Question

The mainwindow.cpp:

#include "ui_mainwindow.h"
#include <QtCore>

/* ****** Thread part ****** */
myThread::myThread(QObject *parent)
    : QThread(parent)
{
}

void myThread::run()
{
    while(1){
        qDebug("thread one----------");
        emit threadSignal1();
        usleep(100000);
    }
    exec();
}

myThread2::myThread2(QObject *parent)
    : QThread(parent)
{
}

void myThread2::run()
{
    while(1){
        qDebug("thread two");
        emit threadSignal2();
        usleep(100000);
    }
    exec();
}
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    onethread = new myThread(this);
    onethread->start(QThread::NormalPriority);

    twothread = new myThread2(this);
    twothread->start(QThread::NormalPriority);

    connect(onethread, SIGNAL(onethread->threadSignal1()),
            this, SLOT(mySlot1()));
    connect(twothread, SIGNAL(threadSignal2()),
            this, SLOT(mySlot2()),Qt::QueuedConnection);
}

void MainWindow::mySlot1()
{
    ui->textEdit1->append("This is thread1");
}

void MainWindow::mySlot2()
{
    ui->textEdit1->append("This is thread2");
}

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

void MainWindow::on_pushButton_clicked()
{
    ui->textEdit1->append("textEdit1");
    ui->textEdit2->append("textEdit2");
}

The mainwindow.h:

#define MAINWINDOW_H

#include <QMainWindow>
#include <QThread>

namespace Ui {
class MainWindow;
}

class myThread : public QThread
{
  Q_OBJECT

public:
    myThread(QObject *parent = 0);
    void run();

signals:
    void threadSignal1();
};

class myThread2 : public QThread
{
  Q_OBJECT

public:
    myThread2(QObject *parent = 0);
    void run();

signals:
    void threadSignal2();
};

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    void mySlot1();
    void mySlot2();

private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;
    myThread *onethread;
    myThread2 *twothread;
};

#endif // MAINWINDOW_H

Please check the above code. It can give the qDebug output normally while the textEdit1/2 have no any output. And it seems to be a multi-thread signal/slot connect issue. Who can help? Thanks!

Answer 1

You need to define the slots as slots:

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

public slots:
    void mySlot1();
    void mySlot2();
...

Also a few other things. You don’t need the exec() call in each of the threads. They are going into an endless loop anyways and would never reach that statement. And, the purpose of exec() is to start an event loop in the thread so that it can receive and process events, like having its own slots. You are just emitting.

Are you sure this connection works?

connect(onethread, SIGNAL(onethread->threadSignal1()),
        this, SLOT(mySlot1()));

It should probably be:

connect(onethread, SIGNAL(threadSignal1()),
        this, SLOT(mySlot1()));

I believe QueuedConnection will be implied for the connections because they are targeting a slot on an owner in a different thread than the emitter.