Development/Tutorials/Kross/Introduction

From KDE TechBase

Kross

The Kross scripting framework provides full Python, Ruby and KDE JavaScript scripting support. The goal was to limit the work needed on applications to have them fully scriptable and to provide a modular way to transparently integrate additional interpreters and in that way extend your application with a new scripting-backend without any new line of code and even without any recompile. To achieve this internally Qt's introspection-functionality like signals, slots, properties, enums, QVariant, QObject, QMetaObject, QMetaType, etc. are used to deal with functionality at runtime.

The Interpreter plugins

Kross offers a plugin-interface to integrate interpreter backends like Python, Ruby and KDE-Javascript. They are loaded on demand at runtime. Neither the application nor Kross needs to know any details about the backends. This clear separation between the application and scripting details enables at the one hand, to deal with scripting at an abstract level without being bound to only one backend, and at the other hand makes it easy to integrate scripting into an already existing application.

Currently Kross comes with support for 3 scripting backends;

Each interpreter plugin needs to implement two abstract classes;

  • The Kross::Interpreter class is a singleton controlled by the Kross::Manager and could be used to setup the interpreter or do other things to share functionality between instances of the Script class.
  • The Kross::Script class handles exactly one script instance. An application is able to deal with multiple scripts at the same time where each of them has it's own instance of the Kross::Script class controlled by an instance of the more abstract Kross::Action class.

The Module plugins

Modules are plugins loaded on demand at runtime to provide additional functionality. They only provide a factory to create and return a QObject instance that is then exposed to the scripting backend.

Since Qt's introspection functionality is used, we are able to throw in just QObject's and have them act as classes/objects within a scripting backend. Slots are membermethods while properties and enumerations are membervariables. If your application also likes to offer DBus support it may be an idea to reuse the QDBusAbstractAdaptor implementations your application has also for scripting, like for example KSpread did.

Let's take a look at the following code that implements the MyObject class which inherits from QObject; 

class MyObject : public QObject {
  Q_OBJECT
  Q_PROPERTY(QString name READ name WRITE setName)
  public:
    MyObject(QObject* parent = 0) : QObject(parent) {}
    virtual ~MyObject() {}
    QString name() const { return objectName(); }
    void setName(const QString& name) {
      return setObjectName(name);
    }
  public slots:
    QObject* create(const QString& name,
                    QObject* parent=0) {
      MyObject* obj = new MyObject(parent);
      obj->setObjectName(name);
      return obj;
    }
    QObject* parent() const { return QObject::parent(); }
    void setParent(QObject* parent) {
      QObject::setParent(parent);
      emit parentChanged(); 
    }
  signals:
    void parentChanged();
};

extern "C" {
  QObject* krossmodule() {
    return new MyObject();
  }
}

At the bottom the krossmodule() function will be called by Kross and returns an instance of MyObject.

Then we just need to have our myobject.h and myobject.cpp files, filled with the content above, defined in the CMakeLists.txt file. The library needs to be named "krossmodule..." where the "..." is then the name the module is accessible as. For our example we use "krossmodulemyobjectmod" and therefore we are able to access the module if installed as "myobjectmod". The example does not depend on Kross, so you may like to replace ${KROSS_INCLUDES} with whatever else your module depends on. 

include_directories(${CMAKE_SOURCE_DIR}
  ${KROSS_INCLUDES})
set(krossmodulemyobjectmod_PART_SRCS
  myobject.cpp)
kde4_automoc(${krossmodulemyobjectmod_PART_SRCS})
kde4_add_plugin(krossmodulemyobjectmod
  ${krossmodulemyobjectmod_PART_SRCS})
target_link_libraries(krossmodulemyobjectmod
  ${KDE4_KDECORE_LIBS} ${KDE4_KROSSCORE_LIBS})
install(TARGETS krossmodulemyobjectmod
  DESTINATION ${PLUGIN_INSTALL_DIR})

The following Python sample code accesses then the module at runtime and uses the QObject, calls it's slots and properties and connects a signal with a python function (e.g. save as file named "myobjecttest.py" and execute with "kross ./myobjecttest.py"); 

#!/usr/bin/env kross
import Kross
m = Kross.module("myobjectmod")
m.name = "MyObjectModuleName"
obj1 = m.create("OtherObjectName")
def myCallbackFunc(args):
    print "The parent of obj1 changed"
obj1.connect("parentChanged()",myCallbackFunc)
obj1.setParent(m)
print "%s %s" % (obj1.name,obj1.parent().name)

Manager, GuiClient, Action

The Kross::Manager class is a singleton that provides access to the interpreters, to actions and to the modules. The Kross::Manager is available within scripting code as module named "Kross". Following Python script uses the Kross module to create a new Kross::Action instance, fills it with JavaScript code and executes that JavaScript code. 

#!/usr/bin/env kross
import Kross
a = Kross.action("MyKjsScript")
a.setInterpreter("javascript")
a.setCode("println(\"Hello world from Kjs\");")
a.trigger()

The Kross::GuiClient class implements KXMLGUIClient to provide GUI functionality, handling of XML configuration files on a more abstract level and offers some predefined actions that could be optionally used in your application.

The Kross::Action class offers an abstract container to deal with scripts like a single standalone scriptfile. Each action holds a reference to the by the matching Kross::Interpreter instance created by Kross::Script instance. Following Python script accesses the Kross module and the self variable which is our Kross::Action instance that provides the context for the running Python script. 

#!/usr/bin/env kross
import Kross
print "objectName=%s" % Kross.objectName()
print "interpreters=%s" % Kross.interpreters()
print "objectName=%s" % self.objectName()
print "text=%s" % self.text
print "enabled=%s" % self.enabled
print "currentPath=%s" % self.currentPath()
print "interpreter=%s" % self.interpreter()
print "description=%s" % self.description()
print "code=%s" % self.code()

DBus and Kross

With the QtDBus module Qt provides a library that a Qt/KDE application is able to use to make Inter-Process Communication using the D-BUS protocol.

The QtDBus module uses the Qt Signals and Slots mechanism. Applications that like to provide parts of there functionality to the DBus world are able to do so by implementing classes that inherit from the QDBusAbstractAdaptor class.

Kross is able to reuse such by an application provided bindings. So, once your application supports dbus, Kross is able to reuse those already existing code and offers transparent access to the scripting-backends to them. How does this differ from e.g. the python-dbus package? Well, first method-calls don't go through the dbus-socket and then it's not dbus-related at all except, that we are able to reuse what your application offers anyway: a clean interface to the outside world. But that's not all, we are not limited to what's possible with dbus. We are also able to exchange instance-pointers to QObject or QWidget instances.

For an example you may like to take a look at how it was done in the KSpread ScriptingModule class.

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