Signal and slot from other class not working

Christian Ehrlicher

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_

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

@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_

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

Cobra91151

@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_

@Cobra91151 Thanks, I will give it a try.

Ucn_

@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

@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_

@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

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

Ucn_

@SGaist what is the viable solution. Cast?

SGaist

Ensure your original parameter is a ClickableLabel instance.

Ucn_

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

SGaist

Since you are using Designer, yes.

Ucn_

@SGaist Thanks.

Ucn_

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

@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.

Ucn_

@Pl45m4

This only works when I setup in main.cpp and when it fires in won't update anything in the function except print. When I set up in the constructor and place the emit in the function it does not fire,

QObject::connect(&timer, SIGNAL(printPerSecond()), &w, SLOT(anotherFuncion()));

emit printPerSecond()

jsulm

@Ucn_ What is timer in your connect?
Is there a warning at runtime that connect failed?
You should use the new connect syntax to get compiler error instead of runtime warning if your connect call is wrong.

Ucn_

@jsulm It does not give any warning. I tried different connects, it just won't execute the function.