Sending messages between two widgets using Signals and Slots

Question

I need to have widgets communicating between each other.

My central Widget has 2 widget instances: widget A (instance of A()), widget B (instance of B()) When a widget A is subject to an event (e.g.: mouseRelease), I need widget A to call a function and to do some computing, processing data. And then I need this processed data to be passed to the widget B, which in turn upon receiving this data from widget A, will call a function with this data, process it, and do something more with it.

Both these widgets are custom widgets derived from QWidget. I am almost certain I need to use signals and slots but I don't understand how to implement function call parts.

I've read over and over again explanations of Qt here: http://qt-project.org/wiki/Signals_and_Slots_in_PySide however I can not figure it out how to connect two widgets which both process data emitted and sent.

I would really appreciate some help here. If you can not answer in Python yet competent with C++, please explain nonetheless using it instead of refraining at all.

Answer 1

Here is a example demonstrating signals/slots connections between two widgets in PyQt4:

#!/usr/bin/env python
#-*- coding:utf-8 -*-

from PyQt4 import QtCore, QtGui

class widgetB(QtGui.QWidget):
    procDone = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetB, self).__init__(parent)

        self.lineEdit = QtGui.QLineEdit(self)
        self.button = QtGui.QPushButton("Send Message to A", self)
        self.layout = QtGui.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

        self.button.clicked.connect(self.on_button_clicked)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procDone.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_procStart(self, message):
        self.lineEdit.setText("From A: " + message)

        self.raise_()

class widgetA(QtGui.QWidget):
    procStart = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(widgetA, self).__init__(parent)

        self.lineEdit = QtGui.QLineEdit(self)
        self.lineEdit.setText("Hello!")

        self.button = QtGui.QPushButton("Send Message to B", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.layout = QtGui.QHBoxLayout(self)
        self.layout.addWidget(self.lineEdit)
        self.layout.addWidget(self.button)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.procStart.emit(self.lineEdit.text())

    @QtCore.pyqtSlot(str)
    def on_widgetB_procDone(self, message):
        self.lineEdit.setText("From B: " + message)

        self.raise_()


class mainwindow(QtGui.QMainWindow):
    def __init__(self, parent=None):
        super(mainwindow, self).__init__(parent)

        self.button = QtGui.QPushButton("Click Me", self)
        self.button.clicked.connect(self.on_button_clicked)

        self.setCentralWidget(self.button)

        self.widgetA = widgetA()
        self.widgetB = widgetB()

        self.widgetA.procStart.connect(self.widgetB.on_procStart)
        self.widgetB.procDone.connect(self.widgetA.on_widgetB_procDone)

    @QtCore.pyqtSlot()
    def on_button_clicked(self):
        self.widgetA.show()
        self.widgetB.show()

        self.widgetA.raise_()


if __name__ == "__main__":
    import sys

    app  = QtGui.QApplication(sys.argv)
    main = mainwindow()
    main.show()
    sys.exit(app.exec_())