following code includes stateful actions above. This program has menus like this:

menu2

The code is as follows.

#include <gtk/gtk.h>

/* The provider below provides application wide CSS data. */
GtkCssProvider *provider;

static void
fullscreen_changed(GSimpleAction *action, GVariant *value, GtkWindow *win) {
  if (g_variant_get_boolean (value))
    gtk_window_maximize (win);
  else
    gtk_window_unmaximize (win);
  g_simple_action_set_state (action, value);
}

static void
color_activated(GSimpleAction *action, GVariant *parameter) {
  char *color = g_strdup_printf ("label.lb {background-color: %s;}", g_variant_get_string (parameter, NULL));
  /* Change the CSS data in the provider. */
  /* Previous data is thrown away. */
  gtk_css_provider_load_from_data (provider, color, -1);
  g_free (color);
  g_action_change_state (G_ACTION (action), parameter);
}

static void
remove_provider (GApplication *app, GtkCssProvider *provider) {
  GdkDisplay *display = gdk_display_get_default();
  
  gtk_style_context_remove_provider_for_display (display, GTK_STYLE_PROVIDER (provider));
  g_object_unref (provider);
}

static void
app_activate (GApplication *app) {
  GtkWindow *win = GTK_WINDOW (gtk_application_window_new (GTK_APPLICATION (app)));
  gtk_window_set_title (win, "menu2");
  gtk_window_set_default_size (win, 400, 300);

  GtkWidget *lb = gtk_label_new (NULL);
  gtk_widget_add_css_class (lb, "lb"); /* the class is used by CSS Selector */
  gtk_window_set_child (win, lb);

  GSimpleAction *act_fullscreen
    = g_simple_action_new_stateful ("fullscreen", NULL, g_variant_new_boolean (FALSE));
  g_signal_connect (act_fullscreen, "change-state", G_CALLBACK (fullscreen_changed), win);
  g_action_map_add_action (G_ACTION_MAP (win), G_ACTION (act_fullscreen));

  gtk_application_window_set_show_menubar (GTK_APPLICATION_WINDOW (win), TRUE);

  gtk_window_present (win);
}

static void
app_startup (GApplication *app) {
  GSimpleAction *act_color
    = g_simple_action_new_stateful ("color", g_variant_type_new("s"), g_variant_new_string ("red"));
  GSimpleAction *act_quit
    = g_simple_action_new ("quit", NULL);
  g_signal_connect (act_color, "activate", G_CALLBACK (color_activated), NULL);
  g_signal_connect_swapped (act_quit, "activate", G_CALLBACK (g_application_quit), app);
  g_action_map_add_action (G_ACTION_MAP (app), G_ACTION (act_color));
  g_action_map_add_action (G_ACTION_MAP (app), G_ACTION (act_quit));

  GMenu *menubar = g_menu_new ();
  GMenu *menu = g_menu_new ();
  GMenu *section1 = g_menu_new ();
  GMenu *section2 = g_menu_new ();
  GMenu *section3 = g_menu_new ();
  GMenuItem *menu_item_fullscreen = g_menu_item_new ("Full Screen", "win.fullscreen");
  GMenuItem *menu_item_red = g_menu_item_new ("Red", "app.color::red");
  GMenuItem *menu_item_green = g_menu_item_new ("Green", "app.color::green");
  GMenuItem *menu_item_blue = g_menu_item_new ("Blue", "app.color::blue");
  GMenuItem *menu_item_quit = g_menu_item_new ("Quit", "app.quit");

  g_menu_append_item (section1, menu_item_fullscreen);
  g_menu_append_item (section2, menu_item_red);
  g_menu_append_item (section2, menu_item_green);
  g_menu_append_item (section2, menu_item_blue);
  g_menu_append_item (section3, menu_item_quit);
  g_object_unref (menu_item_red);
  g_object_unref (menu_item_green);
  g_object_unref (menu_item_blue);
  g_object_unref (menu_item_fullscreen);
  g_object_unref (menu_item_quit);

  g_menu_append_section (menu, NULL, G_MENU_MODEL (section1));
  g_menu_append_section (menu, "Color", G_MENU_MODEL (section2));
  g_menu_append_section (menu, NULL, G_MENU_MODEL (section3));
  g_menu_append_submenu (menubar, "Menu", G_MENU_MODEL (menu));

  gtk_application_set_menubar (GTK_APPLICATION (app), G_MENU_MODEL (menubar));

  provider = gtk_css_provider_new ();
  /* Initialize the css data */
  gtk_css_provider_load_from_data (provider, "label.lb {background-color: red;}", -1);
  /* Add CSS to the default GdkDisplay. */
  GdkDisplay *display = gdk_display_get_default ();
  gtk_style_context_add_provider_for_display (display, GTK_STYLE_PROVIDER (provider),
                                              GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
  g_signal_connect (app, "shutdown", G_CALLBACK (remove_provider), provider);
}

#define APPLICATION_ID "com.github.ToshioCP.menu2"

int
main (int argc, char **argv) {
  GtkApplication *app;
  int stat;

  app = gtk_application_new (APPLICATION_ID, G_APPLICATION_DEFAULT_FLAGS);
  g_signal_connect (app, "startup", G_CALLBACK (app_startup), NULL);
  g_signal_connect (app, "activate", G_CALLBACK (app_activate), NULL);

  stat =g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);
  return stat;
}

Compile

Change your current directory to src/menu.

$ comp menu2
$./a.out

Then, you will see a window and the background color of the content is red. You can change the size to maximum and change again to the original size. You can change the background color to green or blue.

If you run the application again, another window will appear in the same screen. Both of the window have the same background color. Because the act_color action has “app” scope and the CSS is applied to the default display shared by the windows.