Users often use menus to tell a command to the computer. It is like this:
Now let’s analyze the menu above. There are two types of object.
Menus can build a complicated structure thanks to the links of menu items.
GMenuModel is an abstract object which represents a menu. GMenu is a simple implementation of GMenuModel and a child object of GMenuModel.
GObject -- GMenuModel -- GMenu
Because GMenuModel is an abstract object, it isn’t instantiatable.
Therefore, it doesn’t have any functions to create its instance. If you
want to create a menu, use g_menu_new
to create a GMenu
instance. GMenu inherits all the functions of GMenuModel.
GMenuItem is an object directly derived from GObject. GMenuItem and Gmenu (or GMenuModel) don’t have a parent-child relationship.
GObject -- GMenuModel -- GMenu
GObject -- GMenuItem
GMenuItem has attributes. One of the attributes is label. For example, there is a menu item which has “Edit” label in the first diagram. “Cut”, “Copy”, “Paste” and “Select All” are also the labels of the menu items. Other attributes will be explained later.
Some menu items have a link to another GMenu. There are two types of links, submenu and section.
GMenuItem can be inserted, appended or prepended to GMenu. When it is inserted, all of the attributes and link values are copied and stored in the menu. The GMenuItem itself is not really inserted. Therefore, after the insertion, GMenuItem is useless and it should be freed. The same goes for appending or prepending.
The following code shows how to append GMenuItem to GMenu.
GMenu *menu = g_menu_new ();
GMenuItem *menu_item_quit = g_menu_item_new ("Quit", "app.quit");
g_menu_append_item (menu, menu_item_quit);
g_object_unref (menu_item_quit);
One of the attributes of menu items is an action. This attribute points an action object.
There are two action objects, GSimpleAction and GPropertyAction. GSimpleAction is often used. And it is used with a menu item. Only GSimpleAction is described in this section.
An action corresponds to a menu item will be activated when the menu item is clicked. Then the action emits an activate signal.
The following code is an example.
static void
(GSimpleAction *action, GVariant *parameter, gpointer app) { ... ... ...}
quit_activated
*act_quit = g_simple_action_new ("quit", NULL);
GSimpleAction (G_ACTION_MAP (app), G_ACTION (act_quit));
g_action_map_add_action (act_quit, "activate", G_CALLBACK (quit_activated), app);
g_signal_connect *menu_item_quit = g_menu_item_new ("Quit", "app.quit"); GMenuItem
menu_item_quit
points a menu item. It is
actually a pointer, but we often say that menu_item_quit
is a menu item. It has a label “Quit” and is connected to an
action “app.quit”. “app” is a prefix and “quit” is the name of the
action. The prefix “app” means that the action belongs to the
GtkApplication instance.act_quit
is an action. It has a name “quit”. The
function g_simple_action_new
creates a stateless action.
So, act_quit
is stateless. The meaning of stateless will be
explained later. The argument NULL
means that the action
doesn’t have an parameter. Most of the actions are stateless and have no
parameter.act_quit
is added to the GtkApplication
instance with g_action_map_add_action
. So, the action’s
scope is application. The prefix of app.quit
indicates the
scope.quit_activated
.If the menu is clicked, the corresponding action “quit” will be
activated and emits an “activate” signal. Then, the handler
quit_activated
is called.
A menu bar and menus are traditional style. Menu buttons are often used instead of a menu bar lately, but the old style is still used widely.
Applications have only one menu bar. If an application has two or more windows which have menu bars, the menu bars are exactly the same. Because every window refers to the same menubar instance in the application.
An application’s menu bar is usually unchanged once it is set. So, it is appropriate to set it in the “startup” handler. Because it is called only once in the primary application instance.
I think it is good for readers to clarify how applications behave.
Therefore, an “activate” or “open” handler can be called twice or more. On the other hand, a “startup” handler is called once. So, setting a menubar should be done in the “startup” handler.
static void
(GApplication *app) {
app_startup ... ... ...
(GTK_APPLICATION (app), G_MENU_MODEL (menubar));
gtk_application_set_menubar ... ... ...
}
The following is a simple example of menus and actions. The source
file menu1.c
is located at src/menu directory.
@@@include menu/menu1.c @@@
quit_activated
is a handler of the “activate”
signal on the action act_quit
. Handlers of the “activate”
signal have three parameters.
NULL
because the
second argument of g_simple_action_new
(line 23) is
NULL
. You don’ t need to care about it.g_signal_connect
(line 25) that connects the action and the
handler.g_application_quit
immediately quits
the application.app_activate
is an “activate” signal
handler.win
. And sets the
title and the default size.app_startup
is a “startup” signal handleract_quit
. It is stateless.
The first argument of g_simple_action_new
is a name of the
action and the second argument is a parameter. If you don’t need the
parameter, pass NULL
. Therefore, act_quit
has
a name “quit” and no parameter.app
.
GtkApplication implements an interface GActionMap and GActionGroup.
GtkApplication (GActionMap) can have a group of actions and the actions
are added with the function g_action_map_add_action
. This
function is described in Gio API
Reference – g_action_map_add_action. Because this action belongs to
GtkApplication, its scope is “app” and it is referred with “app.quit” if
the prefix (scope) is necessary.quit_activated
.menubar
and menu
are GMenu. menu_item_menu
and
menu_item_quit
are GMenuItem. menu_item_menu
has a label “Menu” and no action. menu_item_quit
has a
label “Quit” and an action “app.quit”.menu_item_quit
to menu
. As
I mentioned before, all the attributes and links are copied and used to
form a new item in menu
. Therefore after the addition,
menu_item_quit
is no longer needed. It is freed by
g_object_unref
.menu_item_menu
to point
menu
.menu_item_menu
to menubar
.
Then frees menu_item_menu
. GMenu and GMenuItem are
connected and finally a menu is made up. The structure of the menu is
shown in the diagram below.Change your current directory to src/menu
. Use comp to
compile menu1.c
.
$ comp menu1
$ ./a.out
Then, a window appears. Click on “Menu” on the menubar, then a menu appears. Click on “Quit” menu, then the application quits.
Let’s try running the application twice. Use &
in
your shell command line, then the application runs concurrently.
$ ./a.out &
[1] 70969
$ ./a.out
$
Then, two windows appear.
./a.out
calls the application and a primary
instance is created. It calls “startup” and “activate” handlers and
shows a window../a.out
calls the the application again and
the created instance is a remote one. It doesn’t emit “startup” signal.
And it activates the application but the “activate” signal is emitted on
the primary instance. The remote instance quits.gtk_application_window_set_show_menubar
function.Both the windows have menu bars. And they are exactly the same. The two windows belong to the primary instance.
If you click on the “Quit” menu, the application (the primary instance) quits.
The second run makes a new window. However, it depends on the “activate” handler. If you create your window in the startup handler and the activate handler just presents the window, no new window is created at the second run. For example, tfe (text file editor) doesn’t create a second window. It just creates a new notebook page. Because its activate handler doesn’t create any window but just creates a new notebook page.
Second or more executions often happen on the desktop applications. If you double-click the icon twice or more, the application is run multiple times. Therefore, you need to think about your startup and activate (open) handler carefully.