How to validate a DBus property when using gdbus-codegen skeleton

[Anders Norman <norman anders gmail com>]

I'm creating a service on DBus using `gdbus` and `gdbus-codegen`.

Introspection is this:

    <!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object
Introspection 1.0//EN"
"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd";>
    <node>
      <interface name="com.example.foo">

        <property name="Bar" type="s" access="readwrite" />

      </interface>
    </node>

I'm executing `gdbus-codegen` like this:

    gdbus-codegen --interface-prefix com.example \
                  --generate-c-code=foo foo.xml

And my main.cpp looks like this:

#include <iostream>

#include "foo.h"

void OnBarChanged(GObject *    gobject,
                  GParamSpec * pspec,
                  gpointer     user_data)
{
  std::cout << "Bar: " << foo_get_bar((Foo *)gobject) << std::endl;
}

void OnBusNameAquired(GDBusConnection * connection,
                      const gchar *     name,
                      gpointer          user_data)
{
  Foo * foo = foo_skeleton_new();

  g_signal_connect(foo, "notify::bar", G_CALLBACK(&OnBarChanged), NULL);

  g_dbus_interface_skeleton_export(G_DBUS_INTERFACE_SKELETON(foo),
                                   connection,
                                   "/com/example/foo",
                                   NULL);
}

int main()
{
  std::cout << "Testing DBus properties" << std::endl;

  GMainLoop * loop;
  loop = g_main_loop_new(NULL, FALSE);
  g_bus_own_name(G_BUS_TYPE_SESSION,
                 "com.example.foo",
                 G_BUS_NAME_OWNER_FLAGS_NONE,
                 NULL,
                 OnBusNameAquired,
                 NULL,
                 NULL,
                 NULL);
  g_main_loop_run(loop);
  return 0;
}

This works as expected and I'm able to set and get properties using:

gdbus call --session --dest com.example.foo \
       --object-path /com/example/foo \
       --method org.freedesktop.DBus.Properties.Set \
         "com.example.foo" "Bar" "<'baz'>"

and

gdbus call --session --dest com.example.foo \
       --object-path /com/example/foo \
       --method org.freedesktop.DBus.Properties.Get \
         "com.example.foo" "Bar"
(<'baz'>,)

The problem:

I want to validate the setting of the property synchronously and be able
to return an error if it fails. How can this be accomplished using the
`gdbus-codegen`-generated code?

PS:

The code leaks and is generally not production ready. I'm fine with that
for now :-)

PPS:

This question is also posted on
http://stackoverflow.com/questions/37541857/how-to-validate-a-dbus-property-when-using-gdbus-codegen-skeleton