docs/reference/gio/migrating-gdbus.xml

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   7 <chapter>
   8   <title>Migrating to GDBus</title>
   9
  10   <section>
  11     <title>Conceptual differences</title>
  12
  13     <para>
  14       The central concepts of D-Bus are modelled in a very similar way
  15       in dbus-glib and GDBus. Both have a objects representing connections,
  16       proxies and method invocations. But there are some important
  17       differences:
  18       <itemizedlist>
  19         <listitem><para>
  20           dbus-glib uses the <ulink
  21           url="http://www.freedesktop.org/wiki/Software/dbus#ReferenceImplementation.28dbus-daemonandlibdbus.29">libdbus
  22           reference implementation</ulink>, GDBus doesn't. Instead, it
  23           relies on GIO streams as transport layer, and has its own
  24           implementation for the D-Bus connection setup and
  25           authentication. Apart from using streams as transport,
  26           avoiding libdbus also lets GDBus avoid some thorny
  27           multithreading issues.
  28         </para></listitem>
  29         <listitem><para>
  30           dbus-glib uses the GObject type system for method arguments and
  31           return values, including a homegrown container specialization
  32           mechanism. GDBus relies on the #GVariant type system which is
  33           explicitly designed to match D-Bus types.
  34         </para></listitem>
  35         <listitem><para>
  36           dbus-glib models only D-Bus interfaces and does not provide
  37           any types for objects. GDBus models both D-Bus interfaces
  38           (via the #GDBusInterface, #GDBusProxy and
  39           #GDBusInterfaceSkeleton types) and objects (via the
  40           #GDBusObject, #GDBusObjectSkeleton and #GDBusObjectProxy types).
  41         </para></listitem>
  42         <listitem><para>
  43           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.
  44         </para></listitem>
  45         <listitem><para>
  46           The typical way to export an object in dbus-glib involves
  47           generating glue code from XML introspection data using
  48           <command>dbus-binding-tool</command>. GDBus provides a
  49           similar tool called <command><link
  50           linkend="gdbus-codegen">gdbus-codegen</link></command> that
  51           can also generate Docbook D-Bus interface documentation.
  52         </para></listitem>
  53         <listitem><para>
  54           dbus-glib doesn't provide any convenience API for owning and
  55           watching bus names, GDBus provides the g_bus_own_name() and
  56           g_bus_watch_name() family of convenience functions.
  57         </para></listitem>
  58         <listitem><para>
  59           GDBus provides API to parse, generate and work with <link
  60           linkend="gio-D-Bus-Introspection-Data">Introspection
  61           XML</link>, dbus-glib doesn't.
  62         </para></listitem>
  63         <listitem><para>
  64           GTestDBus provides API to create isolated unit tests <link
  65           linkend="gio-D-Bus-Test-Scaffolding">GDBus Test Scaffolding</link>.
  66         </para></listitem>
  67       </itemizedlist>
  68     </para>
  69   </section>
  70
  71   <section>
  72     <title>API comparison</title>
  73
  74     <table id="dbus-glib-vs-gdbus">
  75       <title>dbus-glib APIs and their GDBus counterparts</title>
  76       <tgroup cols="2">
  77         <thead>
  78            <row><entry>dbus-glib</entry><entry>GDBus</entry></row>
  79         </thead>
  80         <tbody>
  81           <row><entry>#DBusGConnection</entry><entry>#GDBusConnection</entry></row>
  82           <row><entry>#DBusGProxy</entry><entry>#GDBusProxy, #GDBusInterface - also see #GDBusObjectProxy</entry></row>
  83           <row><entry>#DBusGObject</entry><entry>#GDBusInterfaceSkeleton, #GDBusInterface - also see #GDBusObjectSkeleton</entry></row>
  84           <row><entry>#DBusGMethodInvocation</entry><entry>#GDBusMethodInvocation</entry></row>
  85           <row><entry>dbus_g_bus_get()</entry><entry>g_bus_get_sync(), also see
  86                g_bus_get()</entry></row>
  87           <row><entry>dbus_g_proxy_new_for_name()</entry><entry>g_dbus_proxy_new_sync() and
  88                g_dbus_proxy_new_for_bus_sync(), also see g_dbus_proxy_new()</entry></row>
  89           <row><entry>dbus_g_proxy_add_signal()</entry><entry>not needed, use the generic #GDBusProxy::g-signal</entry></row>
  90           <row><entry>dbus_g_proxy_connect_signal()</entry><entry>use g_signal_connect() with #GDBusProxy::g-signal</entry></row>
  91           <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>
  92           <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>
  93           <row><entry>dbus_g_object_type_install_info()</entry><entry>introspection data is installed while registering
  94                an object, see g_dbus_connection_register_object()</entry></row>
  95           <row><entry>dbus_g_proxy_begin_call()</entry><entry>g_dbus_proxy_call()</entry></row>
  96           <row><entry>dbus_g_proxy_end_call()</entry><entry>g_dbus_proxy_call_finish()</entry></row>
  97           <row><entry>dbus_g_proxy_call()</entry><entry>g_dbus_proxy_call_sync()</entry></row>
  98           <row><entry>dbus_g_error_domain_register()</entry><entry>g_dbus_error_register_error_domain()</entry></row>
  99           <row><entry>dbus_g_error_has_name()</entry><entry>no direct equivalent, see g_dbus_error_get_remote_error()</entry></row>
 100           <row><entry>dbus_g_method_return()</entry><entry>g_dbus_method_invocation_return_value()</entry></row>
 101           <row><entry>dbus_g_method_return_error()</entry><entry>g_dbus_method_invocation_return_error() and variants</entry></row>
 102           <row><entry>dbus_g_method_get_sender()</entry><entry>g_dbus_method_invocation_get_sender()</entry></row>
 103         </tbody>
 104       </tgroup>
 105     </table>
 106   </section>
 107
 108   <section>
 109     <title>Owning bus names</title>
 110     <para>
 111       Using dbus-glib, you typically call RequestName manually
 112       to own a name, like in the following excerpt:
 113       <informalexample><programlisting><![CDATA[
 114   error = NULL;
 115   res = dbus_g_proxy_call (system_bus_proxy,
 116                            "RequestName",
 117                            &error,
 118                            G_TYPE_STRING, NAME_TO_CLAIM,
 119                            G_TYPE_UINT,   DBUS_NAME_FLAG_ALLOW_REPLACEMENT,
 120                            G_TYPE_INVALID,
 121                            G_TYPE_UINT,   &result,
 122                            G_TYPE_INVALID);
 123   if (!res)
 124     {
 125       if (error != NULL)
 126         {
 127           g_warning ("Failed to acquire %s: %s",
 128                      NAME_TO_CLAIM, error->message);
 129           g_error_free (error);
 130         }
 131       else
 132         {
 133           g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
 134         }
 135       goto out;
 136     }
 137
 138   if (result != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
 139     {
 140       if (error != NULL)
 141         {
 142           g_warning ("Failed to acquire %s: %s",
 143                      NAME_TO_CLAIM, error->message);
 144           g_error_free (error);
 145         }
 146       else
 147         {
 148           g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
 149         }
 150       exit (1);
 151     }
 152
 153   dbus_g_proxy_add_signal (system_bus_proxy, "NameLost",
 154                            G_TYPE_STRING, G_TYPE_INVALID);
 155   dbus_g_proxy_connect_signal (system_bus_proxy, "NameLost",
 156                                G_CALLBACK (on_name_lost), NULL, NULL);
 157
 158   /* further setup ... */
 159 ]]>
 160       </programlisting></informalexample>
 161     </para>
 162     <para>
 163     While you can do things this way with GDBus too, using
 164     g_dbus_proxy_call_sync(), it is much nicer to use the high-level API
 165     for this:
 166     <informalexample><programlisting><![CDATA[
 167 static void
 168 on_name_acquired (GDBusConnection *connection,
 169                   const gchar     *name,
 170                   gpointer         user_data)
 171 {
 172   /* further setup ... */
 173 }
 174
 175 /* ... */
 176
 177   owner_id = g_bus_own_name (G_BUS_TYPE_SYSTEM,
 178                              NAME_TO_CLAIM,
 179                              G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT,
 180                              on_bus_acquired,
 181                              on_name_acquired,
 182                              on_name_lost,
 183                              NULL,
 184                              NULL);
 185
 186   g_main_loop_run (loop);
 187
 188   g_bus_unown_name (owner_id);
 189 ]]>
 190     </programlisting></informalexample>
 191     Note that g_bus_own_name() works asynchronously and requires
 192     you to enter your mainloop to await the on_name_aquired()
 193     callback. Also note that in order to avoid race conditions (e.g.
 194     when your service is activated by a method call), you have to export
 195     your manager object <emphasis>before</emphasis> acquiring the
 196     name. The on_bus_acquired() callback is the right place to do
 197     such preparations.
 198     </para>
 199   </section>
 200
 201   <section>
 202     <title>Creating proxies for well-known names</title>
 203     <para>
 204       dbus-glib lets you create proxy objects for well-known names, like the
 205       following example:
 206       <informalexample><programlisting><![CDATA[
 207   proxy = dbus_g_proxy_new_for_name (system_bus_connection,
 208                                      "org.freedesktop.Accounts",
 209                                      "/org/freedesktop/Accounts",
 210                                      "org.freedesktop.Accounts");
 211       ]]>
 212       </programlisting></informalexample>
 213       For a #DBusGProxy constructed like this, method calls will be sent to
 214       the current owner of the name, and that owner can change over time.
 215     </para>
 216     <para>
 217       The same can be achieved with #GDBusProxy:
 218       <informalexample><programlisting><![CDATA[
 219   error = NULL;
 220   proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
 221                                          G_DBUS_PROXY_FLAGS_NONE,
 222                                          NULL, /* GDBusInterfaceInfo */
 223                                          "org.freedesktop.Accounts",
 224                                          "/org/freedesktop/Accounts",
 225                                          "org.freedesktop.Accounts",
 226                                          NULL, /* GCancellable */
 227                                          &error);
 228       ]]>
 229       </programlisting></informalexample>
 230       For an added layer of safety, you can specify what D-Bus
 231       interface the proxy is expected to conform to by using the
 232       #GDBusInterfaceInfo type.  Additionally, #GDBusProxy loads,
 233       caches and tracks changes to the D-Bus properties on the remote
 234       object. It also sets up match rules so D-Bus signals from the
 235       remote object are delivered locally.
 236     </para>
 237     <para>
 238       The #GDBusProxy type normally isn't used directly - instead
 239       proxies subclassing #GDBusProxy generated by <command><link
 240       linkend="gdbus-codegen">gdbus-codegen</link></command> is used, see <xref linkend="gdbus-example-gdbus-codegen"/>
 241     </para>
 242   </section>
 243
 244   <section id="gdbus-example-gdbus-codegen">
 245     <title>Using gdbus-codegen</title>
 246
 247     <para>
 248       dbus-glib comes with <command>dbus-binding-tool</command>, which
 249       can produce somewhat nice client- and server-side wrappers for a D-Bus interface.
 250       With GDBus, <command><link
 251       linkend="gdbus-codegen">gdbus-codegen</link></command> is used and like
 252       its counterpart, it also takes D-Bus Introspection XML as input:
 253     </para>
 254     <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="../gdbus-example-objectmanager.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting></example>
 255     <para>
 256       If this XML is processed like this
 257 <informalexample><programlisting><![CDATA[
 258 gdbus-codegen --interface-prefix org.gtk.GDBus.Example.ObjectManager. \
 259               --generate-c-code generated-code                        \
 260               --c-namespace Example                                   \
 261               --c-generate-object-manager                             \
 262               --generate-docbook generated-docs                       \
 263               gdbus-example-objectmanager.xml
 264 ]]></programlisting></informalexample>
 265       then two files <filename>generated-code.h</filename> and
 266       <filename>generated-code.c</filename> are
 267       generated. Additionally, two XML files
 268       <filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Animal</filename> and
 269       <filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Cat</filename>
 270       with Docbook XML are generated.
 271     </para>
 272     <para>
 273       While the contents of <filename>generated-code.h</filename> and
 274       <filename>generated-code.c</filename> are best described by the
 275       <command><link
 276       linkend="gdbus-codegen">gdbus-codegen</link></command> manual
 277       page, brief examples of how this generated code can be used can be found in
 278       <xref linkend="gdbus-example-codegen-server"/>
 279       and <xref linkend="gdbus-example-codegen-client"/>.
 280     </para>
 281
 282   </section>
 283
 284 </chapter>