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.