Development/Tutorials/Porting to D-Bus

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Revision as of 15:32, 4 March 2007 by Winterz (talk | contribs) (Porting to D-Bus moved to Development/Tutorials/Porting to D-Bus: as discussed with thiago)

Compiling D-Bus

The KDE libraries require D-Bus version 0.62 at least. See http://www.freedesktop.org/wiki/Software_2fdbus for the latest release.

To compile: % ./configure --disable-qt --disable-qt3 --prefix=$DBUSDIR % make % make install

To run the D-Bus daemon, type the command: % eval `PATH=$DBUSDIR/bin $DBUSDIR/bin/dbus-launch --auto-syntax`

Compiling QtDBus

QtDBus depends on D-Bus.

D-Bus is found using pkg-config, so if you did not install it to a standard path in the previous step, set the PKG_CONFIG_PATH environment variable: % PKG_CONFIG_PATH=$DBUSDIR/lib/pkgconfig; export PKG_CONFIG_PATH

Template:Hint

QtDBus now lives in kdesupport and uses cmake. It also requires you to have Qt 4.1.3 in order to compile (qt-copy has it). To compile it: % svn co $SVNROOT/trunk/kdesupport % cd $your_preferred_objdir % cmake $OLDPWD/kdesupport % make

Compiling kdelibs

Remember to set PKG_CONFIG_PATH to where you installed D-Bus to ($DBUSDIR/lib/pkgconfig), where you installed QtDBUS and where Qt is installed ($QTDIR/lib). This is necessary because QtDBUS depends on Qt and is found using pkg-config.

Commands: % cd kdelibs % export PKG_CONFIG_PATH=$QTDIR/lib:$DBUSDIR/lib/pkgconfig % cd $objdir % cmake $OLDPWD % make

Hint for Trolls: pass cmake the flags -DCMAKE_C_COMPILER=/opt/teambuilder2/bin/gcc

-DCMAKE_CXX_COMPILER=/opt/teambuilder2/bin/g++ if you want to build with Teambuilder.

Porting existing DCOP code

Usage of DCOPClient

The most direct replacement of DCOPClient are [1] and [2]. The convenience method QDBus::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 QDBus::sessionBus().busService()).

Hand-written calls using DCOPClient

DCOPClient has a "call" method that takes two QByteArrays 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 sendWithReply. 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).

Calls using DCOPRef and DCOPReply

The direct replacement for DCOPRef is QDBusInterfacePtr, which is just a wrapper around QDBusInterface. The replacement for [3].

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: ~pp~ 

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

~/pp~

Sample code in DBUS: ~pp~ 

   QDBusInterfacePtr kded("org.kde.kded", "/modules/favicons", "org.kde.FaviconsM

odule"); 

   QDBusReply<QString> reply = kded->call("iconForURL", url.url());
   return reply;

~/pp~

Things to note in the code:

  1. use the 3-argument for in QDBusInterfacePtr. The interface name you can usually

obtain by calling "interfaces" on the existing DCOP object (in this case, "dcop kd ed favicons interfaces").

  1. the "application id" becomes a "service name" and it should start with "org.kde"
  2. the "object id" becomes an "object path" and must start with a slash
  3. it's "kded->call", not "kded.call"
  4. you don't write the function signature: just the function name
  5. custom types are not supported (nor ever will be), so you need to convert them t

o basic types

DBUS 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; ~pp~ 

   QDBusInterfacePtr 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. */
   }

~/pp~

Usage of DCOPObject

There is no direct replacement of DCOPObject. It's replaced by normal QObject, wit h or without an adaptor, with explicit registering. So, in order to port, you need 

to follow these steps:
  1. Remove the "virtual public DCOPObject" from the class declaration
  2. 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)

  1. Remove the k_dcop method references.
  2. Make the methods that were DCOP-accessible scriptable slots. That is, you must m

ake it: ~pp~ 

 public Q_SLOTS:
   Q_SCRIPTABLE void methodName();

~/pp~

  1. Change "ASYNC" to "Q_NOREPLY void"
  2. Remove the call to the DCOPObject constructor in the class' constructor.
  3. Add the registering to the constructor.

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

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

   QDBus::sessionBus().registerObject("<object-path>", this, QDBusConnection::Exp

ortSlots); ~/pp~

Normally, "<object-path>" will be the argument the class was passing to the DCOPOb ject constructor. Don't forget the leading slash. Another useful option to the thi rd argument is QDBusConnection::ExportProperties, which will export the scriptable 

properties.

Signals

DCOP had a broken support for signals. They were public methods, while normal sign als in QObject are protected methods. The correct way to port is to refactor the c ode to support signals correctly.

On the current version of QtDBus, you cannot use QDBusConnection::ExportSignals. T his is a known limitation and will be fixed in the future. In order to use signals, you'll need to write an adaptor.

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

way.

You'll declare it as: ~pp~ 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);

}; ~/pp~

And the .cpp file will have: ~pp~ MyAdaptor::MyAdaptor(QObject *parent) 

   : QDBusAbstractAdaptor(parent)

{ 

   setAutoRelaySignals(true);
   /* alternative syntax:
   connect(parent, SIGNAL(signal1()), SIGNAL(signal1()));
   connect(parent, SIGNAL(signal2(QString)), SIGNAL(QString()));
   */

} ~/pp~

In the class using the adaptor, do this: ~pp~ 

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

portAdaptors); ~/pp~

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 Q_SCRIPTABLE. The same goes for slots and pr

operties.

  • 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 whe

n 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 cal led method until later on. QtDBus has the same functionality, under a different na me.

Unlike DCOP, with QtDBus, you need a special parameter to your slot in order to re ceive the information about the call and set up the delayed reply. This parameter is of type "QDBusMessage" and must appear after the last input parameter. You decl are that you want a delayed reply by creating a reply. You will be responsible for 

sending it later.

Sample DCOP code: ~pp~ 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);
   }

}; ~/pp~

The equivalent code with QtDBus would be: ~pp~ class MyClass: public QObject { 

   QDBusMessage reply;

public Q_SLOTS: 

   Q_SCRIPTABLE QString myMethod(const QString &arg1, const QDBusMessage &msg)
   {
       reply = QDBusMessage::methodReply(msg);
       QTimer::singleShot(0, this, SLOT(processLater());
       return QString();       // reply will be ignored
   }

   void processLater()
   {
       reply << QString(QLatin1String("foo"));
       reply.connection().send(reply);
   }

}; ~/pp~

Caveats when porting

  • You cannot pass "long" arguments, so remember to cast any WId parameters to qlon

glong.

  • KDED object paths should start with "/modules/" (i.e., /modules/kssld, /modules/

favicons, etc.)

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