Development/Tutorials/Kross/Connecting Signals and slots in Kross

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Hello world in kross
Tutorial Series   Kross tutorials
Previous   Kross Hello World
What's Next   Scripts as plugins
Further Reading   n/a

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This tutorial shows how to use a system of signals and slots to provide a scripting interface for a KDE application. It builds upon the Kross Hello World tutorial and again follows a 'Hello World' type format.

Update source files

This tutorial is based on the Hello World tutorial and extends the codebase we wrote there with new functionality.


First edit the mainwindow.h to handle the changes to mainwindow.cpp. Note that the change is the addition of a private Kross::Action .

#include <QComboBox>
#include <QLabel>
#include <QLineEdit>

#include <kross/core/action.h>
// The main window to display our combobox and the label.
class MainWindow : public QWidget
    // The constructor.
    MainWindow(QWidget *parent=0);
  private Q_SLOTS:
    // This slot is called when the item in the combobox is changed.
    void interpreterActivated(const QString &);
    QLineEdit* txtInputString;
    QLabel* lblMessage;
    QComboBox* cmbInterpreters;
    // We now have the action as class-member.
    Kross::Action* action;



Using the same CmakeLists.txt and main.cpp from the previous Hello World tutorial, edit the mainwindow.cpp as follows:

#include "mainwindow.h"

#include <QVBoxLayout>
#include <QDebug>

#include <kross/core/manager.h>
#include <kross/core/action.h>

// the constructor.
MainWindow::MainWindow(QWidget *parent) : QWidget(parent)
  txtInputString = new QLineEdit();
  lblMessage = new QLabel("Hello");
  cmbInterpreters = new QComboBox ();
  cmbInterpreters->addItem("Choose Interpreter", "");

  foreach(QString s, Kross::Manager::self().interpreters())

  connect(cmbInterpreters, SIGNAL(activated(const QString &)),
      SLOT(interpreterActivated(const QString &)));

  QVBoxLayout *vLayout = new QVBoxLayout;

  // This time we create the Kross::Action already within the
  // constructor and add the objects that should be accessible
  // from within scripting code.
  action = new Kross::Action(this, "MyScript");

  action->addObject(txtInputString, "MyInputString");
  action->addObject(cmbInterpreters, "MyInterpreter");
  action->addObject(lblMessage, "MyLabel");

// this slot is called when the active item of the combobox changes.
void MainWindow::interpreterActivated(const QString &strSelectedInterpreter)
  if(strSelectedInterpreter.isEmpty()) {

  // this time we are using external script files.
  QString filename;
  if(strSelectedInterpreter == "python")
    filename = "";
  else if(strSelectedInterpreter == "javascript")
    filename = "krossSigsSlots.js";

  // set the script file that should be executed.
  // finally execute the scripting code.

The changes are addition of a text entry field and the reorganisation of logic handling the Kross::Action. Note also that there is no code in mainwindow.cpp to set the label, as there was in the previous tutorial. Instead, Objects are simply made available to the scripting interface through the action->addObject calls, without any knowledge of what the script will use them for. This removes the need to know at the time of writing the application what function the scripts will perform, and is therefore suited to a plugin interface.

This script catches the textChanged SIGNAL of the QLineEdit, and connects it to a simple python function to reverse the string in the QLineEdit and display it in the QLabel.

#!/usr/bin/env kross

import MyLabel
import MyInterpreter
import MyInputString

def reverseString(s):
  s = s[::-1]
  MyLabel.text = s

MyInputString.connect("textChanged(const QString &)", reverseString)


This script does the same as the script above but using the JavaScript scripting language.

function reverseString(s){
    MyLabel.text = s.split("").reverse().join("");

connect(MyInputString, "textChanged(const QString &)", this, "reverseString(const QString &)");
Write similar for other interpreters

Autoconnecting Signals and Slots

It is possible to connect signals from your Object to slots in your script file automatically. Edit the mainwindow.cpp file to add another argument to the addObject function:

action->addObject(txtInputString, "MyInputString",

action->addObject(cmbInterpreters, "MyInterpreter");
action->addObject(lblMessage, "MyLabel");

The Kross::ChildrenInterface::AutoConnectSignals argument causes signals of the object to be automatically connected with scripting functions of the same name. Therefore, the scripts can again be simplified:

Simplified scripts

The following python code provides a function that reverses the string:

import MyLabel

def textChanged(s):
  s = s[::-1]
  MyLabel.text = s

Now follows a javascript function that converts the string to pig latin:

function textChanged(text)
  text = text.replace(/\b([aeiou][a-z]*)\b/gi, "$1way"); // Rule 2
  pigLatin = text.replace(/\b([bcdfghjklmnpqrstvwxyz]+)([a-z]*)\b/gi, "$2$1ay"); // Rule 1
  MyLabel.text = pigLatin;
Write similar for other interpreters

Emitting signals from within scripts

So far this tutorial has described connecting signals in c++ objects with slots in scripts. The objects published to kross scripts also make their signals available to the scripts. The signals can then be emitted by calling them. This is equivalent to calling:

  emit signalName();

in c++ code. To illustrate this, change the python script to emit the setEnabled(bool) signal of the QCombobox:

#!/usr/bin/env kross

import MyLabel
import MyInterpreter

def textChanged(s):
  if s == "off":
  elif s == "on":
  s = MyInputString.text[::-1]
  MyLabel.text = s

If 'off' is written in the text field, the setEnabled signal is emitted and the combobox is greyed out. It is not re-enabled again until 'on' is written in the text field.

This is a simple demonstration of signal-slot relationships using kross. More complex interfaces may be written to complete a plugin architecture.

This page was last modified on 29 June 2011, at 19:47. Content is available under Creative Commons License SA 3.0 as well as the GNU Free Documentation License 1.2 unless otherwise noted.