Inter-Process Communication with Qt. Presented at Qt Developer Days 2007.

1. IPC with Qt

2. About me
 Marius Bugge Monsen
 Senior Software Engineer
 5 years for TT
 Msc. comp. sci. NTNU
 Projects
 Itemviews
 QtDBus
 WebKit
2

3. Contents
 What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
3

4.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
4

5. What is Inter­Process Communication?
I am a
process!
So am I!
Process Process
5

6. What Is Inter­Process Communication?
Exchanging information between processes.
6

7. What Is Inter­Process Communication?
 Techniques
 remote procedure calls (RPC)
 message passing
 synchronization
 shared memory
7

8. What Is Inter­Process Communication?
 Implementations
 CORBA
 SOAP / XML­RPC
 DCOM
 DCOP
 D­Bus
 ...
8

9.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
9

10. What is D­Bus ?
A Message Bus System
10

11. What is D­Bus?
Message Bus Daemon
Process Process Process
11

12. What is D­Bus ?
 Protocol
 Library
 Message bus framework
12

13. D­Bus
 General one­to­one message passing framework
 Single computer
 Computer­to­computer through unencrypted TCP/IP
13

14. D­Bus
 Portable to any *NIX system
 Windows port in progress
 Standard IPC mechanism for Gnome and KDE
14

15. What is D­Bus ?
 System Bus
 system­wide services
 Session Bus
 per­user­login­session services
15

16. Exposing Functionality
 Service
 Objects
 Interfaces
 Methods
 Signals
 Properties
16

17. Exposing Functionality
 Service
 Represent a process connected to a bus.
 Has a unique (base) name chosen by the bus.
 Uses a “:n.nnn” naming scheme (e.g. :1.32).
 Has a well­known name chosen by the process.
 Uses “reverse domain name” naming scheme (e.g.
com.trolltech.appname).
 Multiple services per process
17

18. Exposing Functionality
 Object
 Multiple objects per service.
 Specified by a path.
 /path/to/ObjectName
 The path structure is arbitrary.
 Provides namespaces for objects.
18

19. Exposing Functionality
 Interface
 Multiple interfaces per object.
 Dot­separated name.
 com.trolltech.Assistant.HelpViewer
 Defines a set of methods, signals and properties.
 names
 parameters
 return values
 Can be described by XML.
19

20. Exposing Functionality
 Methods
 A method can be called by sending a message to the process
exposing the functionality (the caller does not have to register
a service).
 Many­to­one.
 Signals
 A signal is emitted by the process which is exposing the
interface and is available to any application on the same bus.
 One­to­many.
20

21. Exposing Functionality
 Properties
 org.freedesktop.DBus.Properties
 The Properties interface that provides Get and Set methods.
21

22. Exposing Functionality
 Address
 Service
 Object
 Interface
 Method or Signal
22

23. D­Bus Address
com.trolltech.Assistant /Assistant com.trolltech.Assistant.HelpViewer showLink
service object path interface method
23

24. Activation
 The Bus can start clients automatically
 by explicitly requesting the Bus to activate the client.
 by invoking a method on an object belonging to the client.
24

25. Introspection
 Objects may provide the introspection interface
 org.freedesktop.DBus.Introspectable
 The Introspect method returns an XML description.
25

26.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
26

27. What is QtDBus ?
 Object­oriented Qt API for D­Bus.
 Maps Qt properties, signals and slots to D­Bus.
 Maps D­Bus properties, signals and methods to Qt.
27

28. QtDBus Tools
 qdbus
 Command­line D­Bus explorer.
 qdbusviewer
 Graphical D­Bus explorer.

28

29. QtDBus Tools
 qdbusxml2cpp
 Compiles XML interface description to an interface or adaptor
class.
 qdbuscpp2xml
 Compiles an interface or adaptor class to an XML interface
description file.
29

30.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
30

31. QtDBus Approaches
 High­level
 Interface representations generated from XML descriptions
 Mid­level
 Direct calls to a generic interface class
 Low­level
 Constructing and sending messages
31

32. Connecting To D­Bus
 QDBusConnection
 Represents a connection to the D­Bus bus daemon.
 QDBusConnection::BusType
 QDBusConnection::SessionBus
 QDBusConnection::SystemBus
 QDBusConnection::ActivationBus
32

33. Connect To The SessionBus
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
...
QDBusConnection connection = QDBusConnection::sessionBus();
...
return app.exec();
}
33

34. Register The Service
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
...
QDBusConnection connection = QDBusConnection::sessionBus();
connection.registerService("com.trolltech.LineEdit");
...
return app.exec();
}
34

35. Exposing Functionality
 Expose QObject slots directly
 Expose interfaces
 doesn't have to map directly to any object
35

36. Exposing All Slots
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QLineEdit le;
QDBusConnection connection = QDBusConnection::sessionBus();
connection.registerService("com.trolltech.LineEdit");
connection.registerObject("/LineEdit", &le,
QDBusConnection::ExportAllSlots);
le.show();
return app.exec();
}
36

37. Exposing Interfaces
 Adaptor Class
 Special class attached to a QObject
 Exposes an interface to the bus
 Lightweight
 Inherits QDBusAbstractAdaptor
37

38. Exposing Interfaces
Adaptor Interface
QObject Adaptor Interface Message Bus
Adaptor Interface
38

39. Exposing An Interface
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QLineEdit le;
new LineEditAdaptor(&le);
QDBusConnection connection = QDBusConnection::sessionBus();
connection.registerService("com.trolltech.LineEdit");
connection.registerObject("/LineEdit", &le,
QDBusConnection::ExportAdaptors);
le.show();
return app.exec();
}
39

40. Exposing Methods
class LineEditAdaptor : public QDBusAbstractAdaptor
{
Q_OBJECT
Q_CLASSINFO("D­Bus Interface", "com.trolltech.LineEdit.Test")
LineEditAdaptor(QLineEdit *w) : QDBusAbstractAdaptor(w), le(w) {}
~LineEditAdaptor() {}
public slots:
inline void show() { le­>show(); }
inline void hide() { le­>hide(); }
private:
QLineEdit *le;
};
40

41. Exposing Properties
class LineEditAdaptor : public QDBusAbstractAdaptor
{
Q_OBJECT
Q_CLASSINFO("D­Bus Interface", "com.trolltech.LineEdit.Test")
Q_PROPERTY(QString text READ text WRITE setText)
public:
LineEditAdaptor(QLineEdit *w) : QDBusAbstractAdaptor(w), le(w) {}
~LineEditAdaptor() {}
inline QString text() const { return le­>text(); }
inline void setText(const QString &text) { le­>setText(text); }
...
};
41

42. Exposing Signals
class LineEditAdaptor : public QDBusAbstractAdaptor
{
Q_OBJECT
Q_CLASSINFO("D­Bus Interface", "com.trolltech.LineEdit.Test")
Q_PROPERTY(QString text READ text WRITE setText)
public:
LineEditAdaptor(QLineEdit *w) : QDBusAbstractAdaptor(w), le(w)
{
QDBusAbstractAdaptor::setAutoRelaySignals(true);
}
...
signals:
void textChanged(const QString &text);
...
};
42

43. XML Interface Description
<node name="/com/trollech/LineEdit">
<interface name="com.trolltech.LineEdit.Test">
<property name="text" type="s" access="readwrite" />
<method name="show"/>
<method name="hide"/>
<signal name="textChanged">
<arg type="s" name="text" />
</signal>
</interface>
</node>
43

44. XML To Adaptor
test.xml
qdbusxml2cpp ­a adaptor test.xml
adaptor.h adaptor.cpp
44

45. XML To Proxy
test.xml
qdbusxml2cpp ­p proxy test.xml
proxy.h proxy.cpp
45

46. Using Interfaces
Interface Proxy
Interface Message Bus Proxy
Interface Proxy
46

47.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
47

48. Using Interfaces
 Proxy Class
 Represents a D­Bus interface
 Represented as a normal QObject
 signals
 slots
 properties
 Inherits QDBusAbstractInterface
48

49. The Generated Proxy
com::trolltech::LineEdit::Test *le =
new com::trolltech::LineEdit::Test( // interface
"com.trolltech.LineEdit", // service
"/LineEdit", // object
QDBusConnection::sessionBus()); // bus
QComboBox box;
box.setEditable(true);
box.setEditText(le­>isValid() ? le­>text() : "empty");
QObject::connect(le, SIGNAL(textChanged(const QString&)),
&box, SLOT(setEditText(const QString&)));
49

50. QDBusInterface
 Generic accessor class
 used to place calls to remote interfaces
 requires org.freedesktop.DBus.Introspectable
50

51. Calling Methods
int main()
{
QDBusInterface iface("com.trolltech.LineEdit",
"/LineEdit",
"com.trolltech.LineEdit.Test",
QDBusConnection::sessionBus());
iface.call("hide");
return 0;
}
51

52. Arguments And Return Values
int main()
{
QString service = "com.trolltech.Calculator";
QString path = "/Calculator";
QString interface = "com.trolltech.Calculator.Computation";
QDBusConnection bus = QDBusConnection::sessionBus();
QDBusInterface iface(service, path, interface, bus);
QDBusMessage result = iface.call("multiply", 4, 3);
QList<QVariant> values = result.arguments();
...
return 0;
}
52

53. Properties
int main()
{
QString service = "com.trolltech.Calculator";
QString path = "/Calculator";
QString interface = "com.trolltech.Calculator.Computation";
QDBusConnection bus = QDBusConnection::sessionBus();
QDBusInterface iface(service, path, interface, bus);
QVariant text = iface.property(“text”);
return 0;
}
53

54. Signals And Slots
int main()
{
QString service = "com.trolltech.Calculator";
QString path = "/Calculator";
QString interface = "com.trolltech.Calculator.Computation";
QDBusConnection bus = QDBusConnection::sessionBus();
QDBusInterface iface(service, path, interface, bus);
QObject::connect(&iface, SIGNAL(textChanged(const QString&)),
&receiver, SLOT(textChanged(const QString&)));
return 0;
}
54

55.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
55

56. QDBusMessage
 Represents D­Bus message.
 MessageType
 MethodCallMessage
 SignalMessage
 ReplyMessage
 ErrorMessage
 InvalidMessage
56

57. Creating A Message
 Static functions
 createError(...)
 createMethodCall(...)
 createSignal(...)
 Non­static functions
 createErrorReply(...)
 createReply(...)
57

58. Sending A Message
int main()
{
QDBusMessage msg = QDBusMessage::createMethodCall(
“com.trolltech.Calculator”, // service
“/Calculator”, // object
“com.trolltech.Calculator.Computation”, // iface
“multiply”); // method
QList<QVariant> args;
args << 4 << 5;
msg.setArguments(args);
QDBusMessage reply = QDBusConnection::sessionBus().call(msg);
...
return 0;
}
58

59. Synchronous vs. Asynchronous
 QDBus::CallMode
 NoBlock
 Block
 BlockWithGui
 AutoDetect
59

60. Sending A Message Asynchronously
int main()
{
...
QDBusMessage msg = QDBusMessage::createMethodCall(
“com.trolltech.LineEdit”, // service
“/LineEdit”, // object
“com.trolltech.LineEdit.Test”, // interface
“hide”); // method
QDBusConnection::sessionBus().send(msg);
...
return 0;
}
60

61. QtDBus Approaches
 High­level
 XML Interface Description
 Adaptor class
 Proxy class
 Mid­level
 QDBusInterface
 Low­level
 QDBusMessage
61

62.  What is Inter­Process Communication ?
 What is D­Bus ?
 What is QtDBus ?
 Exposing Interfaces
 Calling Interfaces
 Messages
 More IPC
62

63. More IPC
 Techniques
 remote procedure calls (RPC)
 message passing
 synchronization
 shared memory
63

64. More Qt IPC (Qt 4.4)
 Synchronization
 QSystemSemaphore
 Shared Memory
 QSharedMemory
 Local Sockets
 QLocalSocket
 QLocalServer
64

65. Resources
http://doc.trolltech.com/4.3/intro­to­dbus.html
http://www.freedesktop.org/wiki/Software/dbus
http://sourceforge.net/projects/windbus
65