Signal and slot from other class not working

Ucn_ · 23 Nov 2019, 16:35

I have two classes in my second ui. I'm trying to connect signal and slot emitted in another class on QLabel click event (I promoted this Label) so I can update QLabel->setText("Some texts") from other class. However it does not work, I have been trying to make it sometimes it will run, but the QLabel->setText("Some texts"); won't update on the ui. Here is my code:

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include "secondform.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();

private:
    SecondForm *captureWig;
    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);


}

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



void MainWindow::on_pushButton_clicked()
{

    captureWig = new SecondForm(this);
    QVBoxLayout *sendWig = new QVBoxLayout;
    sendWig = captureWig->getTreeto();

    QWidget *w = new QWidget();
    w->setLayout(sendWig);

    ui->tabWidget->addTab(w, "Newtab");
}

secondform.h


#ifndef SECONDFORM_H
#define SECONDFORM_H

#include <QObject>
#include <QWidget>
#include <QtWidgets>

class ClickableLabels;

namespace Ui {
class SecondForm;
}

class SecondForm : public QWidget
{
    Q_OBJECT

public:
    explicit SecondForm(QWidget *parent = nullptr);
    ~SecondForm();
    QVBoxLayout *getTreeto();

signals:
void send_fromMain();

private slots:
    void on_Clickme_clicked();
    void ReceivedFromSecondForm();

private:
    ClickableLabels *upLabText;
    QPushButton *button_first;
    Ui::SecondForm *ui;
};


class ClickableLabels : public QLabel {

    Q_OBJECT

public:
    explicit ClickableLabels(QWidget* parent = Q_NULLPTR, Qt::WindowFlags f = Qt::WindowFlags());
    ~ClickableLabels();

signals:
    void clicked();
    void SentFromSecondForm();

protected:
    void mousePressEvent(QMouseEvent* event);

private:

};

#endif // SECONDFORM_H

secondform.cpp


#include "secondform.h"
#include "ui_secondform.h"

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

    upLabText = new ClickableLabels(this);

    QObject::connect (upLabText, SIGNAL (SentFromSecondForm()), this, SLOT(ReceivedFromSecondForm()));
}

void SecondForm::ReceivedFromSecondForm()
{
    ui->label_4->setText("Received");
    qDebug () << "Emitted: ";
}

QVBoxLayout *SecondForm::getTreeto(){

    return ui->verticalLayout_6;
}

ClickableLabels::ClickableLabels(QWidget* parent, Qt::WindowFlags f)
    : QLabel(parent) {

}

ClickableLabels::~ClickableLabels() {}

void ClickableLabels::mousePressEvent(QMouseEvent* ) {

    emit SentFromSecondForm();
    emit clicked();
    qDebug() << "QLabel clicked clicked";

}

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

void SecondForm::on_Clickme_clicked()
{
    ui->label_4->setText("label clicked");
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>657</width>
    <height>663</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QTabWidget" name="tabWidget"/>
    </item>
    <item>
     <widget class="QPushButton" name="pushButton">
      <property name="text">
       <string>add</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>657</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

secondform.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>SecondForm</class>
 <widget class="QWidget" name="SecondForm">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>765</width>
    <height>508</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <widget class="QWidget" name="verticalLayoutWidget_2">
   <property name="geometry">
    <rect>
     <x>10</x>
     <y>30</y>
     <width>279</width>
     <height>137</height>
    </rect>
   </property>
   <layout class="QVBoxLayout" name="verticalLayout_6">
    <item>
     <widget class="QGroupBox" name="groupBox">
      <property name="title">
       <string>GroupBox</string>
      </property>
      <layout class="QVBoxLayout" name="verticalLayout_2">
       <item>
        <widget class="QGroupBox" name="groupBox_2">
         <property name="title">
          <string>GroupBox</string>
         </property>
         <layout class="QHBoxLayout" name="horizontalLayout">
          <item>
           <widget class="QPushButton" name="Clickme">
            <property name="text">
             <string>ckl</string>
            </property>
           </widget>
          </item>
          <item>
           <widget class="QPushButton" name="pushButton">
            <property name="text">
             <string>PushButton</string>
            </property>
           </widget>
          </item>
          <item>
           <widget class="QPushButton" name="pushButton_4">
            <property name="text">
             <string>PushButton</string>
            </property>
           </widget>
          </item>
         </layout>
        </widget>
       </item>
       <item>
        <layout class="QHBoxLayout" name="horizontalLayoutfv"/>
       </item>
       <item>
        <widget class="QLabel" name="label_4">
         <property name="text">
          <string>change here</string>
         </property>
        </widget>
       </item>
       <item>
        <widget class="ClickableLabels" name="label_5">
         <property name="text">
          <string>click here</string>
         </property>
        </widget>
       </item>
      </layout>
     </widget>
    </item>
   </layout>
  </widget>
 </widget>
 <customwidgets>
  <customwidget>
   <class>ClickableLabels</class>
   <extends>QLabel</extends>
   <header location="global">secondform.h</header>
  </customwidget>
 </customwidgets>
 <resources/>
 <connections/>
</ui>

main.cpp

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

int main(int argc, char *argv[])
{

    QApplication a(argc, argv);

//    ClickableLabels sender;
//    SecondForm receiver;

//    QObject::connect(&sender, SIGNAL(SentFromSecondForm()), &receiver, SLOT(ReceivedFromSecondForm()));


    MainWindow w;
    w.show();

    return a.exec();
}

Thanks

Cobra91151 · 23 Nov 2019, 23:11

@Ucn_

Hello!

I have created a simple example to illustrate a clickable label, button and connect using lambdas, so you can simplify your code.

clickablelabel.h

#ifndef CLICKABLELABEL_H
#define CLICKABLELABEL_H

#include <QWidget>
#include <QLabel>
#include <QEvent>

class ClickableLabel : public QLabel
{
    Q_OBJECT
public:
    using QLabel::QLabel;

signals:
    void clicked();

protected:
    void mousePressEvent(QMouseEvent *event) override;
};

#endif // CLICKABLELABEL_H

clickablelabel.cpp

#include "clickablelabel.h"

void ClickableLabel::mousePressEvent(QMouseEvent *event)
{
    emit clicked();
    QLabel::mousePressEvent(event);
}

test.h

#ifndef TEST_H
#define TEST_H

#include <QDialog>
#include <QVBoxLayout>
#include "clickablelabel.h"

namespace Ui {
class Test;
}

class Test : public QDialog
{
    Q_OBJECT

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

private:
    Ui::Test *ui;
};

#endif // TEST_H

test.cpp

#include "test.h"
#include "ui_test.h"

Test::Test(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Test)
{
    ui->setupUi(this);
    setFixedSize(600, 500);
    setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowCloseButtonHint);
    ClickableLabel *testLabel = new ClickableLabel("This is a test label...", this);
    connect(testLabel, &ClickableLabel::clicked, [testLabel]() {
       testLabel->setText("Some text from mouse click");
    });
    ui->pushButton->setText("Test button");
    connect(ui->pushButton, &QPushButton::clicked, [testLabel]() {
       testLabel->setText("Some text from button click");
    });

    QVBoxLayout *testLayout = new QVBoxLayout();
    testLayout->setAlignment(Qt::AlignCenter);
    testLayout->addWidget(testLabel);
    testLayout->addWidget(ui->pushButton);
    setLayout(testLayout);
}

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

Screenshot:

Happy coding!

Pl45m4 · 23 Nov 2019, 17:01

@Ucn_ said in Signal and slot from other class not working:

QObject::connect (upLabText, SIGNAL (SentFromSecondForm() ()), this, SLOT(ReceivedFromSecondForm()));

From just looking at your code: There's one "()" too much in your connection (signal part).

Move your class ClickableLabels to its own header file and try again.

Ucn_ · P Pl45m4 23 Nov 2019, 16:35

@Pl45m4 The "()" was an error while copying, I don't have extra "()" in my actual code. About the QLabel, I click it then emit the signal, where I want to update another QLabel text. The QLabel I click, I promoted because QLabel doesn't have On Clicked attribute. So, I used that custom event for QLabel clicking.

Christian Ehrlicher · wrote on 23 Nov 2019, 17:41

QObject::connect() returns a boolean to indicate if the connect was successful. Also during runtime you will get a warning when the connect is not valid. To avoid this we suggest to use the new signal slot syntax.
Do you actually see your debug output from ClickableLabels::mousePressEvent() ? I would guess no. At least not from the one you create to do the connection from since this is not the one from the ui file...

Ucn_ · 23 Nov 2019, 21:31

The signal and slot connection I commented on the main.cpp fires, but it does not update the Qlabel and it does not emit on button click.

Christian Ehrlicher · 23 Nov 2019, 21:36

@Ucn_ But your current code can not work as I already explained - you are connecting to another instance of ClickableLabels than the one you're displaying.

Ucn_ · 23 Nov 2019, 22:30

@Christian-Ehrlicher I would like to emit from the ClickableLabels class. How do I achieve that? I'm kind of lost.

Cobra91151 · 23 Nov 2019, 23:11

@Ucn_

Hello!

I have created a simple example to illustrate a clickable label, button and connect using lambdas, so you can simplify your code.

clickablelabel.h

#ifndef CLICKABLELABEL_H
#define CLICKABLELABEL_H

#include <QWidget>
#include <QLabel>
#include <QEvent>

class ClickableLabel : public QLabel
{
    Q_OBJECT
public:
    using QLabel::QLabel;

signals:
    void clicked();

protected:
    void mousePressEvent(QMouseEvent *event) override;
};

#endif // CLICKABLELABEL_H

clickablelabel.cpp

#include "clickablelabel.h"

void ClickableLabel::mousePressEvent(QMouseEvent *event)
{
    emit clicked();
    QLabel::mousePressEvent(event);
}

test.h

#ifndef TEST_H
#define TEST_H

#include <QDialog>
#include <QVBoxLayout>
#include "clickablelabel.h"

namespace Ui {
class Test;
}

class Test : public QDialog
{
    Q_OBJECT

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

private:
    Ui::Test *ui;
};

#endif // TEST_H

test.cpp

#include "test.h"
#include "ui_test.h"

Test::Test(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Test)
{
    ui->setupUi(this);
    setFixedSize(600, 500);
    setWindowFlags(Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowCloseButtonHint);
    ClickableLabel *testLabel = new ClickableLabel("This is a test label...", this);
    connect(testLabel, &ClickableLabel::clicked, [testLabel]() {
       testLabel->setText("Some text from mouse click");
    });
    ui->pushButton->setText("Test button");
    connect(ui->pushButton, &QPushButton::clicked, [testLabel]() {
       testLabel->setText("Some text from button click");
    });

    QVBoxLayout *testLayout = new QVBoxLayout();
    testLayout->setAlignment(Qt::AlignCenter);
    testLayout->addWidget(testLabel);
    testLayout->addWidget(ui->pushButton);
    setLayout(testLayout);
}

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

Screenshot:

Happy coding!

Ucn_ · wrote on 23 Nov 2019, 23:11

@Cobra91151 Thanks, I will give it a try.

Ucn_ · 24 Nov 2019, 07:12

@Cobra91151 said in Signal and slot from other class not working:

connect(ui->pushButton, &QPushButton::clicked, testLabel {
testLabel->setText("Some text from button click");
});

The example works, thank. But, is there a way I can pass functions or widgets created in designer to the lambda capture list? Let's say I want to run a function after QLabel click or change a specific label.

mrjj · 24 Nov 2019, 21:32

@Ucn_

Hi
You can capture "this" to allow to use both
UI->xxx and member functions

 connect(ui->pushButton, &QPushButton::clicked, [this]() {
    ui->xxxxx;
    });

Ucn_ · M mrjj 24 Nov 2019, 07:12

@mrjj Thanks, It works. however, I don't understand why it works in one example and in the other says

'QMetaObject::Connection QObject::connect(const QObject *,const char *,const char *,Qt::ConnectionType) const': cannot convert argument 1 from 'QLabel *' to 'const ClickableLabels *'

SGaist · 24 Nov 2019, 22:11

Because you are using a pointer to a QLabel as first parameter and a signal from your ClickableLabel as second parameter.

Ucn_ · S SGaist 24 Nov 2019, 21:36

@SGaist what is the viable solution. Cast?

SGaist · 24 Nov 2019, 22:39

Ensure your original parameter is a ClickableLabel instance.

Ucn_ · S SGaist 24 Nov 2019, 22:13

@SGaist Thanks, I promoted QLabel widget in designer to ClickableLabel and it's working. Is it the correct way?

SGaist · 24 Nov 2019, 22:43

Since you are using Designer, yes.

Ucn_ · S SGaist 24 Nov 2019, 22:42

@SGaist Thanks.

Ucn_ · 25 Nov 2019, 12:50

I know I marked this as solved, and thanks to everyone who helped me understand what I was missing. I'm able to make it work when is widget to widget or promoted with with custom events. However when there is nothing triggered or clicked like a button I'm not able to connect. For example. Function to function, if one function from another class is executed, connect and execute another function in another class. Where the signal is the first executed function and slot is the function executed after the first was triggered. Thanks

Pl45m4 · 25 Nov 2019, 13:27

@Ucn_

Set up the connect in your constructor and emit the signal, you connect to, in your function A to execute function B in your second class.