updating value from mainwindow to dialog

Question

I have a mainwindow that generates random int numbers every second in "numbers" and I have a button that opens a new dialog. In that dialog is a button "Get Number" and a qlabel. If I press "Get Number" I want the current generated number from the mainwindow set to the dialog label text. I tried to write a member function in the mainwindow class that returns the actual value but I cant call it from the dialog and I think that's the wrong way. How can I get this current generated int "numbers" into the dialog? My plain 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 = 0);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();
    void generateNumbers();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "dialog.h"
#include <QTimer>
#include <QDebug>
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QTimer *timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()), this, SLOT(generateNumbers()));
    timer->start(1000);

}

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

void MainWindow::on_pushButton_clicked()
{
    Dialog dialog;
    dialog.setModal(true);
    dialog.exec();
}

void MainWindow::generateNumbers()
{
    int numbers = qrand() % 100;
    qDebug() << numbers;
}

dialog.h

#ifndef DIALOG_H
#define DIALOG_H

#include <QDialog>

namespace Ui {
class Dialog;
}

class Dialog : public QDialog
{
    Q_OBJECT

public:
    explicit Dialog(QWidget *parent = 0);
    ~Dialog();

private:
    Ui::Dialog *ui;
};

#endif // DIALOG_H

dialog.cpp

#include "dialog.h"
#include "ui_dialog.h"

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

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

Answer 1

On mainwindow.h, add a signal:

signals:
    void update_number(int);

On mainwindow.cpp, emit that signal everytime you generate a new number:

void MainWindow::generateNumbers()
{
    int numbers = qrand() % 100;
    emit update_number(numbers); // signals that a new number has been generated
    qDebug() << numbers;
}

and your on_pushButton_clicked() should look like this:

void MainWindow::on_pushButton_clicked()
{
   Dialog dialog(this); // main window as the parent of dialog
   dialog.setModal(true);
   dialog.exec();
}

On dialog.h, add an attribute called number_ and add the following slot:

public slots:
    void on_update_number(int number); // updates the number_

private:
    int number_; // it will be updated with the most recent number
    Ui::Dialog *ui;

On dialog.cpp, connect the signal to the slot:

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
    connect(parent, SIGNAL(update_number(int)), this, SLOT(on_update_number(int)));
}

add the slot code:

void Dialog::on_update_number(int number)
{
    number_ = number;
}

Finally, when the user clicks the button "Get Number" in the dialog, set the label text:

void Dialog::on_pushButton_clicked()
{
    ui->label->setText(QString::number(number_));
}

For more information on Signals & Slots go here.