Creating custom slots and signals

Fuchsiaff

Hi

I have a worker thread and a main GUI. The worker thread reads values every 3 seconds.

I want to emit that value to the main GUI every 3 seconds. How do I do that?

I tried reading the documentation from qt5 but this is as far as I got:

cpu_thread.h

#ifndef CPU_THREAD_H
#define CPU_THREAD_H
#include <QThread>

class cpu_thread : public QThread {
    Q_OBJECT
public:
    cpu_thread();
    void run();
public slots:
    void setValue(int value);

signals:
    void valueChanged(int value);
};

#endif // CPU_THREAD_H

cpu_thread.cpp

#include "cpu_thread.h"
#include <QDebug>
#include <QProcess>
#include <QTest>

cpu_thread::cpu_thread() {

}

void cpu_thread::run() {
    while (true) {

        QThread *thread = new QThread;
        QProcess *process = new QProcess;
        process->moveToThread(thread);
        process->start("cat /proc/loadavg");
        process->waitForFinished(-1);

        QString stdout = process->readAllStandardOutput().trimmed();
        stdout.truncate(4);
        qDebug() << stdout;

        QTest::qWait(3000);
    }
}

void cpu_thread::setValue(int value) {

    emit valueChanged(value);
}

Now the main gui which is supposed to capture the value and update the gui looks like this:

#include "ui_mainwindow.h"
#include <QProcess>
#include <QDebug>
#include <QThread>
#include <cpu_thread.h>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow) {
    ui->setupUi(this);
    cpu_thread *my_thread = new cpu_thread;
    my_thread->start();
    cores = getCpuCores();

}

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

void MainWindow::on_actionExit_triggered() {
    qApp->exit();
}

void MainWindow::mousePressEvent(QMouseEvent *event) {
    m_nMouseClick_X_Coordinate = event->x();
    m_nMouseClick_Y_Coordinate = event->y();
}

void MainWindow::mouseMoveEvent(QMouseEvent *event) {
    move(event->globalX()-m_nMouseClick_X_Coordinate,event->globalY()-m_nMouseClick_Y_Coordinate);
}

int MainWindow::getCpuCores() {

    QProcess *process = new QProcess;

    process->start("nproc");
    process->waitForFinished(-1);

    QString stdout = process->readAllStandardOutput().trimmed();
    QString stderr = process->readAllStandardError().trimmed();

    int cores = stdout.toInt();

    return cores/2;
}

void MainWindow::test() {

    QThread *thread = new QThread;
    QProcess *process = new QProcess;
    process->moveToThread(thread);
    process->start("cat /proc/loadavg");
    process->waitForFinished(-1);

    QString stdout = process->readAllStandardOutput().trimmed();
    qDebug() << stdout;

}

void MainWindow::updateCpu(QString value) {

    ui->lcdNumber->display(value);
}```

sierdzio

@Fuchsiaff said in Creating custom slots and signals:

cpu_thread *my_thread = new cpu_thread;
connect(my_therad, &cpu_thread::valueChanged, this, &MainWindow::someSlotYouCreateInMainWindow);
my_thread->start()

That should do it on UI side. In your thread, you need to emit the value, that would probably look like:

qDebug() << stdout; // BTW that's a HORRIBLE variable name!
emit valueChanged(stdout.toInt());
QTest::qWait(3000);

But, and that is a BIG but - your code is overdoing it massively.

You don't need a thread. QProcess already runs in a separate process - it won't block your UI at all! So you can simplify all this like so:

remove whole cpu_thread class
in MainWindow do this:

QTimer *timer = new QTimer(this);
timer->setInterval(3000);
timer->setSingleShot(false);
// Using a lambda, but you can (should) create your own slot for this, of course
connect(timer, &QTimer::timeout, this, [this] {
  QProcess *process = new QProcess(this);
  process->start("cat /proc/loadavg");
  connect(process, &QProcess::finished, this, &MainWindow::someSlot);
});

// Your slot:
void MainWindow::someSlot() {
  QProcess *process = qobject_cast<QProcess *>(sender());
  ui->lcdNumber->display(process->readAllStandardOutput());
  process->deleteLater();
}

Brain to terminal, not tested of course. But it should give you a general idea.

SGaist

Hi,

Rather than calling cat, why not just use a QFile and read the content of /proc/loadavg ?

Fuchsiaff

Anyway, after using code suggested by sierdzo,

mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow) {
    ui->setupUi(this);
    QTimer *timer = new QTimer(this);
    timer->setInterval(3000);
    timer->setSingleShot(false);
    // Using a lambda, but you can (should) create your own slot for this, of course
    QObject::connect(timer, &QTimer::timeout, this, [this] {
      QProcess *process = new QProcess(this);
      process->start("cat /proc/loadavg");
      QObject::connect(process, &QProcess::finished, this, &MainWindow::updateCpu);
    });
    cores = getCpuCores();

}

void MainWindow::updateCpu(int exitCode, QProcess::ExitStatus exitStatus) {

    QProcess *process = qobject_cast<QProcess *>(sender());
    QString result = process->readAllStandardOutput().trimmed();
    result.truncate(4);
    ui->lcdNumber->display(result);
    process->deleteLater();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMouseEvent>
#include <QMainWindow>
#include <QProcess>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow {
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    int getCpuCores();
    void updateCpu(int exitCode, QProcess::ExitStatus exitStatus);
    void updateRAM(QString value);
    void updateNetwork(QString value);
    void updateFanSpeed(QString value);
    int cores;
    ~MainWindow();

private slots:
    void on_actionExit_triggered();

private:
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    int m_nMouseClick_X_Coordinate;
    int m_nMouseClick_Y_Coordinate;
    Ui::MainWindow *ui;
    bool isMouseDown = false;

};

#endif // MAINWINDOW_H

Qtcreator shows me this:

No matching function call for call to "connect"

sierdzio

Hint from @SGaist is far better :-)

No matching function call for call to "connect"

Which connect? QtC will show you exactly the line where compilation failed.

Fuchsiaff

This part:

 QObject::connect(process, &QProcess::finished, this, &MainWindow::updateCpu);

SGaist

What is the exact error ?

Fuchsiaff

/home/bot/Hardware_mon/mainwindow.cpp:19: error: no matching function for call to ‘MainWindow::connect(QProcess*&, <unresolved overloaded function type>, MainWindow*, void (MainWindow::*)(int, QProcess::ExitStatus))’ QObject::connect(process, &QProcess::finished, this, &MainWindow::updateCpu); ^

sierdzio

The documentation has the proper code for this connect.

Fuchsiaff

So something like this?

connect(process, QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
    [=](int exitCode, QProcess::ExitStatus exitStatus){ 
     QProcess *process = new QProcess(this);
      process->start("cat /proc/loadavg");
      QObject::connect(process, &QProcess::finished, this, &MainWindow::updateCpu);
      
 });

that connect looks even more confusing, i'm even more lost now

aha_1980

Hi @Fuchsiaff: Then try this:

QByteArray avg;
QFile f("/proc/loadavg");
if (f.open(QIODevice::ReadOnly)) {
  avg = f.readAll();
  f.close();
}
// avg now contains the content of /proc/loadavg

SGaist

QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished)

reads as: use the overloaded version of finished having the matching signature: QProcess::finished(int, QProcess::ExitStatus)

Fuchsiaff

I got it to compile with this code

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow) {
    ui->setupUi(this);
    QTimer *timer = new QTimer(this);
    timer->setInterval(3000);
    timer->setSingleShot(false);
    // Using a lambda, but you can (should) create your own slot for this, of course
    QObject::connect(timer, &QTimer::timeout, this, [this] {
      QProcess *process = new QProcess(this);
      process->start("cat /proc/loadavg");
      qDebug() << "hi";
      QObject::connect(process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(updateCpu(int, QProcess::ExitStatus)));
    });
    cores = getCpuCores();

}

But now, looking at the application output section, the string "hi" isn't printed so i'm quessing the timer never reached the timeout signal, but why?

SGaist

From the code you posted, you don't start timer.

Qt6 topics merged with General

Creating custom slots and signals