Help with VoidRealms "Threading done correctly"

[Akiva]

http://www.youtube.com/watch?v=yazMHbIew0Q


So my issue is pretty obvious:

1. I created a new QT GUI Application
2. It is templated as such, that the main thread's constructor creates the UI
3. Thus, when I try to thread as Void Realms does, it creates a duplicate instance of the ui.

This is what I am trying to accomplish in a larger program:

1. A properly threaded program; I understand that the Qt documentation has been troublesome in this area. I am unsure if this is still the case.
2. A thread that communicates with a database, which can make changes to the main UI

The reason why I am using threads

1. To learn. Threading seems like it is a crucial skill to understand.
2. To make my program more efficient.
3. To avoid having my UI hang while it is trying to communicate with the database [which is over a network].

I went ahead and created a new application in QTCreator, File>New File or Project>Applications>QTGui Application
The lines I added from following Void Realms will have a comment beside them, "// Inserted"

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QThread> // Inserted
#include <QDebug> // Inserted
 
namespace Ui {
	class MainWindow;
}
 
class MainWindow : public QMainWindow
{
	Q_OBJECT
 
public:
	explicit MainWindow(QWidget *parent = 0);
	~MainWindow();
	void THREAD_example(QThread &cThread); // Inserted
 
public slots:
	void METHOD_example(); // Inserted
 
private:
	Ui::MainWindow *ui;
};
 
#endif // MAINWINDOW_H

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QThread> // Inserted
#include <QDebug> // Inserted

namespace Ui {
	class MainWindow;
}

class MainWindow : public QMainWindow
{
	Q_OBJECT

public:
	explicit MainWindow(QWidget *parent = 0);
	~MainWindow();
	void THREAD_example(QThread &cThread); // Inserted

public slots:
	void METHOD_example(); // Inserted

private:
	Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

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#include "mainwindow.h"
#include "ui_mainwindow.h"
 
MainWindow::MainWindow(QWidget *parent) :
	QMainWindow(parent),
	ui(new Ui::MainWindow)
{
	ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
	delete ui;
}
 
// Inserted
void MainWindow::THREAD_example(QThread &cThread)
{
	connect(&cThread,SIGNAL(started()),this,SLOT(METHOD_example()));
}
 
// Inserted
void MainWindow::METHOD_example()
{
	qDebug() << "Snakes";
}

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
	QMainWindow(parent),
	ui(new Ui::MainWindow)
{
	ui->setupUi(this);
}

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

// Inserted
void MainWindow::THREAD_example(QThread &cThread)
{
	connect(&cThread,SIGNAL(started()),this,SLOT(METHOD_example()));
}

// Inserted
void MainWindow::METHOD_example()
{
	qDebug() << "Snakes";
}

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#include "mainwindow.h"
#include <QApplication>
 
int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	MainWindow w;
	w.show();
 
// Inserted
	QThread cThread;
	MainWindow cObject;
	cObject.THREAD_example(cThread);
	cObject.moveToThread(&cThread);
	cThread.start();
 
	return a.exec();
}

#include "mainwindow.h"
#include <QApplication>

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

// Inserted
	QThread cThread;
	MainWindow cObject;
	cObject.THREAD_example(cThread);
	cObject.moveToThread(&cThread);
	cThread.start();

	return a.exec();
}

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So how should I change this code, that while still threading "correctly", that I am not creating a duplicate instance of the UI?

[wysota]

You cannot access the user interface from a worker thread.

[Akiva]

Right, I remember that; Give me a half an hour, and I'll try to restructure the application above that the main is setup to receive a signal from the worker.

[wysota]

Just remember that using threads "just because you can" is usually a bad idea. Think if you really benefit from having threads in your program in any way.

[Akiva]

I believe I figured out what I needed to do.

I made a new class, and just added a parameter to my connections which takes the ui object created in main.cpp

Without going too much into it, here is a snippet from my worker class source

void example::THREAD_example(QThread &cThread, MainWindow &w)
{
	connect(&cThread,SIGNAL(started()),&w,SLOT(METHOD_example()));
}

void example::THREAD_example(QThread &cThread, MainWindow &w)
{
	connect(&cThread,SIGNAL(started()),&w,SLOT(METHOD_example()));
}

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METHOD_example() is a public slot in the main thread, which takes signals from my worker class, blah blah blah

anyways, thanks :P

[anda_skoa]

Your original code was almost correct. You can make it work properly by just removing this line

cObject.moveToThread(&cThread);

cObject.moveToThread(&cThread);

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But since the thread is not doing anything it is not very useful

Cheers,
_