April 8, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

Emiting signal does not work

 

Hello. I have got into some problem so I hope you can help me. I have made a simple class where I have set signal to be emited, and a if statement where is checked for condition. If condition is true signal is emited.

Problem is that I run the program it always does the code inside the condition, but it doesn’t emits the signal.

I have tried with diffrent variations inside the signal and inside connect statment, but it simply doesn’t work.

Here is the code of if statment:

  1. if (x>5){
  2.         label->setText("this always run");
  3.         emit signalForStatusBar("this doesnt run");
  4.     }

and here is the code for signal:

  1. signals:
  2.     void signalForStatusBar(QString);

connect statment:

  1. connect(this, SIGNAL(signalForStatusBar(QString)), this, SLOT(testSlot(QString)));

and the slot:

  1. void mainWidget::testSlot(QString string)
  2. {
  3.     label->setText("Slot is running"+string);
  4. }

12 replies

April 8, 2012

koahnig koahnig
Mad Scientist
2101 posts
  • Qt DevNet Moderator
  • Fame monster 3

Did you see an error message on the output when executing the connect statement?

You can also check the connect statements bool return value. It has to be ‘true’.

April 8, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

There is no error message in the output, and I have now checked the output of connect statement and is true.

The output of connect statement is true but the slot is not doing anything????

April 8, 2012

koahnig koahnig
Mad Scientist
2101 posts
  • Qt DevNet Moderator
  • Fame monster 3

If you are running your program in the debugger, you can set a break point at the statement in the slot. Then you will that the slot is triggered.

April 8, 2012

Jeroentje@home Jeroentje@ho..
Hobby Entomologist
235 posts

Just to be clear, but is the signal inside the MainWindow class or somewhere else? That might fail if the ‘this’ is used in the connect statement. You then should you:

  1. connect(variable pointing to the class of signalForStatusBar, SIGNAL(signalForStatusBar(QString)), this, SLOT(testSlot(QString)));

 Signature 

Greetz, Jeroen

April 8, 2012

koahnig koahnig
Mad Scientist
2101 posts
  • Qt DevNet Moderator
  • Fame monster 3

Please post your header file. If it is too big, you may want to skip lines which have nothing to do with the signals and slots.

April 8, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

I have started a new program to see can I make emit to work but is the same principle, as the last program:
main.cpp

  1. #include <QtGui/QApplication>
  2. #include "mainwindow.h"
  3. #include "mainwidget.h"
  4.  
  5. int main(int argc, char *argv[])
  6. {
  7.     QApplication a(argc, argv);
  8.     mainWindow *w=new mainWindow;
  9.     mainWidget *mw=new mainWidget;
  10.  
  11.     w->setCentralWidget(mw);
  12.     w->show();
  13.  
  14.     QObject::connect(mw, SIGNAL(signalForShowMessageInStatusBar(QString)), w, SLOT(slotForStatusBarMessage(QString)));
  15.  
  16.     return a.exec();
  17. }

mainWindow.h

  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5.  
  6. namespace Ui {
  7.     class mainWindow;
  8. }
  9.  
  10. class mainWindow : public QMainWindow
  11. {
  12.     Q_OBJECT
  13.  
  14. public:
  15.     explicit mainWindow(QWidget *parent = 0);
  16.     ~mainWindow();
  17.  
  18. private:
  19.     Ui::mainWindow *ui;
  20.  
  21. private slots:
  22.     void slotForStatusBarMessage(QString);
  23. };
  24.  
  25. #endif // MAINWINDOW_H

mainWindow.cpp

  1. #include "mainwindow.h"
  2. #include "ui_mainwindow.h"
  3.  
  4. mainWindow::mainWindow(QWidget *parent) :
  5.     QMainWindow(parent),
  6.     ui(new Ui::mainWindow)
  7. {
  8.     ui->setupUi(this);
  9. }
  10.  
  11. mainWindow::~mainWindow()
  12. {
  13.     delete ui;
  14. }
  15.  
  16. void mainWindow::slotForStatusBarMessage(QString string)
  17. {
  18.     statusBar()->showMessage(string);
  19. }

mainWidget.h

  1. #ifndef MAINWIDGET_H
  2. #define MAINWIDGET_H
  3.  
  4. #include <QWidget>
  5. #include <QPushButton>
  6. #include <QVBoxLayout>
  7. #include <QHBoxLayout>
  8. #include <QtSql>
  9. #include <QString>
  10.  
  11. #include "mainwindow.h"
  12.  
  13. class mainWidget : public QWidget
  14. {
  15.     Q_OBJECT
  16. public:
  17.     explicit mainWidget(QWidget *parent = 0);
  18.  
  19. signals:
  20.     void signalForShowMessageInStatusBar(QString);
  21.  
  22. public slots:
  23.  
  24. private:
  25.     // varijable
  26.     QPushButton *buttonAddNewRecord;
  27.     QVBoxLayout *layoutMain;
  28.     QHBoxLayout *layoutButton;
  29.  
  30.     // funkcije
  31.     void functionDatabaseOpen();
  32. };
  33.  
  34. #endif // MAINWIDGET_H

mainWidget.cpp

  1. #include "mainwidget.h"
  2.  
  3. mainWidget::mainWidget(QWidget *parent) :
  4.     QWidget(parent)
  5. {
  6.     buttonAddNewRecord=new QPushButton("Add new record", this);
  7.  
  8.     layoutButton=new QHBoxLayout();
  9.     layoutButton->addWidget(buttonAddNewRecord);
  10.  
  11.     layoutMain=new QVBoxLayout();
  12.     layoutMain->addLayout(layoutButton);
  13.  
  14.     functionDatabaseOpen();
  15.  
  16.     setLayout(layoutMain);
  17. }
  18.  
  19. void mainWidget::functionDatabaseOpen()
  20. {
  21.     QSqlDatabase sqlDatabase=QSqlDatabase::addDatabase("QPSQL");
  22.  
  23.     sqlDatabase.setHostName("localhost");
  24.     sqlDatabase.setDatabaseName("postgres");
  25.     sqlDatabase.setUserName("postgres");
  26.     sqlDatabase.setPassword("xxxx");
  27.  
  28.     if (sqlDatabase.open())
  29.     {
  30.         emit signalForShowMessageInStatusBar("true");
  31.     }
  32.     else
  33.     {
  34.         emit signalForShowMessageInStatusBar("false");
  35.     }
  36. }

Thank you for all your help

April 8, 2012

VRonin VRonin
Lab Rat
32 posts

did you try to give a name to the signal argument? like:

signals: void signalForStatusBar(QString abc);
 Signature 

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~Napoleon Bonaparte

April 8, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

@VRodin I have tried still nothing

April 9, 2012

Andre Andre
Area 51 Engineer
6031 posts
  • Certified Specialist
  • Qt DevNet Moderator

The only weird thing I spot, is that you are connecting to a private slot from outside the class itself. Other than that, your code looks fine to me.

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Looking for Qt developers to join our team @ i-Optics: https://qt-project.org/forums/viewthread/25393/

April 9, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

Andre thank you for your help. I have changed the private slot into a public slot but still nothing happnes.

April 9, 2012

Andre Andre
Area 51 Engineer
6031 posts
  • Certified Specialist
  • Qt DevNet Moderator

I think I spotted the issue: The signal is getting emitted before the connection is made. You trigger emitting the signal from the constructor of mainWidget (via the functionDatabaseOpen method), but at that moment the signal is not connected to the slot yet, as you do that only a couple of lines later in your main method.

 Signature 

Looking for Qt developers to join our team @ i-Optics: https://qt-project.org/forums/viewthread/25393/

April 9, 2012

depecheSoul depecheSoul
Lab Rat
28 posts

Andre thanks for your help. I have made some changes in program and it worked.

  1. QApplication a(argc, argv);
  2.     mainWindow *w=new mainWindow;
  3.     mainWidget *mw=new mainWidget;
  4.  
  5.     w->setCentralWidget(mw);
  6.     w->show();
  7.  
  8.     QObject::connect(mw, SIGNAL(signalForShowMessageInStatusBar(QString)), w, SLOT(slotForStatusBarMessage(QString)));
  9.  
  10.     mw->functionDatabaseOpen();
  11.  
  12.     return a.exec();

 
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