Calling a Function when a Signal is Emitted from Another Class

[qtross]

Hello,
I have two ui in different classes.
In classA ui file has; comboBox and StackedWicked with there pages, and
In classB ui file has; pushButton and textEdit.
When I release pushButton in classB, I would like to add the text in textEdit into comboBox which is stated classA.
And I would like to also change the currentIndex of the stackedWidget in classA.

So, there are two changes I would like to do in first class when I release the button. Therefore, I connected a signal my pushButton.released() and a slot which emits.

ClassB.cpp

Second::Second(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::Second)
{
    ui->setupUi(this);
    connect(ui->pushButton, SIGNAL(released()), this, SLOT(emitthesignal()));
}

void Second::emitthesignal() {
     emit;
}

I have tried related solutions found on the previous topics but I could not make it work.

Any suggestion appriciated. Thank you!

[Pablo J. Rogina]

@qtross said in Calling a Function when a Signal is Emitted from Another Class:

void Second::emitthesignal() {
    emit; // it looks you're not emitting anything...
}

Take a look at Signals & Slots introduction to check how to define the signal and its arguments (in your case, you'll be emitting the value of the textEdit from class B)

[qtross]

@Pablo-J-Rogina thank you for your reply.
It is little bit confusing topic, but I have achieved this. However, i did not get any error but it didt work.

in second.h

#include "MainWindow.h"
#include "ui_MainWindow.h"

public:
MainWindow *main;

signals:
textForCombo(QString try);

in second.cpp

void Second::emitsignal(const QString &arg1)

{   
    arg1=ui->textEdit->toPlainText();
    emit textForCombo(arg1); 
    connect(this, SIGNAL(textForCombo(QString)),main, SIGNAL(TextReceive(QString))); 
}

MainWindow.h:

signals:
TextReceive(QString try2);
private slots:
void forstdchange(const QString str);

And in MainWindow.cpp:

{
    ui->setupUi(this);
    connect(this , SIGNAL(TextReceive(QString)),this, SLOT(forstdchange(QString)));
}

void MainWindow::forstdchange(const QString str) {
std::string forstd=str.toStdString();
ui->comboBox->addItem(forstd.c_str());

}

[aha_1980]

@qtross

I'll just add some comments to help you understand your own code:

void Second::emitsignal(const QString &arg1)
{   
    arg1=ui->textEdit->toPlainText(); // How can that work? arg1 is const
    emit textForCombo(arg1); 
    connect(this, SIGNAL(textForCombo(QString)),main, SIGNAL(TextReceive(QString)));  // what is that for?
    // Note that you add an connection everytime the emitsignal function is run, which is NOT what you want
   // Also, you already emit in the line above, so you *don't* need this connect here
}

void MainWindow::forstdchange(const QString str)  // Change to 'const QString &str'
{
  std::string forstd=str.toStdString(); // That looks overly complicated
  ui->comboBox->addItem(forstd.c_str()); // and creates subtile errors with non-ASCII chars

  ui->comboBox->addItem(str); // That is much easier
}

Regards

[qtross]

@aha_1980 Thank you for your reply.

connect(this, SIGNAL(textForCombo(QString)),main, SIGNAL(TextReceive(QString)));

Why I have added this line is I though I need to declare again for accessing first class.ui
with the SIGNAL(TextReceieve(QString)). Therefore, I created void textForCombo.

I did not know I can add directly QString as item into comboBox. I thought I needed to convert it to first std::string then to char.

Now, I did changes you suggested and added this line to mainwindow.cpp,

   connect(this, SIGNAL(emitsignal(QString)), this, SLOT(forstdchange(QString)));

though I added emitsignal in slots-header but I have this error;

Object::connect: No such slot Second::emitthesignal(QString) in ..\untitled\second.cpp:10

Sorry for taking long. I am new to qt and it takes time to understand thank you again.

[aha_1980]

@qtross said in Calling a Function when a Signal is Emitted from Another Class:

connect(this, SIGNAL(textForCombo(QString)),main, SIGNAL(TextReceive(QString)));

To use Signals & Slots effectively, the pattern should always be:

connect(member, SIGNAL(x), this, SLOT(y)); to achive loose coupling. Other way round your objects are harder to re-use.

Object::connect: No such slot Second::emitthesignal(QString) in ..\untitled\second.cpp:10

This error message has nothing to do with this connect, IMHO:

connect(this, SIGNAL(emitsignal(QString)), this, SLOT(forstdchange(QString)));

Also, you should check out the new connect syntax:

https://doc.qt.io/qt-5/signalsandslots.html
https://wiki.qt.io/New_Signal_Slot_Syntax

Regards

[qtross]

Hi again, I updated my code, it seems logical now but I still could not make it work.
I must be missing something simple or doing still something silly.

I also uploaded:
https://ufile.io/dambwl0c
Ui screens:
https://ibb.co/xC6gqTz

and There is my code;

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();
    void addtoCombo(QString text);
private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "secondclass.h"
#include <QDebug>
#include <iostream>
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    SecondClass* second = new SecondClass;
    connect(second, SIGNAL(update(const QString)), this, SLOT(addtoCombo(const QString)));

}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    SecondClass* window = new SecondClass;
    window->show();

}

void MainWindow::addtoCombo(QString text) {

    qDebug() <<"I have been called" << endl;
    ui->comboBox->addItem(text);
    ui->stackedWidget->setCurrentIndex(1);

}

secondclass.h:

#ifndef SECONDCLASS_H
#define SECONDCLASS_H

#include <QWidget>

namespace Ui {
class SecondClass;
}

class SecondClass : public QWidget
{
    Q_OBJECT

public:
    explicit SecondClass(QWidget *parent = nullptr);
    ~SecondClass();

private:
    Ui::SecondClass *ui;
signals:
     void update(const QString text);
private slots:
     void on_pushButton_clicked();
};

#endif // SECONDCLASS_H

seconclass.cpp

#include "secondclass.h"
#include "ui_secondclass.h"

SecondClass::SecondClass(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::SecondClass)
{
    ui->setupUi(this);
}

SecondClass::~SecondClass()
{
    delete ui;
}

void SecondClass::on_pushButton_clicked()
{
     emit update("it adds");
}

[J.Hilk]

@qtross said in Calling a Function when a Signal is Emitted from Another Class:

jup, the problem is you do not connect the signal to the instance that is shown.

in your constructor you create a SecondClass instance

{
ui->setupUi(this);
SecondClass* second = new SecondClass;
connect(second, SIGNAL(update(const QString)), this, SLOT(addtoCombo(const QString)));

}

and on pushbutton clicked, you create yet an other instance of your SecondClass and show that one.
But this one has no connects to it!

void MainWindow::on_pushButton_clicked()
{
SecondClass* window = new SecondClass;
window->show();

}

make SecondClass* second a member of your main window.h

//Constructor than changes to
ui->setupUi(this);
second = new SecondClass;
connect(second, SIGNAL(update(const QString)), this, SLOT(addtoCombo(const QString)));

 void MainWindow::on_pushButton_clicked()
 {
    second->show();
}

[qtross]

@J.Hilk , it worked finally, Thank you so much!