[RESOLVED] Signal from form to form

mrcgnerd

I can't access Ui from Dialog. I am geting this error:
error: 'Ui::MainWindow* MainWindow::ui' is private

@MainWindow *firstForm; // Is it corect ?
firstForm->ui->pushButton->setText("yes");@

I know that seting UI as public is not a good idea. But signals and slots method is too complex for me. Looks like there is no simple wai pf doing that.

mrcgnerd

I Found this: "LINK":http://stackoverflow.com/questions/1056247/passing-variables-from-one-form-to-another-in-qt

It says:
"The dialog box still exists after it closes. So you can, from the main form, do something like this:

QString text = subform->textEdit->text();
This assumes your dialog box is subform and the name you gave to the text edit box is textEdit. Make sure you make textEdit public in the designer.

If you don't want to make textEdit public, then you can add a getter to subform."

How can i make textEdit or other component public ?

ankursaxena

U have to create Object of Second form in the constructor of First Form. and then u can do with second form using object of second.

code_fodder

I think you are trying to directly access the MainWindow from your dialog, which, in theory you can do... but it sort of breaks the rules of encapsulation (i.e reaching in to other objects and accessing their internals... generally a bad practice).

What Qt has is the slot / signal message/event passing system which is made for just this purpose and its really nice and easy to use (once you learn its basics).

So, in your dialog form you press a button and then you want the MainWindow to know that you have pressed a button and do something accordingly. This is how it can be done:

in MainWindow.cpp
@
// Connect a signal from mdialog called somthingHasHappened to a slot in the MainWindow called handleSomthingFromDialog.
QObject::connect(mdialog, SIGNAL(somthingHasHappened()), this SLOT(handleSomthingFromDialog()))

// Make a slot (in the header it comes under "public slots:"
void MainWindow::handleSomthingFromDialog(void)
{
     ui->pushButton->setText("YES");
}

@

in MainWindow.h
@
public slots:
void handleSomthingFromDialog(void);
@

in dialog.cpp
@
void MyDialog::on_radioButton_clicked()
{
// emit the signal somthingHasHappened MainWindow will pick this up
emit somthingHasHappened();
}
@

in dialog.h
@
signals:
void somthingHasHappened(void);
@

The result is that, when radio button is clicked in dialog, it "emits" a signal that is connected to the MainWindow slot.

mrcgnerd

Thank you for help ! Your example is realy helpful.

The program compiles, but not starts. I am geting this line:
The program has unexpectedly finished.
But no error in code. What I am doing wrong ?

@MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QObject::connect(mdialog, SIGNAL(somthingHasHappened(void)), this, SLOT(handleSomthingFromDialog(void)));

}
void MainWindow::handleSomthingFromDialog(void)
{
ui->Form1Button->setText("YES");
}@

I tried to make mdialog public in mainwindow.h:
@public:
Dialog *mdialog;@
And now program starts but when I press radio buton nothing hapens.

@void MyDialog::on_radioButton_clicked()
{
emit somthingHasHappened();
}@

IamSumit

hii
The program has unexpectedly finished.
it might be because you did not initialized mdialog (which is pointer) .
view thread-->
https://qt-project.org/forums/viewthread/19319

code_fodder

yeah, I agree with IamSumit, sounds like a pointer-y like error! (sudden crash - generally means bad memory access).

You must instantiate (create an instance of) mdiaglog before you connect it, or use it in any way at all :)

When you do this, check you debug/output if the connection fails (a slot or signal is missing or not correct) then Qt prints a warning.

mrcgnerd

I started debug. And it shows :

"The iferior stopped becouse it recieved a signal from the Operating System.
Signal name: SIGSEGV
Signal meaning: Segmentation fault"

I don't uderstund how corectly initialize mdialog. Is this not corect:

@public:
Dialog *mdialog;
// with this program started but signal not worked
//But after debuging it's not even starting anymore@

I tried this:
@private:
Dialog *mdialog;
// Same result@

And:

@#include<QDialog>
Publlic:
QDialog* mdialog;
//Same again
@

EDIT:

OK I tried this:
@mdialog = new Dialog(this);
QObject::connect(mdialog, SIGNAL(somthingHasHappened(void)), this, SLOT(handleSomthingFromDialog(void)));
@
And the program starts again but signal not working, I open dialog pres radio button and nothin is happening.

code_fodder

What about the connection debug? - do you see some failure with the connection in the debug output? (when you run the program in Qt Creator it should come up in the debug window.

If you do not, then please print out all code in both files, header and cpp...
Have you declared the signal somthingHasHappened(void) in mdialog? and implemented handleSomthingFromDialog(void) slot?

mrcgnerd

There is no error in debug window.

My code:
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();
void processText();

signals:
void somthingHasHappened(void);

private slots:

void on_radioButton_clicked();

void on_pushButton_clicked();

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;
}

void Dialog::on_radioButton_clicked()
{
emit somthingHasHappened();
}

void Dialog::on_pushButton_clicked()
{
emit somthingHasHappened();

}@

mainwindow.h

@#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <dialog.h>
#include <QDialog>
#include <QtGui>
#include <QtCore>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

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

public slots:
void handleSomthingFromDialog(void);

private slots:
void on_pushButton_clicked();

private:

Ui::MainWindow *ui;

};

#endif // MAINWINDOW_H
@

mainwindow.cpp

@#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
mdialog = new Dialog(this);
QObject::connect(mdialog, SIGNAL(somthingHasHappened(void)), this, SLOT(handleSomthingFromDialog(void)));

}
void MainWindow::handleSomthingFromDialog(void)
{
ui->textEdit->setText("YES");
}

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

void MainWindow::on_pushButton_clicked()
{
Dialog mydialog;
mydialog.exec();

//mdialog = new Dialog(this);
//mdialog->show();

}@

main.cpp

@#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();

return a.exec&#40;&#41;;

}
@

code_fodder

Can you put some debug in
void Dialog::on_radioButton_clicked()
void MainWindow::handleSomthingFromDialog(void)

to see if one or both functions are getting called?

the signal / slots look good to me.

However:
What is this:
@
void MainWindow::on_pushButton_clicked()
{
Dialog mydialog;
mydialog.exec();

//mdialog = new Dialog(this);
//mdialog->show();

}
@

A second dialog? - if this is the dialog that is running, then it is a different instance of the dialog that you have connected your signal/slot to.

mydialog != mdialog

You probably want to do
@
mdialog->exec();
@

this is a guess because I don't know the order of how you do things.

code_fodder

or do:

@
void MainWindow::on_pushButton_clicked()
{
Dialog mydialog;

QObject::connect(&mydialog, SIGNAL(somthingHasHappened(void)), this, SLOT(handleSomthingFromDialog(void)));

mydialog.exec();
}
@

mrcgnerd

[quote author="code_fodder" date="1393943815"]or do:

@
void MainWindow::on_pushButton_clicked()
{
Dialog mydialog;

QObject::connect(&mydialog, SIGNAL(somthingHasHappened(void)), this, SLOT(handleSomthingFromDialog(void)));

mydialog.exec();
}
@[/quote]

YES ! :DDDD
It's working !

THANK YOU !
You helped alot. I think now I will use signals and slots more often in my projects, they are realy useful. I will read more about it.

One last thing, this can sound stupid, becouse I think this is not even Qt but C++ related, but what does & sign before mydialog, in conection line mean ? (connect(&mydialog) ?
Sory I just learning stuf and not facetd with this before.

By the way how can I mark subject as SOLVED ? (God it's even more stupid question :D)

Thanks again for everyone, and sory for my english.

code_fodder

Ah, that is because you have to pass in a pointer in the connect() function.

Dialog mydialog; - is a variable of type Dialog.
Dialog *mdialog; - is a pointer (address) to a Dialog variable.

So to make pass the address of a variable you have to use to the "&" operator, therefore &mydialog becomes (Dialog *) type (address of).

To mark this thread as resolved, just change the title of your post to:
"[RESOLVED] Signal from form to form"

This is the convention