Intermediate D-Bus

The techniques in Accessing D-Bus interfaces work for methods with fairly simple signatures. The more complex interfaces found in practice need the techniques on this page.

#Complex return types

Qt D-Bus needs some setup for methods that return more than single primitive values: the type must be known to Qt's type system so it can be read from the message.

#Lists

D-Bus arrays map to QList. Declare the list type to Qt's type system, for example:

Q_DECLARE_METATYPE(QList<QDBusObjectPath>)

Use Q_DECLARE_METATYPE outside any function or block; the top of the file is the best place.

Register the type with Qt D-Bus as well, before it is used:

qDBusRegisterMetaType<QList<QDBusObjectPath>>();

Qt D-Bus already knows how to read and write QList and QMap of supported types, so you don't need to write streaming operators for them. See The Qt D-Bus Type System.

#Dictionaries

D-Bus dictionaries map to QMap. Declare and register them the same way as lists.

#Several return values

Some D-Bus methods return several values. QDBusReply holds only the first one, so for the others use the QDBusMessage itself. QDBusAbstractInterface::call() and its relatives return a QDBusMessage; a QDBusReply is only built from it.

QDBusMessage::arguments() returns all the values as a QList<QVariant>.

For example, for a method org.foo.DBusTute.Favourites.Get(out INT32 number, out STRING colour, out STRING flavour):

QDBusInterface iface("org.foo.DBusTute",
                     "/org/foo/DBusTute/Favourites",
                     "org.foo.DBusTute.Favourites",
                     QDBusConnection::sessionBus());
QDBusMessage reply = iface.call("Get");
QList<QVariant> values = reply.arguments();
int favouriteNumber = values.takeFirst().toInt();
QString favouriteColour = values.takeFirst().toString();
QString favouriteFlavour = values.takeFirst().toString();

#Interfaces that don't support Introspect

QDBusInterface uses introspection to give you D-Bus signals and properties like a local object's. That needs the remote object to implement org.freedesktop.DBus.Introspectable, which isn't required.

#Properties

Properties read with QObject::property() are found by Introspect. Without it, if you know the names and types of the properties (from the remote interface's source), use the D-Bus properties interface directly:

org.freedesktop.DBus.Properties.Get (in STRING interface_name,
                                     in STRING property_name,
                                     out VARIANT value);
org.freedesktop.DBus.Properties.Set (in STRING interface_name,
                                     in STRING property_name,
                                     in VARIANT value);

#Signals

Without Introspect, QObject::connect() fails at run time with a "no such signal" error.

You can still connect to these signals one level lower, with QDBusConnection::connect(). If you use QDBusInterface for its call() functions, take its connection and connect on that:

QDBusInterface iface("org.foo.DBusTute",
                     "/org/foo/DBusTute/Favourites",
                     "org.foo.DBusTute.Favourites",
                     QDBusConnection::sessionBus());

iface.connection().connect("org.foo.DBusTute",
                           "/org/foo/DBusTute/Favourites",
                           "org.foo.DBusTute.Favourites",
                           "FavouritesChanged", this,
                           SLOT(favouritesChanged()));

This works like the string-based QObject::connect(). QDBusConnection::connect() takes the slot only as a SLOT() string, so function pointers and lambdas can't be used.