docs: Add an actions overview

Add an introduction chapter explaining actions
in detail. Most of this content is taken from
the GAction HowDoI page.

3 files changed, 377 insertions, 1 deletions

diff --git a/docs/reference/gtk/actions.xml b/docs/reference/gtk/actions.xml
new file mode 100644
index 0000000000..3d789b0c79
--- /dev/null
+++ b/docs/reference/gtk/actions.xml
@@ -0,0 +1,373 @@
+<?xml version="1.0"?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
+               "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
+]>
+<refentry id="chap-actions">
+<refmeta>
+<refentrytitle>The GTK Action Model</refentrytitle>
+<manvolnum>3</manvolnum>
+<refmiscinfo>GTK Library</refmiscinfo>
+</refmeta>
+
+<refnamediv>
+<refname>The GTK Action Model</refname>
+<refpurpose>
+    How actions are used in GTK
+</refpurpose>
+</refnamediv>
+
+
+  <refsect1 id="actions-overview">
+    <title>Overview of actions in GTK</title>
+
+  <para>
+   This chapter describes in detail how GTK uses actions to connect
+   activatable UI elements to callbacks. GTK inherits the underlying
+   architecture of GAction and GMenu for describing abstract actions
+   and menus from the GIO library.
+  </para>
+
+  <refsect2>
+    <title>Basics about actions</title>
+
+    <para>
+     A GAction is essentially a way to tell the toolkit about a
+     piece of functionality in your program, and to give it a name.
+   </para>
+
+   <para>
+     Actions are purely functional. They do not contain any
+     presentational information.
+   </para>
+
+   <para>
+     An action has four pieces of information associated with it:
+     <itemizedlist>
+       <listitem><para>
+         a name as an identifier (usually all-lowercase, untranslated
+         English string)
+       </para></listitem>
+       <listitem><para>
+         an enabled flag indicating if the action can be activated or
+         not (like the "sensitive" property on widgets)
+       </para></listitem>
+       <listitem><para>
+         an optional state value, for stateful actions (like a boolean
+         for toggles)
+       </para></listitem>
+       <listitem><para>
+         an optional parameter type, used when activating the action
+       </para></listitem>
+     </itemizedlist>
+   </para>
+
+   <para>
+     An action supports two operations. You can activate it, which
+     requires passing a parameter of the correct type
+     And you can request to change the actions state (for stateful
+     actions) to a new state value of the correct type.
+   </para>
+
+   <para>
+     Here are some rules about an action:
+     <itemizedlist>
+       <listitem><para>
+         the name is immutable (in the sense that it will never
+         change) and it is never %NULL
+       </para></listitem>
+       <listitem><para>
+         the enabled flag can change
+       </para></listitem>
+       <listitem><para>
+         the parameter type is immutable
+       </para></listitem>
+       <listitem><para>
+         the parameter type is optional: it can be %NULL
+       </para></listitem>
+       <listitem><para>
+         if the parameter type is %NULL then action activation must
+         be done without a parameter (ie: a %NULL GVariant pointer)
+       </para></listitem>
+       <listitem><para>
+         if the parameter type is non-%NULL then the parameter must
+         have this type
+       </para></listitem>
+       <listitem><para>
+         the state can change, but it cannot change type
+       </para></listitem>
+       <listitem><para>
+         if the action was stateful when it was created, it will
+         always have a state and it will always have exactly the same
+         type (such as boolean or string)
+       </para></listitem>
+       <listitem><para>
+         if the action was stateless when it was created, it can never
+         have a state
+       </para></listitem>
+       <listitem><para>
+         you can only request state changes on stateful actions and it
+         is only possible to request that the state change to a value
+         of the same type as the existing state
+       </para></listitem>
+     </itemizedlist>
+   </para>
+
+   <para>
+     An action does not have any sort of presentational information
+     such as a label, an icon or a way of creating a widget from it.
+   </para>
+
+   </refsect2>
+
+   <refsect2>
+     <title>Action state and parameters</title>
+
+     <para>
+       Most actions in your application will be stateless actions with
+       no parameters. These typically appear as menu items with no
+       special decoration. An example is "quit".
+     </para>
+
+     <para>
+       Stateful actions are used to represent an action which has a
+       closely-associated state of some kind. A good example is a
+       "fullscreen" action. For this case, you'd expect to see a
+       checkmark next to the menu item when the fullscreen option
+       is active. This is usually called a toggle action, and it has
+       a boolean state. By convention, toggle actions have no parameter
+       type for activation: activating the action always toggles the
+       state.
+     </para>
+
+     <para>
+       Another common case is to have an action representing a
+       enumeration of possible values of a given type (typically
+       string). This is often called a radio action and is usually
+       represented in the user interface with radio buttons or radio
+       menu items, or sometimes a combobox. A good example is
+       "text-justify" with possible values "left", "center", and
+       "right". By convention, these types of actions have a parameter
+       type equal to their state type, and activating them with a
+       particular parameter value is equivalent to changing their
+       state to that value.
+     </para>
+
+     <para>
+       This approach to handling radio buttons is different than many
+       other action systems such as GtkAction. With GAction, there is
+       only one action for "text-justify" and "left", "center" and
+       "right" are possible states on that action. There are not three
+       separate "justify-left", "justify-center" and "justify-right"
+       actions.
+     </para>
+
+     <para>
+       The final common type of action is a stateless action with a
+       parameter. This is typically used for actions like
+       "open-bookmark" where the parameter to the action would be
+       the identifier of the bookmark to open.
+     </para>
+
+     <para>
+       Because some types of actions cannot be invoked without a
+       parameter, it is often important to specify a parameter when
+       referring to the action from a place where it will be invoked
+       (such as from a radio button that sets the state to a particular
+       value or from a menu item that opens a specific bookmark). In
+       these contexts, the value used for the action parameter is
+       typically called the target of the action.
+     </para>
+
+     <para>
+       Even though toggle actions have a state, they do not have a
+       parameter. Therefore, a target value is not needed when
+       referring to them — they will always be toggled on activation.
+     </para>
+
+     <para>
+       Most APIs that allow using a GAction (such as GMenuModel and
+       GtkActionable) allow use of detailed action names. This is a
+       convenient way of specifying an action name and an action target
+       with a single string.
+     </para>
+
+     <para>
+       In the case that the action target is a string with no unusual
+       characters (ie: only alpha-numeric, plus '-' and '.') then you
+       can use a detailed action name of the form "justify::left" to
+       specify the justify action with a target of left.
+     </para>
+
+     <para>
+       In the case that the action target is not a string, or contains
+       unusual characters, you can use the more general format
+       "action-name(5)", where the "5" here is any valid text-format
+       GVariant (ie: a string that can be parsed by g_variant_parse()).
+       Another example is "open-bookmark('http://gnome.org/')".
+     </para>
+
+     <para>
+       You can convert between detailed action names and split-out
+       action names and target values using g_action_parse_detailed_action_name()
+       and g_action_print_detailed_action_name() but usually you will
+       not need to. Most APIs will provide both ways of specifying
+       actions with targets.
+     </para>
+
+   </refsect2>
+
+   <refsect2>
+     <title>Action scopes</title>
+
+     <para>
+       Actions are always scoped to a particular object on which they
+       operate.
+     </para>
+
+     <para>
+       In GTK, actions are typically scoped to either an application
+       or a window, but any widget can have actions associated with it.
+     </para>
+
+     <para>
+       Actions scoped to windows should be the actions that
+       specifically impact that window. These are actions like
+       "fullscreen" and "close", or in the case that a window contains
+       a document, "save" and "print".
+     </para>
+
+     <para>
+       Actions that impact the application as a whole rather than one
+       specific window are scoped to the application. These are actions
+       like "about" and "preferences".
+     </para>
+
+     <para>
+       If a particular action is scoped to a window then it is scoped
+       to a specific window. Another way of saying this: if your
+       application has a "fullscreen" action that applies to windows
+       and it has three windows, then it will have three fullscreen
+       actions: one for each window.
+     </para>
+
+     <para>
+       Having a separate action per-window allows for each window to
+       have a separate state for each instance of the action as well
+       as being able to control the enabled state of the action on a
+       per-window basis.
+     </para>
+
+     <para>
+       Actions are added to their relevant scope (application or
+       window) either using the GActionMap interface, or by using
+       gtk_widget_insert_action_group().
+     </para>
+
+  </refsect2>
+
+  <refsect2>
+    <title>Action groups and action maps</title>
+
+    <para>
+      Actions rarely occurs in isolation. It is common to have groups
+      of related actions, which are represented by instances of the
+      GActionGroup interface.
+    </para>
+
+    <para>
+      Action maps are a variant of action groups that allow to change
+      the name of the action as it is looked up. In GTK, the convention
+      is to add a prefix to the action name to indicate the scope of
+      the actions, such as "app." for the actions with application scope
+      or "win." for those with window scope.
+    </para>
+
+    <para>
+      When referring to actions on a GActionMap only the name of the
+      action itself is used (ie: "quit", not "app.quit"). The
+      "app.quit" form is only used when referring to actions from
+      places like a GMenu or GtkActionable widget where the scope
+      of the action is not already known.
+    </para>
+
+    <para>
+      GtkApplication and GtkApplicationWindow implement the GActionMap
+      interface, so you can just add actions directly to them. For
+      other widgets, use gtk_widget_insert_action_group() to add
+      actions to it.
+    </para>
+
+    <para>
+      If you want to insert several actions at the same time, it is
+      typically faster and easier to use GActionEntry.
+    </para>
+
+  </refsect2>
+
+  <refsect2>
+    <title>Connecting actions to widgets</title>
+
+    <para>
+      Any widget that implements the GtkActionable interface can
+      be connected to an action just by setting the ::action-name
+      property. If the action has a parameter, you will also need
+      to set the ::action-target property.
+      Widgets that implement GtkAction include GtkSwitch, GtkButton,
+      GtkMenuItem and their respective subclasses.
+    </para>
+
+    <para>
+      Another of obtaining widgets that are connected to actions is
+      to create a menu using a GMenu menu model. GMenu provides an
+      abstract way to describe typical menus: nested groups of items
+      where each item can have a label, and icon, and an action.
+    </para>
+
+    <para>
+      Typical uses of GMenu inside GTK are to set up an application
+      menu or menubar with gtk_appication_set_app_menu() or
+      gtk_application_set_menubar(). Another, maybe more common use
+      is to create a popover for a menubutton, using
+      gtk_menu_button_set_menu_model().
+    </para>
+
+    <para>
+      Unlike traditional menus, those created from menu models don't
+      have keyboard accelerators associated with menu items. Instead,
+      GtkApplication offers the gtk_application_set_accels_for_action()
+      API to associate keyboard shortcuts with actions.
+    </para>
+  </refsect2>
+
+  <refsect2>
+    <title>Activation</title>
+
+    <para>
+      When a widget with a connected action is activated, GTK finds
+      the action to activate by walking up the widget hierarchy,
+      looking for a matching action, ending up at the GtkApplication.
+    </para>
+  </refsect2>
+
+  <refsect2>
+    <title>Built-in Actions</title>
+
+    <para>
+      GTK uses actions for its own purposes in a number places. These
+      built-in actions can sometimes be activated by applications, and
+      you should avoid naming conflicts with them when creating your
+      own actions.
+      <variablelist>
+        <varlistentry>
+          <term>default.activate</term>
+          <listitem><para>Activates the default widget in a context
+             (typically a GtkWindow, GtkDialog or GtkPopover)
+          </para></listitem>
+        </varlistentry>
+      </variablelist>
+    </para>
+
+  </refsect2>
+
+  </refsect1>
+</refentry>
diff --git a/docs/reference/gtk/gtk4-docs.xml b/docs/reference/gtk/gtk4-docs.xml
index 17a5337c6f..26f6f028e5 100644
--- a/docs/reference/gtk/gtk4-docs.xml
+++ b/docs/reference/gtk/gtk4-docs.xml
@@ -24,8 +24,9 @@
     <xi:include href="xml/getting_started.xml"/>
     <xi:include href="resources.sgml" />
     <xi:include href="xml/question_index.sgml" />
-    <xi:include href="drawing-model.xml" />
+    <xi:include href="xml/drawing-model.xml" />
     <xi:include href="xml/input-handling.xml" />
+    <xi:include href="xml/actions.xml" />
   </part>
 
   <part id="gtkobjects">
diff --git a/docs/reference/gtk/meson.build b/docs/reference/gtk/meson.build
index 75eb45be35..609cf28faf 100644
--- a/docs/reference/gtk/meson.build
+++ b/docs/reference/gtk/meson.build
@@ -333,6 +333,7 @@ images = [
 ]
 
 content_files = [
+  'actions.xml',
   'broadway.xml',
   'building.sgml',
   'compiling.sgml',
@@ -368,6 +369,7 @@ content_files = [
 ]
 
 expand_content_files = [
+  'actions.xml',
   'compiling.sgml',
   'drawing-model.xml',
   'glossary.xml',