2023-10-10 00:22:20 +02:00
Title: Migrating to GDBus
SPDX-License-Identifier: LGPL-2.1-or-later
SPDX-FileCopyrightText: 2010 Matthias Clasen
SPDX-FileCopyrightText: 2010, 2011 David Zeuthen
# Migrating to GDBus
## Conceptual differences
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:
- dbus-glib uses the libdbus reference implementation, 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.
- 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.
- 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).
- GDBus includes native support for the org.freedesktop.DBus.Properties (via
the GDBusProxy type) and org.freedesktop.DBus.ObjectManager D-Bus
interfaces, dbus-glib doesn't.
- The typical way to export an object in dbus-glib involves generating glue
code from XML introspection data using dbus-binding-tool. GDBus provides a
similar tool called gdbus-codegen that can also generate Docbook D-Bus
interface documentation.
- 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.
- GDBus provides API to parse, generate and work with Introspection XML,
dbus-glib doesn't.
- GTestDBus provides API to create isolated unit tests
## API comparison
| dbus-glib | GDBus |
|-----------|-------|
| `DBusGConnection` | `GDBusConnection` |
| `DBusGProxy` | `GDBusProxy` , `GDBusInterface` - also see `GDBusObjectProxy` |
| `DBusGObject` | `GDBusInterfaceSkeleton` , `GDBusInterface` - also see `GDBusObjectSkeleton` |
| `DBusGMethodInvocation` | `GDBusMethodInvocation` |
| `dbus_g_bus_get()` | `g_bus_get_sync()` , also see `g_bus_get()` |
| `dbus_g_proxy_new_for_name()` | `g_dbus_proxy_new_sync()` and `g_dbus_proxy_new_for_bus_sync()` , also see `g_dbus_proxy_new()` |
| `dbus_g_proxy_add_signal()` | not needed, use the generic “g-signal” |
| `dbus_g_proxy_connect_signal()` | use `g_signal_connect()` with “g-signal” |
| `dbus_g_connection_register_g_object()` | `g_dbus_connection_register_object()` - also see `g_dbus_object_manager_server_export()` |
| `dbus_g_connection_unregister_g_object()` | `g_dbus_connection_unregister_object()` - also see `g_dbus_object_manager_server_unexport()` |
| `dbus_g_object_type_install_info()` | introspection data is installed while registering an object, see `g_dbus_connection_register_object()` |
| `dbus_g_proxy_begin_call()` | `g_dbus_proxy_call()` |
| `dbus_g_proxy_end_call()` | `g_dbus_proxy_call_finish()` |
| `dbus_g_proxy_call()` | `g_dbus_proxy_call_sync()` |
| `dbus_g_error_domain_register()` | `g_dbus_error_register_error_domain()` |
| `dbus_g_error_has_name()` | no direct equivalent, see `g_dbus_error_get_remote_error()` |
| `dbus_g_method_return()` | `g_dbus_method_invocation_return_value()` |
| `dbus_g_method_return_error()` | `g_dbus_method_invocation_return_error()` and variants |
| `dbus_g_method_get_sender()` | `g_dbus_method_invocation_get_sender()` |
## Owning bus names
Using dbus-glib, you typically call RequestName manually to own a name, like in the following excerpt:
```c
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 ... */
```
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:
```c
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);
```
Note that `g_bus_own_name()` works asynchronously and requires you to enter
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your mainloop to await the `on_name_acquired()` callback. Also note that in
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order to avoid race conditions (e.g. when your service is activated by a
method call), you have to export your manager object before acquiring the
name. The `on_bus_acquired()` callback is the right place to do such
preparations.
## Creating proxies for well-known names
dbus-glib lets you create proxy objects for well-known names, like the following example:
```c
proxy = dbus_g_proxy_new_for_name (system_bus_connection,
"org.freedesktop.Accounts",
"/org/freedesktop/Accounts",
"org.freedesktop.Accounts");
```
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.
The same can be achieved with GDBusProxy:
```c
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);
```
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.
The GDBusProxy type normally isn't used directly - instead proxies
subclassing GDBusProxy generated by `gdbus-codegen` is used, see the section
called “Using gdbus-codegen”.
## Generating code and docs
### Using gdbus-codegen
dbus-glib comes with dbus-binding-tool, which can produce somewhat nice
client- and server-side wrappers for a D-Bus interface. With GDBus,
gdbus-codegen is used and like its counterpart, it also takes D-Bus
Introspection XML as input:
#### Example D-Bus Introspection XML
```xml
< node >
<!-- org.gtk.GDBus.Example.ObjectManager.Animal:
@short_description: Example docs generated by gdbus-codegen
@since: 2.30
This D-Bus interface is used to describe a simple animal.
-->
< interface name = "org.gtk.GDBus.Example.ObjectManager.Animal" >
<!-- Mood: The mood of the animal.
@since: 2.30
Known values for this property include
< literal > Happy< / literal > and < literal > Sad< / literal > . Use the
org.gtk.GDBus.Example.ObjectManager.Animal.Poke() method to
change this property.
This property influences how often the animal jumps up and
down, see the
#org .gtk.GDBus.Example.ObjectManager.Animal::Jumped signal
for more details.
-->
< property name = "Mood" type = "s" access = "read" / >
<!--
Poke:
@make_sad: Whether to make the animal sad.
@make_happy: Whether to make the animal happy.
@since: 2.30
Method used to changing the mood of the animal. See also the
#org .gtk.GDBus.Example.ObjectManager.Animal:Mood property.
-->
< method name = "Poke" >
< arg direction = "in" type = "b" name = "make_sad" / >
< arg direction = "in" type = "b" name = "make_happy" / >
< / method >
<!--
Jumped:
@height: Height, in meters, that the animal jumped.
@since: 2.30
Emitted when the animal decides to jump.
-->
< signal name = "Jumped" >
< arg type = "d" name = "height" / >
< / signal >
<!--
Foo:
Property with no < quote > since< / quote > annotation (should inherit the 2.30 from its containing interface).
-->
< property name = "Foo" type = "s" access = "read" / >
<!--
Bar:
@since: 2.36
Property with a later < quote > since< / quote > annotation.
-->
< property name = "Bar" type = "s" access = "read" / >
< / interface >
<!-- org.gtk.GDBus.Example.ObjectManager.Cat:
@short_description: More example docs generated by gdbus-codegen
This D-Bus interface is used to describe a cat. Right now there
are no properties, methods or signals associated with this
interface so it is essentially a < ulink
url="http://en.wikipedia.org/wiki/Marker_interface_pattern">Marker
Interface< / ulink > .
Note that D-Bus objects implementing this interface also
implement the #org .gtk.GDBus.Example.ObjectManager.Animal
interface.
-->
< interface name = "org.gtk.GDBus.Example.ObjectManager.Cat" >
< / interface >
< / node >
```
If this XML is processed like this
```
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
```
then two files generated-code.h and generated-code.c are generated.
Additionally, two XML files
generated-docs-org.gtk.GDBus.Example.ObjectManager.Animal and
generated-docs-org.gtk.GDBus.Example.ObjectManager.Cat with Docbook XML are
generated.
While the contents of `generated-code.h` and `generated-code.c` are best described by the `gdbus-codegen` manual page, here's a brief example of how this generated code can be used:
```c
#include "gdbus-object-manager-example/objectmanager-gen.h"
/* ---------------------------------------------------------------------------------------------------- */
static GDBusObjectManagerServer *manager = NULL;
static gboolean
on_animal_poke (ExampleAnimal *animal,
GDBusMethodInvocation *invocation,
gboolean make_sad,
gboolean make_happy,
gpointer user_data)
{
if ((make_sad & & make_happy) || (!make_sad & & !make_happy))
{
g_dbus_method_invocation_return_dbus_error (invocation,
"org.gtk.GDBus.Examples.ObjectManager.Error.Failed",
"Exactly one of make_sad or make_happy must be TRUE");
goto out;
}
if (make_sad)
{
if (g_strcmp0 (example_animal_get_mood (animal), "Sad") == 0)
{
g_dbus_method_invocation_return_dbus_error (invocation,
"org.gtk.GDBus.Examples.ObjectManager.Error.SadAnimalIsSad",
"Sad animal is already sad");
goto out;
}
example_animal_set_mood (animal, "Sad");
example_animal_complete_poke (animal, invocation);
goto out;
}
if (make_happy)
{
if (g_strcmp0 (example_animal_get_mood (animal), "Happy") == 0)
{
g_dbus_method_invocation_return_dbus_error (invocation,
"org.gtk.GDBus.Examples.ObjectManager.Error.HappyAnimalIsHappy",
"Happy animal is already happy");
goto out;
}
example_animal_set_mood (animal, "Happy");
example_animal_complete_poke (animal, invocation);
goto out;
}
g_assert_not_reached ();
out:
return G_DBUS_METHOD_INVOCATION_HANDLED;
}
static void
on_bus_acquired (GDBusConnection *connection,
const gchar *name,
gpointer user_data)
{
ExampleObjectSkeleton *object;
guint n;
g_print ("Acquired a message bus connection\n");
/* Create a new org.freedesktop.DBus.ObjectManager rooted at /example/Animals */
manager = g_dbus_object_manager_server_new ("/example/Animals");
for (n = 0; n < 10 ; n + + )
{
gchar *s;
ExampleAnimal *animal;
/* Create a new D-Bus object at the path /example/Animals/N where N is 000..009 */
s = g_strdup_printf ("/example/Animals/%03d", n);
object = example_object_skeleton_new (s);
g_free (s);
/* Make the newly created object export the interface
* org.gtk.GDBus.Example.ObjectManager.Animal (note
* that @object takes its own reference to @animal ).
*/
animal = example_animal_skeleton_new ();
example_animal_set_mood (animal, "Happy");
example_object_skeleton_set_animal (object, animal);
g_object_unref (animal);
/* Cats are odd animals - so some of our objects implement the
* org.gtk.GDBus.Example.ObjectManager.Cat interface in addition
* to the .Animal interface
*/
if (n % 2 == 1)
{
ExampleCat *cat;
cat = example_cat_skeleton_new ();
example_object_skeleton_set_cat (object, cat);
g_object_unref (cat);
}
/* Handle Poke() D-Bus method invocations on the .Animal interface */
g_signal_connect (animal,
"handle-poke",
G_CALLBACK (on_animal_poke),
NULL); /* user_data */
/* Export the object (@manager takes its own reference to @object ) */
g_dbus_object_manager_server_export (manager, G_DBUS_OBJECT_SKELETON (object));
g_object_unref (object);
}
/* Export all objects */
g_dbus_object_manager_server_set_connection (manager, connection);
}
static void
on_name_acquired (GDBusConnection *connection,
const gchar *name,
gpointer user_data)
{
g_print ("Acquired the name %s\n", name);
}
static void
on_name_lost (GDBusConnection *connection,
const gchar *name,
gpointer user_data)
{
g_print ("Lost the name %s\n", name);
}
gint
main (gint argc, gchar *argv[])
{
GMainLoop *loop;
guint id;
loop = g_main_loop_new (NULL, FALSE);
id = g_bus_own_name (G_BUS_TYPE_SESSION,
"org.gtk.GDBus.Examples.ObjectManager",
G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT |
G_BUS_NAME_OWNER_FLAGS_REPLACE,
on_bus_acquired,
on_name_acquired,
on_name_lost,
loop,
NULL);
g_main_loop_run (loop);
g_bus_unown_name (id);
g_main_loop_unref (loop);
return 0;
}
```
This, on the other hand, is a client-side application using generated code:
```c
#include "gdbus-object-manager-example/objectmanager-gen.h"
/* ---------------------------------------------------------------------------------------------------- */
static void
print_objects (GDBusObjectManager *manager)
{
GList *objects;
GList *l;
g_print ("Object manager at %s\n", g_dbus_object_manager_get_object_path (manager));
objects = g_dbus_object_manager_get_objects (manager);
for (l = objects; l != NULL; l = l->next)
{
ExampleObject *object = EXAMPLE_OBJECT (l->data);
GList *interfaces;
GList *ll;
g_print (" - Object at %s\n", g_dbus_object_get_object_path (G_DBUS_OBJECT (object)));
interfaces = g_dbus_object_get_interfaces (G_DBUS_OBJECT (object));
for (ll = interfaces; ll != NULL; ll = ll->next)
{
GDBusInterface *interface = G_DBUS_INTERFACE (ll->data);
g_print (" - Interface %s\n", g_dbus_interface_get_info (interface)->name);
/* Note that @interface is really a GDBusProxy instance - and additionally also
* an ExampleAnimal or ExampleCat instance - either of these can be used to
* invoke methods on the remote object. For example, the generated function
*
* void example_animal_call_poke_sync (ExampleAnimal *proxy,
* gboolean make_sad,
* gboolean make_happy,
* GCancellable *cancellable,
* GError **error);
*
* can be used to call the Poke() D-Bus method on the .Animal interface.
* Additionally, the generated function
*
* const gchar *example_animal_get_mood (ExampleAnimal *object);
*
* can be used to get the value of the :Mood property.
*/
}
g_list_free_full (interfaces, g_object_unref);
}
g_list_free_full (objects, g_object_unref);
}
static void
on_object_added (GDBusObjectManager *manager,
GDBusObject *object,
gpointer user_data)
{
gchar *owner;
owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
g_print ("Added object at %s (owner %s)\n", g_dbus_object_get_object_path (object), owner);
g_free (owner);
}
static void
on_object_removed (GDBusObjectManager *manager,
GDBusObject *object,
gpointer user_data)
{
gchar *owner;
owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
g_print ("Removed object at %s (owner %s)\n", g_dbus_object_get_object_path (object), owner);
g_free (owner);
}
static void
on_notify_name_owner (GObject *object,
GParamSpec *pspec,
gpointer user_data)
{
GDBusObjectManagerClient *manager = G_DBUS_OBJECT_MANAGER_CLIENT (object);
gchar *name_owner;
name_owner = g_dbus_object_manager_client_get_name_owner (manager);
g_print ("name-owner: %s\n", name_owner);
g_free (name_owner);
}
static void
on_interface_proxy_properties_changed (GDBusObjectManagerClient *manager,
GDBusObjectProxy *object_proxy,
GDBusProxy *interface_proxy,
GVariant *changed_properties,
const gchar *const *invalidated_properties,
gpointer user_data)
{
GVariantIter iter;
const gchar *key;
GVariant *value;
gchar *s;
g_print ("Properties Changed on %s:\n", g_dbus_object_get_object_path (G_DBUS_OBJECT (object_proxy)));
g_variant_iter_init (& iter, changed_properties);
while (g_variant_iter_next (& iter, "{& sv}", & key, & value))
{
s = g_variant_print (value, TRUE);
g_print (" %s -> %s\n", key, s);
g_variant_unref (value);
g_free (s);
}
}
gint
main (gint argc, gchar *argv[])
{
GDBusObjectManager *manager;
GMainLoop *loop;
GError *error;
gchar *name_owner;
manager = NULL;
loop = NULL;
loop = g_main_loop_new (NULL, FALSE);
error = NULL;
manager = example_object_manager_client_new_for_bus_sync (G_BUS_TYPE_SESSION,
G_DBUS_OBJECT_MANAGER_CLIENT_FLAGS_NONE,
"org.gtk.GDBus.Examples.ObjectManager",
"/example/Animals",
NULL, /* GCancellable */
&error);
if (manager == NULL)
{
g_printerr ("Error getting object manager client: %s", error->message);
g_error_free (error);
goto out;
}
name_owner = g_dbus_object_manager_client_get_name_owner (G_DBUS_OBJECT_MANAGER_CLIENT (manager));
g_print ("name-owner: %s\n", name_owner);
g_free (name_owner);
print_objects (manager);
g_signal_connect (manager,
"notify::name-owner",
G_CALLBACK (on_notify_name_owner),
NULL);
g_signal_connect (manager,
"object-added",
G_CALLBACK (on_object_added),
NULL);
g_signal_connect (manager,
"object-removed",
G_CALLBACK (on_object_removed),
NULL);
g_signal_connect (manager,
"interface-proxy-properties-changed",
G_CALLBACK (on_interface_proxy_properties_changed),
NULL);
g_main_loop_run (loop);
out:
if (manager != NULL)
g_object_unref (manager);
if (loop != NULL)
g_main_loop_unref (loop);
return 0;
}
```
