Development/Tutorials/D-Bus/Creating Interfaces

Creating D-Bus Interfaces

Tutorial Series	D-Bus
Previous	Introduction
What's Next	Autostart Services
Further Reading	n/a

Abstract

D-Bus allows applications to expose internal API to the outside world by means of remotely callable interfaces. This tutorial shows how to create and implement such interfaces in your applications.

Lights: Defining The Interface

D-Bus interfaces generally reflect the API of one or more classes in the providing application. The interface as seen on the bus can be described using a standard XML format that is described in the D-Bus specification.

Such XML might look like this example:

<!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="org.foo.Background">
   <signal name="backgroundChanged">
   </signal>
   <method name="refreshBackground">
   </method>
   <method name="currentBackground">
     <arg type="s" direction="out"/>
   </method>
   <method name="setBackground">
     <arg type="b" direction="out"/>
     <arg name="name" type="s" direction="in"/>
   </method>
 </interface>

</node>

One can construct this XML by hand, mapping it to the API of a class by hand, but not only is this error prone and time consuming it's not much fun. Fortunately there are ways to automate the process.

Defining Methods

We will be using the (fictitious) example of an interface that lets the user set the background wallpaper and query the current settings. We will be providing three methods in this interface, which can be seen in the following class definition:

1. include <QObject>

class Background : QObject {

   Q_OBJECT

   public:
       Background(QObject* parent);

       void doNotExportToDBus();

       void refreshBackground();
       QString currentBackground();

   Q_SIGNALS:
       void doNotExportThisSignal();
       void backgroundChanged();

   public Q_SLOTS:
       bool setBackground(QString name);

   protected Q_SLOTS:
       void dbusCanNotSeeMe();

};

Next we need to mark which of the above methods we wish to expose via D-Bus. Fortunately, this is quite simple with the following options available to us:

• export all signals
• export all public slots
• export all properties
• export only scriptable signals
• export only scriptable public slots
• export only scriptable properties

We can also combine the above as we wish. To achieve the desired results in the above example then, we might adjust the class definition thusly:

1. include <QObject>

class Background : QObject {

   Q_OBJECT

   public:
       Background(QObject* parent);

       void doNotExportToDBus();

   Q_SIGNALS:
       void doNotExportThisSignal();
       Q_SCRIPTABLE void backgroundChanged();

   public Q_SLOTS:
       void refreshBackground();
       QString currentBackground();
       bool setBackground(QString name);

   protected Q_SLOTS:
       void dbusCanNotSeeMe();

};

Note how we moved the methods we wish to export to to be public slots and marked the signal we want to export with Q_SCRIPTABLE. We will later then choose to create and interface that exports all the public slots and all scriptable signals.

We would then go about creating an implementation of this class as defined above.

Tip

When exposing an API to other applications via D-Bus other applications and users, via scripting, may come to rely on the calls available in the interface. Changing the D-Bus interface can therefore cause breakage for others. For this reason it is recommended to keep compatibility with publicly advertised D-Bus APIs over the lifespan of a major release of your application.

Naming The Interface

The next step after having defined our interface is to come up with a name that it will appear as on the bus. These names by convention take on the form of reverse domain names so as to prevent name collisions. Therefore if the domain for your project website is http://foo.org you should prefix your interface names with org.foo. This is a very common approach to namespacing such exported components.

Therefore, we may choose to call our interface example org.foo.Background. The easiest way to define this is to add a Q_CLASSINFO macro entry to our class definition:

class Background : QObject {

   Q_OBJECT
   Q_CLASSINFO("D-Bus Interface", "org.foo.Background")

The interface will now be known as org.foo.Background.

Camera: Generating the Interface

Now that we have set up the interface in our class, we will want to generate an adaptor class that mediates between D-Bus and our application's objects. The first step is to generate the XML seen at the outset of this tutorial.

qdbuscpp2xml

To generate the XML we will be using the qdbuscpp2xml command line tool that comes with Qt. This program takes a C++ source file and generates a D-Bus XML definition of the interface for us. It let's us define which methods to export using the following command line switches:

qdbuscpp2xml switches

Switch	Exports !
-S	all signals
-M	all public slots
-P	all properties
-A	all exportable items
-s	scriptable signals
-m	scriptable public slots
-p	scriptable properties
-a	all scriptable items

In our example above we want to export all the public slots but only scriptable signals. Therefore we would use this command line:

$> qdbuscpp2xml -M -s background.h -o org.foo.Background.xml

This produces a file named org.foo.Backgroudn.xml which contains 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="org.foo.Background">
   <signal name="backgroundChanged">
   </signal>
   <method name="refreshBackground">
   </method>
   <method name="currentBackground">
     <arg type="s" direction="out"/>
   </method>
   <method name="setBackground">
     <arg type="b" direction="out"/>
     <arg name="name" type="s" direction="in"/>
   </method>
 </interface>

</node>

This file should be shipped with your project's source distribution.

CMake

Next we add this XML file to our project. This is done by adding the following line to the CMakeLists.txt file:

QT4_ADD_DBUS_ADAPTOR(foo_SRCS org.foo.Background.xml

                    background.h Background)

This will cause two files, in this case backgroundadaptor.h and backgroundadaptor.cpp, to be generated in the build directory, built and added to the application at build time. You should not ship these files with your project's source distribution.

The D-Bus XML description file will also be installed. This allows users to examine it as a reference and other applications to use this file to generate interface classes using qdbusxml2cpp as seen in the tutorial on accessing D-Bus interfaces.

Action: Instantiating the Interface At Runtime

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