QThread and MainWindow (Qt designer)

[darksoul_e]

Hi!

I am developing my first application with Qt and I would like to ask you the way to update the MainWindow from a thread.

The main window has been created with Qt designer, and now I want to update one of its Widgets (QTextEdit) from a thread. I have managed to do that in Mac OS X, creating a public function for MainWindow that updates QTextEdit. The pointer to MainWindow is passed to the thread and the thread is using the function. I am sure that is not the best way to do this. It seems that when I am passing the pointer of the MainWindow to the thread and using the function the program is not working.

How could I update the QTextEdit? (I want to append a QString)

Thank you in advance!

[Zombieisme]

Hi darksoul_e!

You can use the signal / slot to connect between your thread and main thread. emit signal from your thread and slot in main thread will update QTextEdit.

[darksoul_e]

Thank you for your answer Zombieisme!

I was thinking in that option, but I do not understand how can I pass the QString using signals. I will try to do it, otherwise any suggestion or help is welcome

Thank you!

[darksoul_e]

Ok! reading the Qt book again, I realized that I can pass parameters in the signals... This way I can pass the QString

[divanov]

I was thinking in that option, but I do not understand how can I pass the QString using signals. I will try to do it, otherwise any suggestion or help is welcome

Refer to the documentation
http://doc.trolltech.com/4.6/signalsandslots.html
This is how you emit a signal with QString as a parameter:

QString stringParameter = "Test string";
emit signalName(stringParameter);

[darksoul_e]

I think I am missing something important with signals

I am using the book C++ GUI programming with Qt3, but the example is very similar to the one in the link. MyThread.h looks like this:

#include "mainwindow.h"
#include <QObject>
#include <QThread>

class MyThread : public QThread
    {
    Q_OBJECT

    public:
        MyThread(QObject* parent = 0);
        virtual ~MyThread();

    public: // From QThread
        void run();
        QString str;

    signals:
        void sendString(const QString &str);
    public slots:

    private:
    };

The slot of the MainWindow is:

#include <QMainWindow>

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow {
    Q_OBJECT
public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();
    void UpdateChat(QString str);

protected:
    void changeEvent(QEvent *e);

private:
    Ui::MainWindow *ui;
private slots:
   (many)
public slots:
    void display(const QString &str);
};

And now, in MyThread.cpp I do not know how to connect the signal

    connect(this, SIGNAL(sendString(const QString &)),
            ?, SLOT(display(const QString &)));

Do I have to pass the reference of the instance of MainWindow to the thread to send the signal? How can I connect both, the thread and the MainWindow?

The files that I have are: mainwindow.cpp , main.cpp and mythread.cpp

Thank you in advance

[divanov]

And now, in MyThread.cpp I do not know how to connect the signal

    connect(this, SIGNAL(sendString(const QString &)),
            ?, SLOT(display(const QString &)));

You connect Thread's signal o Window's slot in main.cpp.

[darksoul_e]

Sorry! I am doing the connection within the thread, so this refers to my thread.

[darksoul_e]

Finally I achieved that, but I do not know if the solution is very elegant...

From the main I have passed to the Thread the reference of MainWindow:

class MyThread : public QThread
    {
    Q_OBJECT

    public:
        MyThread(QObject* parent = 0);
        virtual ~MyThread();

    public: // From QThread
        void run();
        QString str;
        MainWindow *w;

    signals:
        void sendString(const QString &str);
    public slots:

    private:
    };

Main:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    MyThread *thread;

    w.show(); 
    thread = new MyThread();
    thread->w=&w;
    thread->start();
    //thread.wait();
    return a.exec();
}

And I connect this way in MyThread.cpp

QObject::connect(this, SIGNAL(sendString(const QString &)),
            w, SLOT(display(const QString &)));

[divanov]

Could you consider this way of doing a thing?

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    MyThread thread;

    QObject::connect(&thread, SIGNAL(sendString(const QString &)),
                     &w, SLOT(display(const QString &)));
    w.show(); 
    thread.start();

    return a.exec();
}

[darksoul_e]

Yes This way is better. Thank you!

I have changed your code to use the heap for the thread.

Could you consider this way of doing a thing?

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    MyThread *thread;

    thread=new MyThread();
    QObject::connect(thread, SIGNAL(sendString(const QString &)),
                     &w, SLOT(display(const QString &)));
    w.show(); 
    thread.start();

    return a.exec();
}

[divanov]

There is no really good reason to use heap, but if you allocating your thread object dynamically, then you should free memory at some point, otherwise you have a memory leak.