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glib/docs/reference/gio/migrating-gdbus.xml
Emmanuele Bassi 2993bb8275 docs: Fix xinclude directives 
For legacy reasons, Meson's gtk-doc helper script launches the
gtkdoc-mkhtml tool in the `html` directory under the build root. This
means that all paths must be relative to that location.
2020-10-30 16:41:07 +00:00

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<?xml version="1.0"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % local.common.attrib "xmlns:xi CDATA #FIXED 'http://www.w3.org/2003/XInclude'">
<!ENTITY version SYSTEM "version.xml">
]>
<chapter>
<title>Migrating to GDBus</title>
<section>
<title>Conceptual differences</title>
<para>
The central concepts of D-Bus are modelled in a very similar way
in dbus-glib and GDBus. Both have objects representing connections,
proxies and method invocations. But there are some important
differences:
<itemizedlist>
<listitem><para>
dbus-glib uses the <ulink
url="http://www.freedesktop.org/wiki/Software/dbus#ReferenceImplementation.28dbus-daemonandlibdbus.29">libdbus
reference implementation</ulink>, GDBus doesn't. Instead, it
relies on GIO streams as transport layer, and has its own
implementation for the D-Bus connection setup and
authentication. Apart from using streams as transport,
avoiding libdbus also lets GDBus avoid some thorny
multithreading issues.
</para></listitem>
<listitem><para>
dbus-glib uses the GObject type system for method arguments and
return values, including a homegrown container specialization
mechanism. GDBus relies on the #GVariant type system which is
explicitly designed to match D-Bus types.
</para></listitem>
<listitem><para>
dbus-glib models only D-Bus interfaces and does not provide
any types for objects. GDBus models both D-Bus interfaces
(via the #GDBusInterface, #GDBusProxy and
#GDBusInterfaceSkeleton types) and objects (via the
#GDBusObject, #GDBusObjectSkeleton and #GDBusObjectProxy types).
</para></listitem>
<listitem><para>
GDBus includes native support for the <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-properties">org.freedesktop.DBus.Properties</ulink> (via the #GDBusProxy type) and <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager">org.freedesktop.DBus.ObjectManager</ulink> D-Bus interfaces, dbus-glib doesn't.
</para></listitem>
<listitem><para>
The typical way to export an object in dbus-glib involves
generating glue code from XML introspection data using
<command>dbus-binding-tool</command>. GDBus provides a
similar tool called <command><link
linkend="gdbus-codegen">gdbus-codegen</link></command> that
can also generate Docbook D-Bus interface documentation.
</para></listitem>
<listitem><para>
dbus-glib doesn't provide any convenience API for owning and
watching bus names, GDBus provides the g_bus_own_name() and
g_bus_watch_name() family of convenience functions.
</para></listitem>
<listitem><para>
GDBus provides API to parse, generate and work with <link
linkend="gio-D-Bus-Introspection-Data">Introspection
XML</link>, dbus-glib doesn't.
</para></listitem>
<listitem><para>
GTestDBus provides API to create isolated unit tests <link
linkend="gio-D-Bus-Test-Scaffolding">GDBus Test Scaffolding</link>.
</para></listitem>
</itemizedlist>
</para>
</section>
<section>
<title>API comparison</title>
<table id="dbus-glib-vs-gdbus">
<title>dbus-glib APIs and their GDBus counterparts</title>
<tgroup cols="2">
<thead>
<row><entry>dbus-glib</entry><entry>GDBus</entry></row>
</thead>
<tbody>
<row><entry>#DBusGConnection</entry><entry>#GDBusConnection</entry></row>
<row><entry>#DBusGProxy</entry><entry>#GDBusProxy, #GDBusInterface - also see #GDBusObjectProxy</entry></row>
<row><entry>#DBusGObject</entry><entry>#GDBusInterfaceSkeleton, #GDBusInterface - also see #GDBusObjectSkeleton</entry></row>
<row><entry>#DBusGMethodInvocation</entry><entry>#GDBusMethodInvocation</entry></row>
<row><entry>dbus_g_bus_get()</entry><entry>g_bus_get_sync(), also see
g_bus_get()</entry></row>
<row><entry>dbus_g_proxy_new_for_name()</entry><entry>g_dbus_proxy_new_sync() and
g_dbus_proxy_new_for_bus_sync(), also see g_dbus_proxy_new()</entry></row>
<row><entry>dbus_g_proxy_add_signal()</entry><entry>not needed, use the generic #GDBusProxy::g-signal</entry></row>
<row><entry>dbus_g_proxy_connect_signal()</entry><entry>use g_signal_connect() with #GDBusProxy::g-signal</entry></row>
<row><entry>dbus_g_connection_register_g_object()</entry><entry>g_dbus_connection_register_object() - also see g_dbus_object_manager_server_export()</entry></row>
<row><entry>dbus_g_connection_unregister_g_object()</entry><entry>g_dbus_connection_unregister_object() - also see g_dbus_object_manager_server_unexport()</entry></row>
<row><entry>dbus_g_object_type_install_info()</entry><entry>introspection data is installed while registering
an object, see g_dbus_connection_register_object()</entry></row>
<row><entry>dbus_g_proxy_begin_call()</entry><entry>g_dbus_proxy_call()</entry></row>
<row><entry>dbus_g_proxy_end_call()</entry><entry>g_dbus_proxy_call_finish()</entry></row>
<row><entry>dbus_g_proxy_call()</entry><entry>g_dbus_proxy_call_sync()</entry></row>
<row><entry>dbus_g_error_domain_register()</entry><entry>g_dbus_error_register_error_domain()</entry></row>
<row><entry>dbus_g_error_has_name()</entry><entry>no direct equivalent, see g_dbus_error_get_remote_error()</entry></row>
<row><entry>dbus_g_method_return()</entry><entry>g_dbus_method_invocation_return_value()</entry></row>
<row><entry>dbus_g_method_return_error()</entry><entry>g_dbus_method_invocation_return_error() and variants</entry></row>
<row><entry>dbus_g_method_get_sender()</entry><entry>g_dbus_method_invocation_get_sender()</entry></row>
</tbody>
</tgroup>
</table>
</section>
<section>
<title>Owning bus names</title>
<para>
Using dbus-glib, you typically call RequestName manually
to own a name, like in the following excerpt:
<informalexample><programlisting><![CDATA[
error = NULL;
res = dbus_g_proxy_call (system_bus_proxy,
"RequestName",
&error,
G_TYPE_STRING, NAME_TO_CLAIM,
G_TYPE_UINT, DBUS_NAME_FLAG_ALLOW_REPLACEMENT,
G_TYPE_INVALID,
G_TYPE_UINT, &result,
G_TYPE_INVALID);
if (!res)
{
if (error != NULL)
{
g_warning ("Failed to acquire %s: %s",
NAME_TO_CLAIM, error->message);
g_error_free (error);
}
else
{
g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
}
goto out;
}
if (result != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
{
if (error != NULL)
{
g_warning ("Failed to acquire %s: %s",
NAME_TO_CLAIM, error->message);
g_error_free (error);
}
else
{
g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
}
exit (1);
}
dbus_g_proxy_add_signal (system_bus_proxy, "NameLost",
G_TYPE_STRING, G_TYPE_INVALID);
dbus_g_proxy_connect_signal (system_bus_proxy, "NameLost",
G_CALLBACK (on_name_lost), NULL, NULL);
/* further setup ... */
]]>
</programlisting></informalexample>
</para>
<para>
While you can do things this way with GDBus too, using
g_dbus_proxy_call_sync(), it is much nicer to use the high-level API
for this:
<informalexample><programlisting><![CDATA[
static void
on_name_acquired (GDBusConnection *connection,
const gchar *name,
gpointer user_data)
{
/* further setup ... */
}
/* ... */
owner_id = g_bus_own_name (G_BUS_TYPE_SYSTEM,
NAME_TO_CLAIM,
G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT,
on_bus_acquired,
on_name_acquired,
on_name_lost,
NULL,
NULL);
g_main_loop_run (loop);
g_bus_unown_name (owner_id);
]]>
</programlisting></informalexample>
Note that g_bus_own_name() works asynchronously and requires
you to enter your mainloop to await the on_name_aquired()
callback. Also note that in order to avoid race conditions (e.g.
when your service is activated by a method call), you have to export
your manager object <emphasis>before</emphasis> acquiring the
name. The on_bus_acquired() callback is the right place to do
such preparations.
</para>
</section>
<section>
<title>Creating proxies for well-known names</title>
<para>
dbus-glib lets you create proxy objects for well-known names, like the
following example:
<informalexample><programlisting><![CDATA[
proxy = dbus_g_proxy_new_for_name (system_bus_connection,
"org.freedesktop.Accounts",
"/org/freedesktop/Accounts",
"org.freedesktop.Accounts");
]]>
</programlisting></informalexample>
For a #DBusGProxy constructed like this, method calls will be sent to
the current owner of the name, and that owner can change over time.
</para>
<para>
The same can be achieved with #GDBusProxy:
<informalexample><programlisting><![CDATA[
error = NULL;
proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
G_DBUS_PROXY_FLAGS_NONE,
NULL, /* GDBusInterfaceInfo */
"org.freedesktop.Accounts",
"/org/freedesktop/Accounts",
"org.freedesktop.Accounts",
NULL, /* GCancellable */
&error);
]]>
</programlisting></informalexample>
For an added layer of safety, you can specify what D-Bus
interface the proxy is expected to conform to by using the
#GDBusInterfaceInfo type. Additionally, #GDBusProxy loads,
caches and tracks changes to the D-Bus properties on the remote
object. It also sets up match rules so D-Bus signals from the
remote object are delivered locally.
</para>
<para>
The #GDBusProxy type normally isn't used directly - instead
proxies subclassing #GDBusProxy generated by <command><link
linkend="gdbus-codegen">gdbus-codegen</link></command> is used, see <xref linkend="gdbus-example-gdbus-codegen"/>
</para>
</section>
<section>
<title>Generating code and docs</title>
<section id="gdbus-example-gdbus-codegen">
<title>Using gdbus-codegen</title>
<para>
dbus-glib comes with <command>dbus-binding-tool</command>, which
can produce somewhat nice client- and server-side wrappers for a D-Bus interface.
With GDBus, <command><link
linkend="gdbus-codegen">gdbus-codegen</link></command> is used and like
its counterpart, it also takes D-Bus Introspection XML as input:
</para>
<example id="gdbus-example-codegen-input"><title>Example D-Bus Introspection XML</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../../../../gio/tests/gdbus-object-manager-example/gdbus-example-objectmanager.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: gdbus-example-objectmanager.xml</xi:fallback></xi:include></programlisting></example>
<para>
If this XML is processed like this
<informalexample><programlisting><![CDATA[
gdbus-codegen --interface-prefix org.gtk.GDBus.Example.ObjectManager. \
--generate-c-code generated-code \
--c-namespace Example \
--c-generate-object-manager \
--generate-docbook generated-docs \
gdbus-example-objectmanager.xml
]]></programlisting></informalexample>
then two files <filename>generated-code.h</filename> and
<filename>generated-code.c</filename> are
generated. Additionally, two XML files
<filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Animal</filename> and
<filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Cat</filename>
with Docbook XML are generated. For an example of what the docs look
like see <link
linkend="gdbus-interface-org-gtk-GDBus-Example-ObjectManager-Animal">the Animal D-Bus interface documentation</link>.
and
<link
linkend="gdbus-interface-org-gtk-GDBus-Example-ObjectManager-Cat">the Cat D-Bus interface documentation</link>.
</para>
<para>
While the contents of <filename>generated-code.h</filename> and
<filename>generated-code.c</filename> are best described by the
<command><link
linkend="gdbus-codegen">gdbus-codegen</link></command> manual
page, brief examples of how this generated code can be used can be found in
<xref linkend="gdbus-example-codegen-server"/>
and <xref
linkend="gdbus-example-codegen-client"/>. Additionally, since
the generated code has 100% gtk-doc coverage, see
#ExampleAnimal, #ExampleCat, #ExampleObject and
#ExampleObjectManagerClient pages for documentation.
</para>
<example id="gdbus-example-codegen-server"><title>Server-side application using generated code</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../../../../gio/tests/gdbus-example-objectmanager-server.c"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: gdbus-example-objectmanager-server.c</xi:fallback></xi:include></programlisting></example>
<example id="gdbus-example-codegen-client"><title>Client-side application using generated code</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../../../../gio/tests/gdbus-example-objectmanager-client.c"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: gdbus-example-objectmanager-client.c</xi:fallback></xi:include></programlisting></example>
</section>
<!-- All XInclude paths are relative to the html/ directory under the build root directory -->
<xi:include href="../../../../gio/tests/gdbus-object-manager-example/objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Animal.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Animal.xml</xi:fallback></xi:include>
<xi:include href="../../../../gio/tests/gdbus-object-manager-example/objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Cat.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: objectmanager-gen-org.gtk.GDBus.Example.ObjectManager.Cat.xml</xi:fallback></xi:include>
<xi:include href="../gdbus-object-manager-example/xml/ExampleAnimal.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: ExampleAnimal.xml</xi:fallback></xi:include>
<xi:include href="../gdbus-object-manager-example/xml/ExampleCat.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: ExampleCat.xml</xi:fallback></xi:include>
<xi:include href="../gdbus-object-manager-example/xml/ExampleObject.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: ExampleObject.xml</xi:fallback></xi:include>
<xi:include href="../gdbus-object-manager-example/xml/ExampleObjectManagerClient.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT: ExampleObjectManagerClient.xml</xi:fallback></xi:include>
</section>
</chapter>
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