Example of calling a function to parent?

Panoss

I 'm searching for a simple example of calling (from a form) a function to another form.

Let me explain: I have FormA with a button on it. Clicking this button I open FormB which also has a button.
When I click on FormB's button I want to call a function in FormA.

From what I read it will be done with Signal, Slots and emit() (but to tell the truth I don't understand much).
Is there some example?

Panoss

Ok, I found a python example that works.

#!/usr/bin/python3
# Threading example with QThread and moveToThread (PyQt5)
import sys
import time
from PyQt5 import QtWidgets, QtCore


class WorkerThread(QtCore.QObject):
    signalExample = QtCore.pyqtSignal(str, int)

    def __init__(self):
        super().__init__()

    @QtCore.pyqtSlot()
    def run(self):
        while True:
            # Long running task ...
            self.signalExample.emit("leet", 1337)
            time.sleep(5)


class Main(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        self.worker = WorkerThread()
        self.workerThread = QtCore.QThread()
        self.workerThread.started.connect(self.worker.run)  # Init worker run() at startup (optional)
        self.worker.signalExample.connect(self.signalExample)  # Connect your signals/slots
        self.worker.moveToThread(self.workerThread)  # Move the Worker object to the Thread object
        self.workerThread.start()

    def signalExample(self, text, number):
        print(text)
        print(number)


if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    gui = Main()
    sys.exit(app.exec_())

I want to modify it (in python code for now) so that the WorkerThread class' code will 'move' to mainWindow (and WorkerThread class to be completely removed).
How can I do this?

JonB

@Panoss
You have picked an example with threads. Why? That is the last thing you should be doing as a newcomer. In any case you cannot perform any UI actions in anything but the main thread in Qt, so what you describe will not work anyway.

Have you read https://doc.qt.io/qt-5/signalsandslots.html ? That shows the sort of thing you were asking about, and does not mention threads.

Panoss

@JonB said in Example of calling a function to parent?:

@Panoss
You have picked an example with threads. Why? That is the last thing you should be doing as a newcomer.

Yes I know, but this was the only one I found that works.

Panoss

@JonB said in Example of calling a function to parent?:

Have you read https://doc.qt.io/qt-5/signalsandslots.html ? That shows the sort of thing you were asking about, and does not mention threads.

This was the first I read, but as I mentioned, didn't understand much. This is why I'm looking for a working, simple, example.

Panoss

Ok, I have an example in python which is exactly as I want it:

from PySide6 import QtWidgets, QtCore


class FormB(QtWidgets.QWidget):
    form_done = QtCore.Signal(str)

    def __init__(self, *args, steps=5, **kwargs):
        super().__init__(*args, **kwargs)

        self.entry = QtWidgets.QLineEdit("Enter value here")
        self.button = QtWidgets.QPushButton("Done")
        self.button.clicked.connect(self.button_pressed)
        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.entry)
        layout.addWidget(self.button)

    def button_pressed(self):
        #self.hide()
        self.form_done.emit(self.entry.text())


class FormA(QtWidgets.QWidget):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.form_b = None
        self.button = QtWidgets.QPushButton("Draw B")
        self.button.clicked.connect(self.draw_b)
        self.label = QtWidgets.QLabel("I display the result in this label")
        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.button)
        layout.addWidget(self.label)

    def draw_b(self):
        if self.form_b is None:
            self.form_b = FormB()
            self.form_b.form_done.connect(self.b_done)
        self.form_b.show()

    def b_done(self, text):
        self.label.setText(text)


def main():
    app = QtWidgets.QApplication([])
    form_a = FormA()
    form_a.show()
    app.exec()


main()

So, how do I convert this python code:

form_done = QtCore.Signal(str)

to c++?

SGaist

Hi,

The Qt documentation contains quite a lot of examples using signals and slots. You should go through the ones from the widgets module since it's the one you are implementing your application with.

Panoss

@SGaist said in Example of calling a function to parent?:

The Qt documentation contains quite a lot of examples using signals and slots

Didn't find a full working example, only fragments of code I 'm trying to put together.
If you have found a full working example please post it!

SGaist

Here you have the full list of the widgets examples.

Panoss

@SGaist impressive selection of examples, I confess I had not discovered it!
But reading their description I see nothing that suits my needs (two classes (Forms) communicating with Signals and Slots)

Panoss

Ok, let me show you what I tried so far:

In my FormB 's class declaration added:

public slots:
    void setValue(int value);
signals:
    void valueChanged(int newValue);

Am I on the correct way?

SGaist

These lines are to be found in the class declaration.

Panoss

Yes you 're right, I corrected it!

Panoss

And this is the function in my FormB: ()

void FormB::setValue(int value)
{       
        emit valueChanged(value);   
}

Correct?

Panoss

And now this:

QObject::connect(&a, &Counter::valueChanged,
                     &b, &Counter::setValue);

I modified it to (this code is in the constructor of FormB):

connect(&parent, &parent::valueChanged,
                         this, &FormB::setValue);

???
It's wrong...The 'parent' I mean.
The sender is FormB...so..

mpergand

@Panoss
Everything is upside down :)

let's recap, you want the main window (formA) to be inform when a value has changed in second window (formB)

The main window register to receive a signal from the second window.

In formA:

connect(formB, &FormB::valueChanged, this , &FormA::valueChangedInFormB);

Panoss

@mpergand it works!!!
Thank you all!

The actual name of FormA is 'ArticlesWindow' and of FormB 'positionsForm'.
(FormB is the sender)
I post the code in case someone needs it:

articleswindow.h
private slots:    
    void positionChanged();

articleswindow.cpp
void ArticlesWindow::positionChanged(){
    qDebug() << "ArticlesWindow::positionChanged";
}
void ArticlesWindow::openPositionsForm(){    
    class positionsForm *form = new class positionsForm(model, this);
    connect(form, &positionsForm::valueChanged, this , &ArticlesWindow::positionChanged);

And:

positionsform.h
  public slots:
    void setValue(int value);

  signals:
    void valueChanged(int newValue);

positionsform.cpp
 void positionsForm::setValue(int value)
 {          
   emit valueChanged(value);    
 }

Panoss

It works but the message is printed twice, like if function ArticlesWindow::positionChanged is called twice??

mpergand

@Panoss

class positionsForm *form = new class positionsForm(model, this);

What is that ?

You seem to create an new instance each time openPositionsForm() is called ...

Panoss

Why is this wrong?
Every time I want to open the form I create an instance of it 's class.
When I close the form the instance is destroyed, right?

What 's the correct way?