Development/Tutorials/Porting to D-Bus

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This tutorial will explain how to convert code that uses DCOP for its interprocess communication (IPC) to D-Bus. D-Bus is the new IPC system for KDE 4.

Porting from DCOP to D-Bus

Converting from DCOPClient

The most direct replacement of DCOPClient are QDBusConnection and QDBusConnectionInterface. The convenience method QDBusConnection::sessionBus() usually replaces all occurrences of KApplication::dcopClient().

The methods in DCOPClient that are related to listing existing applications on the bus are in QDBusBusService (which you can access with QDBusConnection::sessionBus().interface()).

Converting Hand-written DCOPClient calls

DCOPClient has a "call" method that takes two QByteArray's containing the data to be transmitted and the data that was received. The most direct replacement for this is using QDBusMessage and QDBusConnection::send or call. You most likely don't want to use that.

Instead, drop the QByteArray and QDataStream variables and use the dynamic call mode (see next section).

Converting Calls using DCOPRef and DCOPReply

The direct replacement for DCOPRef is QDBusInterface. The replacement for DCOPReply is QDBusReply.

However, there are some important differences to be noticed:

  • QDBusInterface is not "cheap": it constructs an entire QObject. So, if you can, store the value somewhere for later reuse.
  • QDBusReply is a template class, so you must know ahead of time what your reply type is.
  • If you create a QDBusInterface without specifying the third argument (the interface name), this will generate a round-trip to the remote application and will allocate non-shared memory. So, wherever possible, use the interface name to make it cached.

Sample code in DCOP: 

DCOPRef kded("kded", "favicons")
DCOPReply reply = kded.call("iconForURL(KUrl)", url);
QString icon;
if (reply.isValid())
    reply.get(icon);
return icon;


Sample code in D-Bus: 

QDBusInterface kded("org.kde.kded", "/modules/favicons",      
                    "org.kde.FaviconsModule");
QDBusReply<QString> reply = kded.call("iconForURL", url.url());
return reply;

Things to note in the code:

  • use the 3-argument version of QDBusInterface. The interface name you can usually obtain by calling "interfaces" on the existing DCOP object (in this case, "dcop kded favicons interfaces").
  • the "application id" becomes a "service name" and it should start with "org.kde"
  • the "object id" becomes an "object path" and must start with a slash
  • you don't write the function signature: just the function name
  • custom types are not supported (nor ever will be), so you need to convert them to basic types

D-Bus also supports multiple return values. You will normally not find this kind of construct in DCOP, since it didn't support that functionality. However, this may show up in the form of a struct being returned. In this case, you may want to use the functionality of multiple return arguments. You'll need to use QDBusMessage in this case:

Sample; 

QDBusInterface interface("org.kde.myapp", "/MyObject",  
                         "org.kde.MyInterface");
QDBusMessage reply = interface.call("myFunction", argument1, argument2);
if (reply.type() == QDBusMessage::ReplyMessage)
{
    returnvalue1 = reply.at(0).toString();
    returnvalue2 = reply.at(1).toInt();
        /* etc. */
}

Converting from DCOPObject

There is no direct replacement for DCOPObject. It's replaced by a normal QObject with explict registering. You may use this new QObject with or without an adaptor. So, in order to port, you need to follow these steps:

  • Remove the "virtual public DCOPObject" from the class declaration
  • Replace the K_DCOP macro with Q_CLASSINFO("D-Bus Interface", "<interfacename>") where <interfacename> is the name of the interface you're declaring (generally, it'll be "org.kde" followed by the class name itself)
  • Remove the k_dcop method references.
  • Make the methods that were DCOP-accessible scriptable slots. That is, you must make it:
public Q_SLOTS:
    Q_SCRIPTABLE void methodName();
  • Change "ASYNC" to "Q_NOREPLY void"
  • Remove the call to the DCOPObject constructor in the class' constructor.
  • Register the QObject with D-Bus in the constructor.

Note that the Q_CLASSINFO macro is case-sensitive. Do not misspell "D-Bus Interface" (that's capital I and D-Bus has a dash).

In order to register the object, you'll need to do: 

QDBusConnection::sessionBus().registerObject("<object-path>", this,
                            QDBusConnection::ExportScriptableSlots);

Normally, "<object-path>" will be the argument the class was passing to the DCOPObject constructor. Don't forget the leading slash. Another useful option to the third argument is QDBusConnection::ExportScriptableProperties, which will export the scriptable properties.

Signals

DCOP had broken support for signals. They were public methods, while normal signals in QObject are protected methods. The correct way to port from DCOP signals is to refactor the code to support signals correctly.

Adaptors are a special QObject that you attach to your normal QObject and whose sole purpose is to relay things to and from the bus. You'll generally use it to export more than one interface or when you need to translate the call arguments in any way.

You'll declare it as: 

class MyAdaptor: public QDBusAbstractAdaptor
{
    Q_OBJECT
    Q_CLASSINFO("D-Bus Interface", "org.kde.MyAdaptor")
public:
    MyAdaptor(QObject *parent);

signals:
    void signal1();
    void signal2(const QString &argument);
};

And the implementation will look like: 

MyAdaptor::MyAdaptor(QObject *parent)
 : QDBusAbstractAdaptor(parent)
{
    setAutoRelaySignals(true);
    /* alternative syntax:
    connect(parent, SIGNAL(signal1()), SIGNAL(signal1()));
    connect(parent, SIGNAL(signal2(QString)), SIGNAL(QString()));
    */
}

In the class using the adaptor, do this: 

new MyAdaptor(this);
QDBusConnection::sessionBus().registerObject("/<object-path>", this,
                                             QDBusConnection::ExportAdaptors);

Things to notice:

  • MyAdaptor is not exported. This is a private class and the .h file should be a _p.h as well.
  • There's no need to mark the signals in the adaptor as Q_SCRIPTABLE. The same goes for the slots and properties in the adaptor
  • The "setAutoRelaySignals" function just connects all signals in "parent" to the signals of the same name and arguments in the adaptor.
  • There's no need to store the adaptor pointer, it'll be deleted automatically when needed. Therefore, do not delete it and, especially, do not reparent it.
  • You can create more adaptors later, if you need. There's no need to re-register the object.

DCOP Transactions

DCOP had the capability of doing transactions: that is, delay the reply from a called method until later on. QtDBus has the same functionality, under a different name.

Unlike DCOP, with QtDBus, you need to declare that your class may want to do transactions. For that, you must make your object derive from QDBusContext as well as from your your normal inheritance. This will provide you with a few extra methods in your class. You will want to call setDelayedReply to notify QtDBus that the reply will come at a later point in time. You also want to keep the original connection object and message so as to be able to send the proper reply on the proper connection.

Sample DCOP code: 

class MyClass: public QObject, public DCOPObject
{
    DCOPTransaction *xact;
    DCOPClient *client;

k_dcop:
    QString myMethod(const QString &arg1)
    {
        client = callingDcopClient();
        xact = client->beginTransaction();
        QTimer::singleShot(0, this, SLOT(processLater());
        return QString();       // reply will be ignored
    }

public Q_SLOTS:
    void processLater()
    {
        QByteArray replyData;
        QDataStream stream(&replyData, QIODevice::WriteOnly);
        stream << QString(QLatin1String("foo"));
        client->endTransaction(xact, "QString", replyData);
    }
};

The equivalent code with QtDBus would be: 

class MyClass: public QObject, private QDBusContext
{
    QDBusMessage reply;
    QDBusConnection connection;

public Q_SLOTS:
    Q_SCRIPTABLE QString myMethod(const QString &arg1)
    {
        // connection(), message() and setDelayedReply() 
        // come from QDBusContext
        connection = connection();
        reply = QDBusMessage::createReply(message());
        setDelayedReply(true);
        QTimer::singleShot(0, this, SLOT(processLater());
        return QString();       // reply will be ignored
    }

    void processLater()
    {
        reply << QString::fromLatin1("foo");
        connection.send(reply);
    }
};

Caveats when porting

  • You cannot pass "long" arguments, so remember to cast any WId parameters to qlonglong.
  • KDED object paths should start with "/modules/" (i.e., /modules/kssld, /modules/favicons, etc.)
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